postgres/src/backend/access/transam/xlog.c

/*-------------------------------------------------------------------------
 *
 * xlog.c
 *		PostgreSQL transaction log manager
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xlog.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <time.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/multixact.h"
#include "access/subtrans.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_database.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "postmaster/startup.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "storage/barrier.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/reinit.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/ps_status.h"
#include "utils/relmapper.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
#include "pg_trace.h"

extern uint32 bootstrap_data_checksum_version;

/* File path names (all relative to $PGDATA) */
#define RECOVERY_COMMAND_FILE	"recovery.conf"
#define RECOVERY_COMMAND_DONE	"recovery.done"
#define PROMOTE_SIGNAL_FILE "promote"
#define FAST_PROMOTE_SIGNAL_FILE "fast_promote"


/* User-settable parameters */
int			CheckPointSegments = 3;
int			wal_keep_segments = 0;
int			XLOGbuffers = -1;
int			XLogArchiveTimeout = 0;
bool		XLogArchiveMode = false;
char	   *XLogArchiveCommand = NULL;
bool		EnableHotStandby = false;
bool		fullPageWrites = true;
bool		log_checkpoints = false;
int			sync_method = DEFAULT_SYNC_METHOD;
int			wal_level = WAL_LEVEL_MINIMAL;
int			CommitDelay = 0;	/* precommit delay in microseconds */
int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
int			num_xloginsert_slots = 8;

#ifdef WAL_DEBUG
bool		XLOG_DEBUG = false;
#endif

/*
 * XLOGfileslop is the maximum number of preallocated future XLOG segments.
 * When we are done with an old XLOG segment file, we will recycle it as a
 * future XLOG segment as long as there aren't already XLOGfileslop future
 * segments; else we'll delete it.  This could be made a separate GUC
 * variable, but at present I think it's sufficient to hardwire it as
 * 2*CheckPointSegments+1.	Under normal conditions, a checkpoint will free
 * no more than 2*CheckPointSegments log segments, and we want to recycle all
 * of them; the +1 allows boundary cases to happen without wasting a
 * delete/create-segment cycle.
 */
#define XLOGfileslop	(2*CheckPointSegments + 1)


/*
 * GUC support
 */
const struct config_enum_entry sync_method_options[] = {
	{"fsync", SYNC_METHOD_FSYNC, false},
#ifdef HAVE_FSYNC_WRITETHROUGH
	{"fsync_writethrough", SYNC_METHOD_FSYNC_WRITETHROUGH, false},
#endif
#ifdef HAVE_FDATASYNC
	{"fdatasync", SYNC_METHOD_FDATASYNC, false},
#endif
#ifdef OPEN_SYNC_FLAG
	{"open_sync", SYNC_METHOD_OPEN, false},
#endif
#ifdef OPEN_DATASYNC_FLAG
	{"open_datasync", SYNC_METHOD_OPEN_DSYNC, false},
#endif
	{NULL, 0, false}
};

/*
 * Statistics for current checkpoint are collected in this global struct.
 * Because only the background writer or a stand-alone backend can perform
 * checkpoints, this will be unused in normal backends.
 */
CheckpointStatsData CheckpointStats;

/*
 * ThisTimeLineID will be same in all backends --- it identifies current
 * WAL timeline for the database system.
 */
TimeLineID	ThisTimeLineID = 0;

/*
 * Are we doing recovery from XLOG?
 *
 * This is only ever true in the startup process; it should be read as meaning
 * "this process is replaying WAL records", rather than "the system is in
 * recovery mode".  It should be examined primarily by functions that need
 * to act differently when called from a WAL redo function (e.g., to skip WAL
 * logging).  To check whether the system is in recovery regardless of which
 * process you're running in, use RecoveryInProgress() but only after shared
 * memory startup and lock initialization.
 */
bool		InRecovery = false;

/* Are we in Hot Standby mode? Only valid in startup process, see xlog.h */
HotStandbyState standbyState = STANDBY_DISABLED;

static XLogRecPtr LastRec;

/* Local copy of WalRcv->receivedUpto */
static XLogRecPtr receivedUpto = 0;
static TimeLineID receiveTLI = 0;

/*
 * During recovery, lastFullPageWrites keeps track of full_page_writes that
 * the replayed WAL records indicate. It's initialized with full_page_writes
 * that the recovery starting checkpoint record indicates, and then updated
 * each time XLOG_FPW_CHANGE record is replayed.
 */
static bool lastFullPageWrites;

/*
 * Local copy of SharedRecoveryInProgress variable. True actually means "not
 * known, need to check the shared state".
 */
static bool LocalRecoveryInProgress = true;

/*
 * Local copy of SharedHotStandbyActive variable. False actually means "not
 * known, need to check the shared state".
 */
static bool LocalHotStandbyActive = false;

/*
 * Local state for XLogInsertAllowed():
 *		1: unconditionally allowed to insert XLOG
 *		0: unconditionally not allowed to insert XLOG
 *		-1: must check RecoveryInProgress(); disallow until it is false
 * Most processes start with -1 and transition to 1 after seeing that recovery
 * is not in progress.	But we can also force the value for special cases.
 * The coding in XLogInsertAllowed() depends on the first two of these states
 * being numerically the same as bool true and false.
 */
static int	LocalXLogInsertAllowed = -1;

/*
 * When ArchiveRecoveryRequested is set, archive recovery was requested,
 * ie. recovery.conf file was present. When InArchiveRecovery is set, we are
 * currently recovering using offline XLOG archives. These variables are only
 * valid in the startup process.
 *
 * When ArchiveRecoveryRequested is true, but InArchiveRecovery is false, we're
 * currently performing crash recovery using only XLOG files in pg_xlog, but
 * will switch to using offline XLOG archives as soon as we reach the end of
 * WAL in pg_xlog.
*/
bool		ArchiveRecoveryRequested = false;
bool		InArchiveRecovery = false;

/* Was the last xlog file restored from archive, or local? */
static bool restoredFromArchive = false;

/* options taken from recovery.conf for archive recovery */
char	   *recoveryRestoreCommand = NULL;
static char *recoveryEndCommand = NULL;
static char *archiveCleanupCommand = NULL;
static RecoveryTargetType recoveryTarget = RECOVERY_TARGET_UNSET;
static bool recoveryTargetInclusive = true;
static bool recoveryPauseAtTarget = true;
static TransactionId recoveryTargetXid;
static TimestampTz recoveryTargetTime;
static char *recoveryTargetName;

/* options taken from recovery.conf for XLOG streaming */
static bool StandbyModeRequested = false;
static char *PrimaryConnInfo = NULL;
static char *TriggerFile = NULL;

/* are we currently in standby mode? */
bool		StandbyMode = false;

/* whether request for fast promotion has been made yet */
static bool fast_promote = false;

/* if recoveryStopsHere returns true, it saves actual stop xid/time/name here */
static TransactionId recoveryStopXid;
static TimestampTz recoveryStopTime;
static char recoveryStopName[MAXFNAMELEN];
static bool recoveryStopAfter;

/*
 * During normal operation, the only timeline we care about is ThisTimeLineID.
 * During recovery, however, things are more complicated.  To simplify life
 * for rmgr code, we keep ThisTimeLineID set to the "current" timeline as we
 * scan through the WAL history (that is, it is the line that was active when
 * the currently-scanned WAL record was generated).  We also need these
 * timeline values:
 *
 * recoveryTargetTLI: the desired timeline that we want to end in.
 *
 * recoveryTargetIsLatest: was the requested target timeline 'latest'?
 *
 * expectedTLEs: a list of TimeLineHistoryEntries for recoveryTargetTLI and the timelines of
 * its known parents, newest first (so recoveryTargetTLI is always the
 * first list member).	Only these TLIs are expected to be seen in the WAL
 * segments we read, and indeed only these TLIs will be considered as
 * candidate WAL files to open at all.
 *
 * curFileTLI: the TLI appearing in the name of the current input WAL file.
 * (This is not necessarily the same as ThisTimeLineID, because we could
 * be scanning data that was copied from an ancestor timeline when the current
 * file was created.)  During a sequential scan we do not allow this value
 * to decrease.
 */
static TimeLineID recoveryTargetTLI;
static bool recoveryTargetIsLatest = false;
static List *expectedTLEs;
static TimeLineID curFileTLI;

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 */
static XLogRecPtr ProcLastRecPtr = InvalidXLogRecPtr;

XLogRecPtr	XactLastRecEnd = InvalidXLogRecPtr;

/*
 * RedoRecPtr is this backend's local copy of the REDO record pointer
 * (which is almost but not quite the same as a pointer to the most recent
 * CHECKPOINT record).	We update this from the shared-memory copy,
 * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
 * hold an insertion slot).  See XLogInsert for details.  We are also allowed
 * to update from XLogCtl->RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.  A freshly spawned backend obtains the value during
 * InitXLOGAccess.
 */
static XLogRecPtr RedoRecPtr;

/*
 * RedoStartLSN points to the checkpoint's REDO location which is specified
 * in a backup label file, backup history file or control file. In standby
 * mode, XLOG streaming usually starts from the position where an invalid
 * record was found. But if we fail to read even the initial checkpoint
 * record, we use the REDO location instead of the checkpoint location as
 * the start position of XLOG streaming. Otherwise we would have to jump
 * backwards to the REDO location after reading the checkpoint record,
 * because the REDO record can precede the checkpoint record.
 */
static XLogRecPtr RedoStartLSN = InvalidXLogRecPtr;

/*----------
 * Shared-memory data structures for XLOG control
 *
 * LogwrtRqst indicates a byte position that we need to write and/or fsync
 * the log up to (all records before that point must be written or fsynced).
 * LogwrtResult indicates the byte positions we have already written/fsynced.
 * These structs are identical but are declared separately to indicate their
 * slightly different functions.
 *
 * To read XLogCtl->LogwrtResult, you must hold either info_lck or
 * WALWriteLock.  To update it, you need to hold both locks.  The point of
 * this arrangement is that the value can be examined by code that already
 * holds WALWriteLock without needing to grab info_lck as well.  In addition
 * to the shared variable, each backend has a private copy of LogwrtResult,
 * which is updated when convenient.
 *
 * The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
 * (protected by info_lck), but we don't need to cache any copies of it.
 *
 * info_lck is only held long enough to read/update the protected variables,
 * so it's a plain spinlock.  The other locks are held longer (potentially
 * over I/O operations), so we use LWLocks for them.  These locks are:
 *
 * WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
 * It is only held while initializing and changing the mapping.  If the
 * contents of the buffer being replaced haven't been written yet, the mapping
 * lock is released while the write is done, and reacquired afterwards.
 *
 * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
 * XLogFlush).
 *
 * ControlFileLock: must be held to read/update control file or create
 * new log file.
 *
 * CheckpointLock: must be held to do a checkpoint or restartpoint (ensures
 * only one checkpointer at a time; currently, with all checkpoints done by
 * the checkpointer, this is just pro forma).
 *
 *----------
 */

typedef struct XLogwrtRqst
{
	XLogRecPtr	Write;			/* last byte + 1 to write out */
	XLogRecPtr	Flush;			/* last byte + 1 to flush */
} XLogwrtRqst;

typedef struct XLogwrtResult
{
	XLogRecPtr	Write;			/* last byte + 1 written out */
	XLogRecPtr	Flush;			/* last byte + 1 flushed */
} XLogwrtResult;


/*
 * A slot for inserting to the WAL. This is similar to an LWLock, the main
 * difference is that there is an extra xlogInsertingAt field that is protected
 * by the same mutex. Unlike an LWLock, a slot can only be acquired in
 * exclusive mode.
 *
 * The xlogInsertingAt field is used to advertise to other processes how far
 * the slot owner has progressed in inserting the record. When a backend
 * acquires a slot, it initializes xlogInsertingAt to 1, because it doesn't
 * yet know where it's going to insert the record. That's conservative
 * but correct; the new insertion is certainly going to go to a byte position
 * greater than 1. If another backend needs to flush the WAL, it will have to
 * wait for the new insertion. xlogInsertingAt is updated after finishing the
 * insert or when crossing a page boundary, which will wake up anyone waiting
 * for it, whether the wait was necessary in the first place or not.
 *
 * A process can wait on a slot in two modes: LW_EXCLUSIVE or
 * LW_WAIT_UNTIL_FREE. LW_EXCLUSIVE works like in an lwlock; when the slot is
 * released, the first LW_EXCLUSIVE waiter in the queue is woken up. Processes
 * waiting in LW_WAIT_UNTIL_FREE mode are woken up whenever the slot is
 * released, or xlogInsertingAt is updated. In other words, a process in
 * LW_WAIT_UNTIL_FREE mode is woken up whenever the inserter makes any progress
 * copying the record in place. LW_WAIT_UNTIL_FREE waiters are always added to
 * the front of the queue, while LW_EXCLUSIVE waiters are appended to the end.
 *
 * To join the wait queue, a process must set MyProc->lwWaitMode to the mode
 * it wants to wait in, MyProc->lwWaiting to true, and link MyProc to the head
 * or tail of the wait queue. The same mechanism is used to wait on an LWLock,
 * see lwlock.c for details.
 */
typedef struct
{
	slock_t		mutex;			/* protects the below fields */
	XLogRecPtr	xlogInsertingAt; /* insert has completed up to this point */

	PGPROC	   *owner;			/* for debugging purposes */

	bool		releaseOK;		/* T if ok to release waiters */
	char		exclusive;		/* # of exclusive holders (0 or 1) */
	PGPROC	   *head;			/* head of list of waiting PGPROCs */
	PGPROC	   *tail;			/* tail of list of waiting PGPROCs */
	/* tail is undefined when head is NULL */
} XLogInsertSlot;

/*
 * All the slots are allocated as an array in shared memory. We force the
 * array stride to be a power of 2, which saves a few cycles in indexing, but
 * more importantly also ensures that individual slots don't cross cache line
 * boundaries.	(Of course, we have to also ensure that the array start
 * address is suitably aligned.)
 */
typedef union XLogInsertSlotPadded
{
	XLogInsertSlot slot;
	char		pad[64];
} XLogInsertSlotPadded;

/*
 * Shared state data for XLogInsert.
 */
typedef struct XLogCtlInsert
{
	slock_t		insertpos_lck;	/* protects CurrBytePos and PrevBytePos */

	/*
	 * CurrBytePos is the end of reserved WAL. The next record will be inserted
	 * at that position. PrevBytePos is the start position of the previously
	 * inserted (or rather, reserved) record - it is copied to the the prev-
	 * link of the next record. These are stored as "usable byte positions"
	 * rather than XLogRecPtrs (see XLogBytePosToRecPtr()).
	 */
	uint64		CurrBytePos;
	uint64		PrevBytePos;

	/* insertion slots, see above for details */
	XLogInsertSlotPadded *insertSlots;

	/*
	 * fullPageWrites is the master copy used by all backends to determine
	 * whether to write full-page to WAL, instead of using process-local one.
	 * This is required because, when full_page_writes is changed by SIGHUP,
	 * we must WAL-log it before it actually affects WAL-logging by backends.
	 * Checkpointer sets at startup or after SIGHUP.
	 *
	 * To read these fields, you must hold an insertion slot. To modify them,
	 * you must hold ALL the slots.
	 */
	XLogRecPtr	RedoRecPtr;		/* current redo point for insertions */
	bool		forcePageWrites;	/* forcing full-page writes for PITR? */
	bool		fullPageWrites;

	/*
	 * exclusiveBackup is true if a backup started with pg_start_backup() is
	 * in progress, and nonExclusiveBackups is a counter indicating the number
	 * of streaming base backups currently in progress. forcePageWrites is set
	 * to true when either of these is non-zero. lastBackupStart is the latest
	 * checkpoint redo location used as a starting point for an online backup.
	 */
	bool		exclusiveBackup;
	int			nonExclusiveBackups;
	XLogRecPtr	lastBackupStart;
} XLogCtlInsert;

/*
 * Total shared-memory state for XLOG.
 */
typedef struct XLogCtlData
{
	XLogCtlInsert Insert;

	/* Protected by info_lck: */
	XLogwrtRqst LogwrtRqst;
	XLogRecPtr	RedoRecPtr;		/* a recent copy of Insert->RedoRecPtr */
	uint32		ckptXidEpoch;	/* nextXID & epoch of latest checkpoint */
	TransactionId ckptXid;
	XLogRecPtr	asyncXactLSN;	/* LSN of newest async commit/abort */
	XLogSegNo	lastRemovedSegNo;		/* latest removed/recycled XLOG
										 * segment */

	/* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */
	XLogRecPtr	unloggedLSN;
	slock_t		ulsn_lck;

	/* Time of last xlog segment switch. Protected by WALWriteLock. */
	pg_time_t	lastSegSwitchTime;

	/*
	 * Protected by info_lck and WALWriteLock (you must hold either lock to
	 * read it, but both to update)
	 */
	XLogwrtResult LogwrtResult;

	/*
	 * Latest initialized page in the cache (last byte position + 1).
	 *
	 * To change the identity of a buffer (and InitializedUpTo), you need to
	 * hold WALBufMappingLock.  To change the identity of a buffer that's still
	 * dirty, the old page needs to be written out first, and for that you
	 * need WALWriteLock, and you need to ensure that there are no in-progress
	 * insertions to the page by calling WaitXLogInsertionsToFinish().
	 */
	XLogRecPtr	InitializedUpTo;

	/*
	 * These values do not change after startup, although the pointed-to pages
	 * and xlblocks values certainly do.  xlblock values are protected by
	 * WALBufMappingLock.
	 */
	char	   *pages;			/* buffers for unwritten XLOG pages */
	XLogRecPtr *xlblocks;		/* 1st byte ptr-s + XLOG_BLCKSZ */
	int			XLogCacheBlck;	/* highest allocated xlog buffer index */

	/*
	 * Shared copy of ThisTimeLineID. Does not change after end-of-recovery.
	 * If we created a new timeline when the system was started up,
	 * PrevTimeLineID is the old timeline's ID that we forked off from.
	 * Otherwise it's equal to ThisTimeLineID.
	 */
	TimeLineID	ThisTimeLineID;
	TimeLineID	PrevTimeLineID;

	/*
	 * archiveCleanupCommand is read from recovery.conf but needs to be in
	 * shared memory so that the checkpointer process can access it.
	 */
	char		archiveCleanupCommand[MAXPGPATH];

	/*
	 * SharedRecoveryInProgress indicates if we're still in crash or archive
	 * recovery.  Protected by info_lck.
	 */
	bool		SharedRecoveryInProgress;

	/*
	 * SharedHotStandbyActive indicates if we're still in crash or archive
	 * recovery.  Protected by info_lck.
	 */
	bool		SharedHotStandbyActive;

	/*
	 * WalWriterSleeping indicates whether the WAL writer is currently in
	 * low-power mode (and hence should be nudged if an async commit occurs).
	 * Protected by info_lck.
	 */
	bool		WalWriterSleeping;

	/*
	 * recoveryWakeupLatch is used to wake up the startup process to continue
	 * WAL replay, if it is waiting for WAL to arrive or failover trigger file
	 * to appear.
	 */
	Latch		recoveryWakeupLatch;

	/*
	 * During recovery, we keep a copy of the latest checkpoint record here.
	 * Used by the background writer when it wants to create a restartpoint.
	 *
	 * Protected by info_lck.
	 */
	XLogRecPtr	lastCheckPointRecPtr;
	CheckPoint	lastCheckPoint;

	/*
	 * lastReplayedEndRecPtr points to end+1 of the last record successfully
	 * replayed. When we're currently replaying a record, ie. in a redo
	 * function, replayEndRecPtr points to the end+1 of the record being
	 * replayed, otherwise it's equal to lastReplayedEndRecPtr.
	 */
	XLogRecPtr	lastReplayedEndRecPtr;
	TimeLineID	lastReplayedTLI;
	XLogRecPtr	replayEndRecPtr;
	TimeLineID	replayEndTLI;
	/* timestamp of last COMMIT/ABORT record replayed (or being replayed) */
	TimestampTz recoveryLastXTime;
	/* current effective recovery target timeline */
	TimeLineID	RecoveryTargetTLI;

	/*
	 * timestamp of when we started replaying the current chunk of WAL data,
	 * only relevant for replication or archive recovery
	 */
	TimestampTz currentChunkStartTime;
	/* Are we requested to pause recovery? */
	bool		recoveryPause;

	/*
	 * lastFpwDisableRecPtr points to the start of the last replayed
	 * XLOG_FPW_CHANGE record that instructs full_page_writes is disabled.
	 */
	XLogRecPtr	lastFpwDisableRecPtr;

	slock_t		info_lck;		/* locks shared variables shown above */
} XLogCtlData;

static XLogCtlData *XLogCtl = NULL;

/*
 * We maintain an image of pg_control in shared memory.
 */
static ControlFileData *ControlFile = NULL;

/*
 * Calculate the amount of space left on the page after 'endptr'. Beware
 * multiple evaluation!
 */
#define INSERT_FREESPACE(endptr)	\
	(((endptr) % XLOG_BLCKSZ == 0) ? 0 : (XLOG_BLCKSZ - (endptr) % XLOG_BLCKSZ))

/* Macro to advance to next buffer index. */
#define NextBufIdx(idx)		\
		(((idx) == XLogCtl->XLogCacheBlck) ? 0 : ((idx) + 1))

/*
 * XLogRecPtrToBufIdx returns the index of the WAL buffer that holds, or
 * would hold if it was in cache, the page containing 'recptr'.
 */
#define XLogRecPtrToBufIdx(recptr)	\
	(((recptr) / XLOG_BLCKSZ) % (XLogCtl->XLogCacheBlck + 1))

/*
 * These are the number of bytes in a WAL page and segment usable for WAL data.
 */
#define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)
#define UsableBytesInSegment ((XLOG_SEG_SIZE / XLOG_BLCKSZ) * UsableBytesInPage - (SizeOfXLogLongPHD - SizeOfXLogShortPHD))

/*
 * Private, possibly out-of-date copy of shared LogwrtResult.
 * See discussion above.
 */
static XLogwrtResult LogwrtResult = {0, 0};

/*
 * Codes indicating where we got a WAL file from during recovery, or where
 * to attempt to get one.
 */
typedef enum
{
	XLOG_FROM_ANY = 0,			/* request to read WAL from any source */
	XLOG_FROM_ARCHIVE,			/* restored using restore_command */
	XLOG_FROM_PG_XLOG,			/* existing file in pg_xlog */
	XLOG_FROM_STREAM,			/* streamed from master */
} XLogSource;

/* human-readable names for XLogSources, for debugging output */
static const char *xlogSourceNames[] = {"any", "archive", "pg_xlog", "stream"};

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * When it's open, openLogOff is the current seek offset in the file.
 * openLogSegNo identifies the segment.  These variables are only
 * used to write the XLOG, and so will normally refer to the active segment.
 */
static int	openLogFile = -1;
static XLogSegNo openLogSegNo = 0;
static uint32 openLogOff = 0;

/*
 * These variables are used similarly to the ones above, but for reading
 * the XLOG.  Note, however, that readOff generally represents the offset
 * of the page just read, not the seek position of the FD itself, which
 * will be just past that page. readLen indicates how much of the current
 * page has been read into readBuf, and readSource indicates where we got
 * the currently open file from.
 */
static int	readFile = -1;
static XLogSegNo readSegNo = 0;
static uint32 readOff = 0;
static uint32 readLen = 0;
static XLogSource readSource = 0;		/* XLOG_FROM_* code */

/*
 * Keeps track of which source we're currently reading from. This is
 * different from readSource in that this is always set, even when we don't
 * currently have a WAL file open. If lastSourceFailed is set, our last
 * attempt to read from currentSource failed, and we should try another source
 * next.
 */
static XLogSource currentSource = 0;	/* XLOG_FROM_* code */
static bool lastSourceFailed = false;

typedef struct XLogPageReadPrivate
{
	int			emode;
	bool		fetching_ckpt;	/* are we fetching a checkpoint record? */
	bool		randAccess;
} XLogPageReadPrivate;

/*
 * These variables track when we last obtained some WAL data to process,
 * and where we got it from.  (XLogReceiptSource is initially the same as
 * readSource, but readSource gets reset to zero when we don't have data
 * to process right now.  It is also different from currentSource, which
 * also changes when we try to read from a source and fail, while
 * XLogReceiptSource tracks where we last successfully read some WAL.)
 */
static TimestampTz XLogReceiptTime = 0;
static XLogSource XLogReceiptSource = 0;		/* XLOG_FROM_* code */

/* State information for XLOG reading */
static XLogRecPtr ReadRecPtr;	/* start of last record read */
static XLogRecPtr EndRecPtr;	/* end+1 of last record read */

static XLogRecPtr minRecoveryPoint;		/* local copy of
										 * ControlFile->minRecoveryPoint */
static TimeLineID minRecoveryPointTLI;
static bool updateMinRecoveryPoint = true;

/*
 * Have we reached a consistent database state? In crash recovery, we have
 * to replay all the WAL, so reachedConsistency is never set. During archive
 * recovery, the database is consistent once minRecoveryPoint is reached.
 */
bool		reachedConsistency = false;

static bool InRedo = false;

/* Have we launched bgwriter during recovery? */
static bool bgwriterLaunched = false;

/* For WALInsertSlotAcquire/Release functions */
static int	MySlotNo = 0;
static bool holdingAllSlots = false;

static void readRecoveryCommandFile(void);
static void exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo);
static bool recoveryStopsHere(XLogRecord *record, bool *includeThis);
static void recoveryPausesHere(void);
static void SetLatestXTime(TimestampTz xtime);
static void SetCurrentChunkStartTime(TimestampTz xtime);
static void CheckRequiredParameterValues(void);
static void XLogReportParameters(void);
static void checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI,
					TimeLineID prevTLI);
static void LocalSetXLogInsertAllowed(void);
static void CreateEndOfRecoveryRecord(void);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);
static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo);

static bool XLogCheckBuffer(XLogRecData *rdata, bool holdsExclusiveLock,
				XLogRecPtr *lsn, BkpBlock *bkpb);
static Buffer RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb,
						 char *blk, bool get_cleanup_lock, bool keep_buffer);
static void AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic);
static bool XLogCheckpointNeeded(XLogSegNo new_segno);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible);
static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
					   bool find_free, int *max_advance,
					   bool use_lock);
static int XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
			 int source, bool notexistOk);
static int	XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source);
static int XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
			 int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
			 TimeLineID *readTLI);
static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
							bool fetching_ckpt, XLogRecPtr tliRecPtr);
static int	emode_for_corrupt_record(int emode, XLogRecPtr RecPtr);
static void XLogFileClose(void);
static void PreallocXlogFiles(XLogRecPtr endptr);
static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr);
static void UpdateLastRemovedPtr(char *filename);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force);
static XLogRecord *ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
		   int emode, bool fetching_ckpt);
static void CheckRecoveryConsistency(void);
static XLogRecord *ReadCheckpointRecord(XLogReaderState *xlogreader,
					 XLogRecPtr RecPtr, int whichChkpti, bool report);
static bool rescanLatestTimeLine(void);
static void WriteControlFile(void);
static void ReadControlFile(void);
static char *str_time(pg_time_t tnow);
static bool CheckForStandbyTrigger(void);

#ifdef WAL_DEBUG
static void xlog_outrec(StringInfo buf, XLogRecord *record);
#endif
static void pg_start_backup_callback(int code, Datum arg);
static bool read_backup_label(XLogRecPtr *checkPointLoc,
				  bool *backupEndRequired, bool *backupFromStandby);
static void rm_redo_error_callback(void *arg);
static int	get_sync_bit(int method);

static void CopyXLogRecordToWAL(int write_len, bool isLogSwitch,
				  XLogRecData *rdata,
				  XLogRecPtr StartPos, XLogRecPtr EndPos);
static void ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos,
						  XLogRecPtr *EndPos, XLogRecPtr *PrevPtr);
static bool ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos,
				  XLogRecPtr *PrevPtr);
static XLogRecPtr WaitXLogInsertionsToFinish(XLogRecPtr upto);
static void WakeupWaiters(XLogRecPtr EndPos);
static char *GetXLogBuffer(XLogRecPtr ptr);
static XLogRecPtr XLogBytePosToRecPtr(uint64 bytepos);
static XLogRecPtr XLogBytePosToEndRecPtr(uint64 bytepos);
static uint64 XLogRecPtrToBytePos(XLogRecPtr ptr);

static void WALInsertSlotAcquire(bool exclusive);
static void WALInsertSlotAcquireOne(int slotno);
static void WALInsertSlotRelease(void);
static void WALInsertSlotReleaseOne(int slotno);

/*
 * Insert an XLOG record having the specified RMID and info bytes,
 * with the body of the record being the data chunk(s) described by
 * the rdata chain (see xlog.h for notes about rdata).
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 *
 * NB: this routine feels free to scribble on the XLogRecData structs,
 * though not on the data they reference.  This is OK since the XLogRecData
 * structs are always just temporaries in the calling code.
 */
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
{
	XLogCtlInsert *Insert = &XLogCtl->Insert;
	XLogRecData *rdt;
	XLogRecData *rdt_lastnormal;
	Buffer		dtbuf[XLR_MAX_BKP_BLOCKS];
	bool		dtbuf_bkp[XLR_MAX_BKP_BLOCKS];
	BkpBlock	dtbuf_xlg[XLR_MAX_BKP_BLOCKS];
	XLogRecPtr	dtbuf_lsn[XLR_MAX_BKP_BLOCKS];
	XLogRecData dtbuf_rdt1[XLR_MAX_BKP_BLOCKS];
	XLogRecData dtbuf_rdt2[XLR_MAX_BKP_BLOCKS];
	XLogRecData dtbuf_rdt3[XLR_MAX_BKP_BLOCKS];
	XLogRecData hdr_rdt;
	pg_crc32	rdata_crc;
	uint32		len,
				write_len;
	unsigned	i;
	bool		doPageWrites;
	bool		isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);
	bool		inserted;
	uint8		info_orig = info;
	static XLogRecord *rechdr;
	XLogRecPtr	StartPos;
	XLogRecPtr	EndPos;

	if (rechdr == NULL)
	{
		rechdr = malloc(SizeOfXLogRecord);
		if (rechdr == NULL)
			elog(ERROR, "out of memory");
		MemSet(rechdr, 0, SizeOfXLogRecord);
	}

	/* cross-check on whether we should be here or not */
	if (!XLogInsertAllowed())
		elog(ERROR, "cannot make new WAL entries during recovery");

	/* info's high bits are reserved for use by me */
	if (info & XLR_INFO_MASK)
		elog(PANIC, "invalid xlog info mask %02X", info);

	TRACE_POSTGRESQL_XLOG_INSERT(rmid, info);

	/*
	 * In bootstrap mode, we don't actually log anything but XLOG resources;
	 * return a phony record pointer.
	 */
	if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
	{
		EndPos = SizeOfXLogLongPHD;		/* start of 1st chkpt record */
		return EndPos;
	}

	/*
	 * Here we scan the rdata chain, to determine which buffers must be backed
	 * up.
	 *
	 * We may have to loop back to here if a race condition is detected below.
	 * We could prevent the race by doing all this work while holding an
	 * insertion slot, but it seems better to avoid doing CRC calculations
	 * while holding one.
	 *
	 * We add entries for backup blocks to the chain, so that they don't need
	 * any special treatment in the critical section where the chunks are
	 * copied into the WAL buffers. Those entries have to be unlinked from the
	 * chain if we have to loop back here.
	 */
begin:;
	for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
	{
		dtbuf[i] = InvalidBuffer;
		dtbuf_bkp[i] = false;
	}

	/*
	 * Decide if we need to do full-page writes in this XLOG record: true if
	 * full_page_writes is on or we have a PITR request for it.  Since we
	 * don't yet have an insertion slot, fullPageWrites and forcePageWrites
	 * could change under us, but we'll recheck them once we have a slot.
	 */
	doPageWrites = Insert->fullPageWrites || Insert->forcePageWrites;

	len = 0;
	for (rdt = rdata;;)
	{
		if (rdt->buffer == InvalidBuffer)
		{
			/* Simple data, just include it */
			len += rdt->len;
		}
		else
		{
			/* Find info for buffer */
			for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
			{
				if (rdt->buffer == dtbuf[i])
				{
					/* Buffer already referenced by earlier chain item */
					if (dtbuf_bkp[i])
					{
						rdt->data = NULL;
						rdt->len = 0;
					}
					else if (rdt->data)
						len += rdt->len;
					break;
				}
				if (dtbuf[i] == InvalidBuffer)
				{
					/* OK, put it in this slot */
					dtbuf[i] = rdt->buffer;
					if (doPageWrites && XLogCheckBuffer(rdt, true,
										   &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
					{
						dtbuf_bkp[i] = true;
						rdt->data = NULL;
						rdt->len = 0;
					}
					else if (rdt->data)
						len += rdt->len;
					break;
				}
			}
			if (i >= XLR_MAX_BKP_BLOCKS)
				elog(PANIC, "can backup at most %d blocks per xlog record",
					 XLR_MAX_BKP_BLOCKS);
		}
		/* Break out of loop when rdt points to last chain item */
		if (rdt->next == NULL)
			break;
		rdt = rdt->next;
	}

	/*
	 * NOTE: We disallow len == 0 because it provides a useful bit of extra
	 * error checking in ReadRecord.  This means that all callers of
	 * XLogInsert must supply at least some not-in-a-buffer data.  However, we
	 * make an exception for XLOG SWITCH records because we don't want them to
	 * ever cross a segment boundary.
	 */
	if (len == 0 && !isLogSwitch)
		elog(PANIC, "invalid xlog record length %u", len);

	/*
	 * Make additional rdata chain entries for the backup blocks, so that we
	 * don't need to special-case them in the write loop.  This modifies the
	 * original rdata chain, but we keep a pointer to the last regular entry,
	 * rdt_lastnormal, so that we can undo this if we have to loop back to the
	 * beginning.
	 *
	 * At the exit of this loop, write_len includes the backup block data.
	 *
	 * Also set the appropriate info bits to show which buffers were backed
	 * up. The XLR_BKP_BLOCK(N) bit corresponds to the N'th distinct buffer
	 * value (ignoring InvalidBuffer) appearing in the rdata chain.
	 */
	rdt_lastnormal = rdt;
	write_len = len;
	for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
	{
		BkpBlock   *bkpb;
		char	   *page;

		if (!dtbuf_bkp[i])
			continue;

		info |= XLR_BKP_BLOCK(i);

		bkpb = &(dtbuf_xlg[i]);
		page = (char *) BufferGetBlock(dtbuf[i]);

		rdt->next = &(dtbuf_rdt1[i]);
		rdt = rdt->next;

		rdt->data = (char *) bkpb;
		rdt->len = sizeof(BkpBlock);
		write_len += sizeof(BkpBlock);

		rdt->next = &(dtbuf_rdt2[i]);
		rdt = rdt->next;

		if (bkpb->hole_length == 0)
		{
			rdt->data = page;
			rdt->len = BLCKSZ;
			write_len += BLCKSZ;
			rdt->next = NULL;
		}
		else
		{
			/* must skip the hole */
			rdt->data = page;
			rdt->len = bkpb->hole_offset;
			write_len += bkpb->hole_offset;

			rdt->next = &(dtbuf_rdt3[i]);
			rdt = rdt->next;

			rdt->data = page + (bkpb->hole_offset + bkpb->hole_length);
			rdt->len = BLCKSZ - (bkpb->hole_offset + bkpb->hole_length);
			write_len += rdt->len;
			rdt->next = NULL;
		}
	}

	/*
	 * Calculate CRC of the data, including all the backup blocks
	 *
	 * Note that the record header isn't added into the CRC initially since we
	 * don't know the prev-link yet.  Thus, the CRC will represent the CRC of
	 * the whole record in the order: rdata, then backup blocks, then record
	 * header.
	 */
	INIT_CRC32(rdata_crc);
	for (rdt = rdata; rdt != NULL; rdt = rdt->next)
		COMP_CRC32(rdata_crc, rdt->data, rdt->len);

	/*
	 * Construct record header (prev-link is filled in later, after reserving
	 * the space for the record), and make that the first chunk in the chain.
	 *
	 * The CRC calculated for the header here doesn't include prev-link,
	 * because we don't know it yet. It will be added later.
	 */
	rechdr->xl_xid = GetCurrentTransactionIdIfAny();
	rechdr->xl_tot_len = SizeOfXLogRecord + write_len;
	rechdr->xl_len = len;		/* doesn't include backup blocks */
	rechdr->xl_info = info;
	rechdr->xl_rmid = rmid;
	rechdr->xl_prev = InvalidXLogRecPtr;
	COMP_CRC32(rdata_crc, ((char *) rechdr), offsetof(XLogRecord, xl_prev));

	hdr_rdt.next = rdata;
	hdr_rdt.data = (char *) rechdr;
	hdr_rdt.len = SizeOfXLogRecord;
	write_len += SizeOfXLogRecord;

	/*----------
	 *
	 * We have now done all the preparatory work we can without holding a
	 * lock or modifying shared state. From here on, inserting the new WAL
	 * record to the shared WAL buffer cache is a two-step process:
	 *
	 * 1. Reserve the right amount of space from the WAL. The current head of
	 *    reserved space is kept in Insert->CurrBytePos, and is protected by
	 *    insertpos_lck.
	 *
	 * 2. Copy the record to the reserved WAL space. This involves finding the
	 *    correct WAL buffer containing the reserved space, and copying the
	 *    record in place. This can be done concurrently in multiple processes.
	 *
	 * To keep track of which insertions are still in-progress, each concurrent
	 * inserter allocates an "insertion slot", which tells others how far the
	 * inserter has progressed. There is a small fixed number of insertion
	 * slots, determined by the num_xloginsert_slots GUC. When an inserter
	 * finishes, it updates the xlogInsertingAt of its slot to the end of the
	 * record it inserted, to let others know that it's done. xlogInsertingAt
	 * is also updated when crossing over to a new WAL buffer, to allow the
	 * the previous buffer to be flushed.
	 *
	 * Holding onto a slot also protects RedoRecPtr and fullPageWrites from
	 * changing until the insertion is finished.
	 *
	 * Step 2 can usually be done completely in parallel. If the required WAL
	 * page is not initialized yet, you have to grab WALBufMappingLock to
	 * initialize it, but the WAL writer tries to do that ahead of insertions
	 * to avoid that from happening in the critical path.
	 *
	 *----------
	 */
	START_CRIT_SECTION();
	WALInsertSlotAcquire(isLogSwitch);

	/*
	 * Check to see if my RedoRecPtr is out of date.  If so, may have to go
	 * back and recompute everything.  This can only happen just after a
	 * checkpoint, so it's better to be slow in this case and fast otherwise.
	 *
	 * If we aren't doing full-page writes then RedoRecPtr doesn't actually
	 * affect the contents of the XLOG record, so we'll update our local copy
	 * but not force a recomputation.
	 */
	if (RedoRecPtr != Insert->RedoRecPtr)
	{
		Assert(RedoRecPtr < Insert->RedoRecPtr);
		RedoRecPtr = Insert->RedoRecPtr;

		if (doPageWrites)
		{
			for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
			{
				if (dtbuf[i] == InvalidBuffer)
					continue;
				if (dtbuf_bkp[i] == false &&
					dtbuf_lsn[i] <= RedoRecPtr)
				{
					/*
					 * Oops, this buffer now needs to be backed up, but we
					 * didn't think so above.  Start over.
					 */
					WALInsertSlotRelease();
					END_CRIT_SECTION();
					rdt_lastnormal->next = NULL;
					info = info_orig;
					goto begin;
				}
			}
		}
	}

	/*
	 * Also check to see if fullPageWrites or forcePageWrites was just turned
	 * on; if we weren't already doing full-page writes then go back and
	 * recompute. (If it was just turned off, we could recompute the record
	 * without full pages, but we choose not to bother.)
	 */
	if ((Insert->fullPageWrites || Insert->forcePageWrites) && !doPageWrites)
	{
		/* Oops, must redo it with full-page data. */
		WALInsertSlotRelease();
		END_CRIT_SECTION();
		rdt_lastnormal->next = NULL;
		info = info_orig;
		goto begin;
	}

	/*
	 * Reserve space for the record in the WAL. This also sets the xl_prev
	 * pointer.
	 */
	if (isLogSwitch)
		inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
	else
	{
		ReserveXLogInsertLocation(write_len, &StartPos, &EndPos,
								  &rechdr->xl_prev);
		inserted = true;
	}

	if (inserted)
	{
		/*
		 * Now that xl_prev has been filled in, finish CRC calculation of the
		 * record header.
		 */
		COMP_CRC32(rdata_crc, ((char *) &rechdr->xl_prev), sizeof(XLogRecPtr));
		FIN_CRC32(rdata_crc);
		rechdr->xl_crc = rdata_crc;

		/*
		 * All the record data, including the header, is now ready to be
		 * inserted. Copy the record in the space reserved.
		 */
		CopyXLogRecordToWAL(write_len, isLogSwitch, &hdr_rdt, StartPos, EndPos);
	}
	else
	{
		/*
		 * This was an xlog-switch record, but the current insert location was
		 * already exactly at the beginning of a segment, so there was no need
		 * to do anything.
		 */
	}

	/*
	 * Done! Let others know that we're finished.
	 */
	WALInsertSlotRelease();

	END_CRIT_SECTION();

	/*
	 * Update shared LogwrtRqst.Write, if we crossed page boundary.
	 */
	if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		/* advance global request to include new block(s) */
		if (xlogctl->LogwrtRqst.Write < EndPos)
			xlogctl->LogwrtRqst.Write = EndPos;
		/* update local result copy while I have the chance */
		LogwrtResult = xlogctl->LogwrtResult;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/*
	 * If this was an XLOG_SWITCH record, flush the record and the empty
	 * padding space that fills the rest of the segment, and perform
	 * end-of-segment actions (eg, notifying archiver).
	 */
	if (isLogSwitch)
	{
		TRACE_POSTGRESQL_XLOG_SWITCH();
		XLogFlush(EndPos);
		/*
		 * Even though we reserved the rest of the segment for us, which is
		 * reflected in EndPos, we return a pointer to just the end of the
		 * xlog-switch record.
		 */
		if (inserted)
		{
			EndPos = StartPos + SizeOfXLogRecord;
			if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
			{
				if (EndPos % XLOG_SEG_SIZE == EndPos % XLOG_BLCKSZ)
					EndPos += SizeOfXLogLongPHD;
				else
					EndPos += SizeOfXLogShortPHD;
			}
		}
	}

#ifdef WAL_DEBUG
	if (XLOG_DEBUG)
	{
		StringInfoData buf;

		initStringInfo(&buf);
		appendStringInfo(&buf, "INSERT @ %X/%X: ",
						 (uint32) (EndPos >> 32), (uint32) EndPos);
		xlog_outrec(&buf, rechdr);
		if (rdata->data != NULL)
		{
			appendStringInfo(&buf, " - ");
			RmgrTable[rechdr->xl_rmid].rm_desc(&buf, rechdr->xl_info, rdata->data);
		}
		elog(LOG, "%s", buf.data);
		pfree(buf.data);
	}
#endif

	/*
	 * Update our global variables
	 */
	ProcLastRecPtr = StartPos;
	XactLastRecEnd = EndPos;

	return EndPos;
}

/*
 * Reserves the right amount of space for a record of given size from the WAL.
 * *StartPos is set to the beginning of the reserved section, *EndPos to
 * its end+1. *PrevPtr is set to the beginning of the previous record; it is
 * used to set the xl_prev of this record.
 *
 * This is the performance critical part of XLogInsert that must be serialized
 * across backends. The rest can happen mostly in parallel. Try to keep this
 * section as short as possible, insertpos_lck can be heavily contended on a
 * busy system.
 *
 * NB: The space calculation here must match the code in CopyXLogRecordToWAL,
 * where we actually copy the record to the reserved space.
 */
static void
ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos, XLogRecPtr *EndPos,
						  XLogRecPtr *PrevPtr)
{
	volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
	uint64		startbytepos;
	uint64		endbytepos;
	uint64		prevbytepos;

	size = MAXALIGN(size);

	/* All (non xlog-switch) records should contain data. */
	Assert(size > SizeOfXLogRecord);

	/*
	 * The duration the spinlock needs to be held is minimized by minimizing
	 * the calculations that have to be done while holding the lock. The
	 * current tip of reserved WAL is kept in CurrBytePos, as a byte position
	 * that only counts "usable" bytes in WAL, that is, it excludes all WAL
	 * page headers. The mapping between "usable" byte positions and physical
	 * positions (XLogRecPtrs) can be done outside the locked region, and
	 * because the usable byte position doesn't include any headers, reserving
	 * X bytes from WAL is almost as simple as "CurrBytePos += X".
	 */
	SpinLockAcquire(&Insert->insertpos_lck);

	startbytepos = Insert->CurrBytePos;
	endbytepos = startbytepos + size;
	prevbytepos = Insert->PrevBytePos;
	Insert->CurrBytePos = endbytepos;
	Insert->PrevBytePos = startbytepos;

	SpinLockRelease(&Insert->insertpos_lck);

	*StartPos = XLogBytePosToRecPtr(startbytepos);
	*EndPos = XLogBytePosToEndRecPtr(endbytepos);
	*PrevPtr = XLogBytePosToRecPtr(prevbytepos);

	/*
	 * Check that the conversions between "usable byte positions" and
	 * XLogRecPtrs work consistently in both directions.
	 */
	Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
	Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
	Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);
}

/*
 * Like ReserveXLogInsertLocation(), but for an xlog-switch record.
 *
 * A log-switch record is handled slightly differently. The rest of the
 * segment will be reserved for this insertion, as indicated by the returned
 * *EndPos value. However, if we are already at the beginning of the current
 * segment, *StartPos and *EndPos are set to the current location without
 * reserving any space, and the function returns false.
*/
static bool
ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos, XLogRecPtr *PrevPtr)
{
	volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
	uint64		startbytepos;
	uint64		endbytepos;
	uint64		prevbytepos;
	uint32		size = SizeOfXLogRecord;
	XLogRecPtr	ptr;
	uint32		segleft;

	/*
	 * These calculations are a bit heavy-weight to be done while holding a
	 * spinlock, but since we're holding all the WAL insertion slots, there
	 * are no other inserters competing for it. GetXLogInsertRecPtr() does
	 * compete for it, but that's not called very frequently.
	 */
	SpinLockAcquire(&Insert->insertpos_lck);

	startbytepos = Insert->CurrBytePos;

	ptr = XLogBytePosToEndRecPtr(startbytepos);
	if (ptr % XLOG_SEG_SIZE == 0)
	{
		SpinLockRelease(&Insert->insertpos_lck);
		*EndPos = *StartPos = ptr;
		return false;
	}

	endbytepos = startbytepos + size;
	prevbytepos = Insert->PrevBytePos;

	*StartPos = XLogBytePosToRecPtr(startbytepos);
	*EndPos = XLogBytePosToEndRecPtr(endbytepos);

	segleft = XLOG_SEG_SIZE - ((*EndPos) % XLOG_SEG_SIZE);
	if (segleft != XLOG_SEG_SIZE)
	{
		/* consume the rest of the segment */
		*EndPos += segleft;
		endbytepos = XLogRecPtrToBytePos(*EndPos);
	}
	Insert->CurrBytePos = endbytepos;
	Insert->PrevBytePos = startbytepos;

	SpinLockRelease(&Insert->insertpos_lck);

	*PrevPtr = XLogBytePosToRecPtr(prevbytepos);

	Assert((*EndPos) % XLOG_SEG_SIZE == 0);
	Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
	Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
	Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);

	return true;
}

/*
 * Subroutine of XLogInsert.  Copies a WAL record to an already-reserved
 * area in the WAL.
 */
static void
CopyXLogRecordToWAL(int write_len, bool isLogSwitch, XLogRecData *rdata,
					XLogRecPtr StartPos, XLogRecPtr EndPos)
{
	char	   *currpos;
	int			freespace;
	int			written;
	XLogRecPtr	CurrPos;
	XLogPageHeader pagehdr;

	/* The first chunk is the record header */
	Assert(rdata->len == SizeOfXLogRecord);

	/*
	 * Get a pointer to the right place in the right WAL buffer to start
	 * inserting to.
	 */
	CurrPos = StartPos;
	currpos = GetXLogBuffer(CurrPos);
	freespace = INSERT_FREESPACE(CurrPos);

	/*
	 * there should be enough space for at least the first field (xl_tot_len)
	 * on this page.
	 */
	Assert(freespace >= sizeof(uint32));

	/* Copy record data */
	written = 0;
	while (rdata != NULL)
	{
		char	   *rdata_data = rdata->data;
		int			rdata_len = rdata->len;

		while (rdata_len > freespace)
		{
			/*
			 * Write what fits on this page, and continue on the next page.
			 */
			Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || freespace == 0);
			memcpy(currpos, rdata_data, freespace);
			rdata_data += freespace;
			rdata_len -= freespace;
			written += freespace;
			CurrPos += freespace;

			/*
			 * Get pointer to beginning of next page, and set the xlp_rem_len
			 * in the page header. Set XLP_FIRST_IS_CONTRECORD.
			 *
			 * It's safe to set the contrecord flag and xlp_rem_len without a
			 * lock on the page. All the other flags were already set when the
			 * page was initialized, in AdvanceXLInsertBuffer, and we're the
			 * only backend that needs to set the contrecord flag.
			 */
			currpos = GetXLogBuffer(CurrPos);
			pagehdr = (XLogPageHeader) currpos;
			pagehdr->xlp_rem_len = write_len - written;
			pagehdr->xlp_info |= XLP_FIRST_IS_CONTRECORD;

			/* skip over the page header */
			if (CurrPos % XLogSegSize == 0)
			{
				CurrPos += SizeOfXLogLongPHD;
				currpos += SizeOfXLogLongPHD;
			}
			else
			{
				CurrPos += SizeOfXLogShortPHD;
				currpos += SizeOfXLogShortPHD;
			}
			freespace = INSERT_FREESPACE(CurrPos);
		}

		Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || rdata_len == 0);
		memcpy(currpos, rdata_data, rdata_len);
		currpos += rdata_len;
		CurrPos += rdata_len;
		freespace -= rdata_len;
		written += rdata_len;

		rdata = rdata->next;
	}
	Assert(written == write_len);

	/* Align the end position, so that the next record starts aligned */
	CurrPos = MAXALIGN(CurrPos);

	/*
	 * If this was an xlog-switch, it's not enough to write the switch record,
	 * we also have to consume all the remaining space in the WAL segment.
	 * We have already reserved it for us, but we still need to make sure it's
	 * allocated and zeroed in the WAL buffers so that when the caller (or
	 * someone else) does XLogWrite(), it can really write out all the zeros.
	 */
	if (isLogSwitch && CurrPos % XLOG_SEG_SIZE != 0)
	{
		/* An xlog-switch record doesn't contain any data besides the header */
		Assert(write_len == SizeOfXLogRecord);

		/*
		 * We do this one page at a time, to make sure we don't deadlock
		 * against ourselves if wal_buffers < XLOG_SEG_SIZE.
		 */
		Assert(EndPos % XLogSegSize == 0);

		/* Use up all the remaining space on the first page */
		CurrPos += freespace;

		while (CurrPos < EndPos)
		{
			/* initialize the next page (if not initialized already) */
			WakeupWaiters(CurrPos);
			AdvanceXLInsertBuffer(CurrPos, false);
			CurrPos += XLOG_BLCKSZ;
		}
	}

	if (CurrPos != EndPos)
		elog(PANIC, "space reserved for WAL record does not match what was written");
}

/*
 * Allocate a slot for insertion.
 *
 * In exclusive mode, all slots are reserved for the current process. That
 * blocks all concurrent insertions.
 */
static void
WALInsertSlotAcquire(bool exclusive)
{
	int			i;

	if (exclusive)
	{
		for (i = 0; i < num_xloginsert_slots; i++)
			WALInsertSlotAcquireOne(i);
		holdingAllSlots = true;
	}
	else
		WALInsertSlotAcquireOne(-1);
}

/*
 * Workhorse of WALInsertSlotAcquire. Acquires the given slot, or an arbitrary
 * one if slotno == -1. The index of the slot that was acquired is stored in
 * MySlotNo.
 *
 * This is more or less equivalent to LWLockAcquire().
 */
static void
WALInsertSlotAcquireOne(int slotno)
{
	volatile XLogInsertSlot *slot;
	PGPROC	   *proc = MyProc;
	bool		retry = false;
	int			extraWaits = 0;
	static int	slotToTry = -1;

	/*
	 * Try to use the slot we used last time. If the system isn't particularly
	 * busy, it's a good bet that it's available, and it's good to have some
	 * affinity to a particular slot so that you don't unnecessarily bounce
	 * cache lines between processes when there is no contention.
	 *
	 * If this is the first time through in this backend, pick a slot
	 * (semi-)randomly. This allows the slots to be used evenly if you have a
	 * lot of very short connections.
	 */
	if (slotno != -1)
		MySlotNo = slotno;
	else
	{
		if (slotToTry == -1)
			slotToTry = MyProc->pgprocno % num_xloginsert_slots;
		MySlotNo = slotToTry;
	}

	/*
	 * We can't wait if we haven't got a PGPROC.  This should only occur
	 * during bootstrap or shared memory initialization.  Put an Assert here
	 * to catch unsafe coding practices.
	 */
	Assert(MyProc != NULL);

	/*
	 * Lock out cancel/die interrupts until we exit the code section protected
	 * by the slot.  This ensures that interrupts will not interfere with
	 * manipulations of data structures in shared memory. There is no cleanup
	 * mechanism to release the slot if the backend dies while holding one,
	 * so make this a critical section.
	 */
	START_CRIT_SECTION();

	/*
	 * Loop here to try to acquire slot after each time we are signaled by
	 * WALInsertSlotRelease.
	 */
	for (;;)
	{
		bool		mustwait;

		slot = &XLogCtl->Insert.insertSlots[MySlotNo].slot;

		/* Acquire mutex.  Time spent holding mutex should be short! */
		SpinLockAcquire(&slot->mutex);

		/* If retrying, allow WALInsertSlotRelease to release waiters again */
		if (retry)
			slot->releaseOK = true;

		/* If I can get the slot, do so quickly. */
		if (slot->exclusive == 0)
		{
			slot->exclusive++;
			mustwait = false;
		}
		else
			mustwait = true;

		if (!mustwait)
			break;				/* got the lock */

		Assert(slot->owner != MyProc);

		/*
		 * Add myself to wait queue.
		 */
		proc->lwWaiting = true;
		proc->lwWaitMode = LW_EXCLUSIVE;
		proc->lwWaitLink = NULL;
		if (slot->head == NULL)
			slot->head = proc;
		else
			slot->tail->lwWaitLink = proc;
		slot->tail = proc;

		/* Can release the mutex now */
		SpinLockRelease(&slot->mutex);

		/*
		 * Wait until awakened.
		 *
		 * Since we share the process wait semaphore with the regular lock
		 * manager and ProcWaitForSignal, and we may need to acquire a slot
		 * while one of those is pending, it is possible that we get awakened
		 * for a reason other than being signaled by WALInsertSlotRelease. If
		 * so, loop back and wait again.  Once we've gotten the slot,
		 * re-increment the sema by the number of additional signals received,
		 * so that the lock manager or signal manager will see the received
		 * signal when it next waits.
		 */
		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		/* Now loop back and try to acquire lock again. */
		retry = true;
	}

	slot->owner = proc;

	/*
	 * Normally, we initialize the xlogInsertingAt value of the slot to 1,
	 * because we don't yet know where in the WAL we're going to insert. It's
	 * not critical what it points to right now - leaving it to a too small
	 * value just means that WaitXlogInsertionsToFinish() might wait on us
	 * unnecessarily, until we update the value (when we finish the insert or
	 * move to next page).
	 *
	 * If we're grabbing all the slots, however, stamp all but the last one
	 * with InvalidXLogRecPtr, meaning there is no insert in progress. The last
	 * slot is the one that we will update as we proceed with the insert, the
	 * rest are held just to keep off other inserters.
	 */
	if (slotno != -1 && slotno != num_xloginsert_slots - 1)
		slot->xlogInsertingAt = InvalidXLogRecPtr;
	else
		slot->xlogInsertingAt = 1;

	/* We are done updating shared state of the slot itself. */
	SpinLockRelease(&slot->mutex);

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);

	/*
	 * If we couldn't get the slot immediately, try another slot next time.
	 * On a system with more insertion slots than concurrent inserters, this
	 * causes all the inserters to eventually migrate to a slot that no-one
	 * else is using. On a system with more inserters than slots, it still
	 * causes the inserters to be distributed quite evenly across the slots.
	 */
	if (slotno != -1 && retry)
		slotToTry = (slotToTry + 1) % num_xloginsert_slots;
}

/*
 * Wait for the given slot to become free, or for its xlogInsertingAt location
 * to change to something else than 'waitptr'. In other words, wait for the
 * inserter using the given slot to finish its insertion, or to at least make
 * some progress.
 */
static void
WaitOnSlot(volatile XLogInsertSlot *slot, XLogRecPtr waitptr)
{
	PGPROC	   *proc = MyProc;
	int			extraWaits = 0;

	/*
	 * Lock out cancel/die interrupts while we sleep on the slot. There is
	 * no cleanup mechanism to remove us from the wait queue if we got
	 * interrupted.
	 */
	HOLD_INTERRUPTS();

	/*
	 * Loop here to try to acquire lock after each time we are signaled.
	 */
	for (;;)
	{
		bool		mustwait;

		/* Acquire mutex.  Time spent holding mutex should be short! */
		SpinLockAcquire(&slot->mutex);

		/* If I can get the lock, do so quickly. */
		if (slot->exclusive == 0 || slot->xlogInsertingAt != waitptr)
			mustwait = false;
		else
			mustwait = true;

		if (!mustwait)
			break;				/* the lock was free */

		Assert(slot->owner != MyProc);

		/*
		 * Add myself to wait queue.
		 */
		proc->lwWaiting = true;
		proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
		proc->lwWaitLink = NULL;

		/* waiters are added to the front of the queue */
		proc->lwWaitLink = slot->head;
		if (slot->head == NULL)
			slot->tail = proc;
		slot->head = proc;

		/* Can release the mutex now */
		SpinLockRelease(&slot->mutex);

		/*
		 * Wait until awakened.
		 *
		 * Since we share the process wait semaphore with other things, like
		 * the regular lock manager and ProcWaitForSignal, and we may need to
		 * acquire an LWLock while one of those is pending, it is possible that
		 * we get awakened for a reason other than being signaled by
		 * LWLockRelease. If so, loop back and wait again.  Once we've gotten
		 * the LWLock, re-increment the sema by the number of additional
		 * signals received, so that the lock manager or signal manager will
		 * see the received signal when it next waits.
		 */
		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		/* Now loop back and try to acquire lock again. */
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&slot->mutex);

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();
}

/*
 * Wake up all processes waiting for us with WaitOnSlot(). Sets our
 * xlogInsertingAt value to EndPos, without releasing the slot.
 */
static void
WakeupWaiters(XLogRecPtr EndPos)
{
	volatile XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[MySlotNo].slot;
	PGPROC	   *head;
	PGPROC	   *proc;
	PGPROC	   *next;

	/*
	 * If we have already reported progress up to the same point, do nothing.
	 * No other process can modify xlogInsertingAt, so we can check this before
	 * grabbing the spinlock.
	 */
	if (slot->xlogInsertingAt == EndPos)
		return;
	/* xlogInsertingAt should not go backwards */
	Assert(slot->xlogInsertingAt < EndPos);

	/* Acquire mutex.  Time spent holding mutex should be short! */
	SpinLockAcquire(&slot->mutex);

	/* we should own the slot */
	Assert(slot->exclusive == 1 && slot->owner == MyProc);

	slot->xlogInsertingAt = EndPos;

	/*
	 * See if there are any waiters that need to be woken up.
	 */
	head = slot->head;

	if (head != NULL)
	{
		proc = head;

		/* LW_WAIT_UNTIL_FREE waiters are always in the front of the queue */
		next = proc->lwWaitLink;
		while (next && next->lwWaitMode == LW_WAIT_UNTIL_FREE)
		{
			proc = next;
			next = next->lwWaitLink;
		}

		/* proc is now the last PGPROC to be released */
		slot->head = next;
		proc->lwWaitLink = NULL;
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&slot->mutex);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}
}

/*
 * Release our insertion slot (or slots, if we're holding them all).
 */
static void
WALInsertSlotRelease(void)
{
	int			i;

	if (holdingAllSlots)
	{
		for (i = 0; i < num_xloginsert_slots; i++)
			WALInsertSlotReleaseOne(i);
		holdingAllSlots = false;
	}
	else
		WALInsertSlotReleaseOne(MySlotNo);
}

static void
WALInsertSlotReleaseOne(int slotno)
{
	volatile XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[slotno].slot;
	PGPROC	   *head;
	PGPROC	   *proc;

	/* Acquire mutex.  Time spent holding mutex should be short! */
	SpinLockAcquire(&slot->mutex);

	/* we must be holding it */
	Assert(slot->exclusive == 1 && slot->owner == MyProc);

	slot->xlogInsertingAt = InvalidXLogRecPtr;

	/* Release my hold on the slot */
	slot->exclusive = 0;
	slot->owner = NULL;

	/*
	 * See if I need to awaken any waiters..
	 */
	head = slot->head;
	if (head != NULL)
	{
		if (slot->releaseOK)
		{
			/*
			 * Remove the to-be-awakened PGPROCs from the queue.
			 */
			bool		releaseOK = true;

			proc = head;

			/*
			 * First wake up any backends that want to be woken up without
			 * acquiring the lock. These are always in the front of the queue.
			 */
			while (proc->lwWaitMode == LW_WAIT_UNTIL_FREE && proc->lwWaitLink)
				proc = proc->lwWaitLink;

			/*
			 * Awaken the first exclusive-waiter, if any.
			 */
			if (proc->lwWaitLink)
			{
				Assert(proc->lwWaitLink->lwWaitMode == LW_EXCLUSIVE);
				proc = proc->lwWaitLink;
				releaseOK = false;
			}
			/* proc is now the last PGPROC to be released */
			slot->head = proc->lwWaitLink;
			proc->lwWaitLink = NULL;

			slot->releaseOK = releaseOK;
		}
		else
			head = NULL;
	}

	/* We are done updating shared state of the slot itself. */
	SpinLockRelease(&slot->mutex);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	END_CRIT_SECTION();
}


/*
 * Wait for any WAL insertions < upto to finish.
 *
 * Returns the location of the oldest insertion that is still in-progress.
 * Any WAL prior to that point has been fully copied into WAL buffers, and
 * can be flushed out to disk. Because this waits for any insertions older
 * than 'upto' to finish, the return value is always >= 'upto'.
 *
 * Note: When you are about to write out WAL, you must call this function
 * *before* acquiring WALWriteLock, to avoid deadlocks. This function might
 * need to wait for an insertion to finish (or at least advance to next
 * uninitialized page), and the inserter might need to evict an old WAL buffer
 * to make room for a new one, which in turn requires WALWriteLock.
 */
static XLogRecPtr
WaitXLogInsertionsToFinish(XLogRecPtr upto)
{
	uint64		bytepos;
	XLogRecPtr	reservedUpto;
	XLogRecPtr	finishedUpto;
	volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
	int			i;

	if (MyProc == NULL)
		elog(PANIC, "cannot wait without a PGPROC structure");

	/* Read the current insert position */
	SpinLockAcquire(&Insert->insertpos_lck);
	bytepos = Insert->CurrBytePos;
	SpinLockRelease(&Insert->insertpos_lck);
	reservedUpto = XLogBytePosToEndRecPtr(bytepos);

	/*
	 * No-one should request to flush a piece of WAL that hasn't even been
	 * reserved yet. However, it can happen if there is a block with a bogus
	 * LSN on disk, for example. XLogFlush checks for that situation and
	 * complains, but only after the flush. Here we just assume that to mean
	 * that all WAL that has been reserved needs to be finished. In this
	 * corner-case, the return value can be smaller than 'upto' argument.
	 */
	if (upto > reservedUpto)
	{
		elog(LOG, "request to flush past end of generated WAL; request %X/%X, currpos %X/%X",
			 (uint32) (upto >> 32), (uint32) upto,
			 (uint32) (reservedUpto >> 32), (uint32) reservedUpto);
		upto = reservedUpto;
	}

	/*
	 * finishedUpto is our return value, indicating the point upto which
	 * all the WAL insertions have been finished. Initialize it to the head
	 * of reserved WAL, and as we iterate through the insertion slots, back it
	 * out for any insertion that's still in progress.
	 */
	finishedUpto = reservedUpto;

	/*
	 * Loop through all the slots, sleeping on any in-progress insert older
	 * than 'upto'.
	 */
	for (i = 0; i < num_xloginsert_slots; i++)
	{
		volatile XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[i].slot;
		XLogRecPtr insertingat;

	retry:
		/*
		 * We can check if the slot is in use without grabbing the spinlock.
		 * The spinlock acquisition of insertpos_lck before this loop acts
		 * as a memory barrier. If someone acquires the slot after that, it
		 * can't possibly be inserting to anything < reservedUpto. If it was
		 * acquired before that, an unlocked test will return true.
		 */
		if (!slot->exclusive)
			continue;

		SpinLockAcquire(&slot->mutex);
		/* re-check now that we have the lock */
		if (!slot->exclusive)
		{
			SpinLockRelease(&slot->mutex);
			continue;
		}
		insertingat = slot->xlogInsertingAt;
		SpinLockRelease(&slot->mutex);

		if (insertingat == InvalidXLogRecPtr)
		{
			/*
			 * slot is reserved just to hold off other inserters, there is no
			 * actual insert in progress.
			 */
			continue;
		}

		/*
		 * This insertion is still in progress. Do we need to wait for it?
		 *
		 * When an inserter acquires a slot, it doesn't reset 'insertingat', so
		 * it will initially point to the old value of some already-finished
		 * insertion. The inserter will update the value as soon as it finishes
		 * the insertion, moves to the next page, or has to do I/O to flush an
		 * old dirty buffer. That means that when we see a slot with
		 * insertingat value < upto, we don't know if that insertion is still
		 * truly in progress, or if the slot is reused by a new inserter that
		 * hasn't updated the insertingat value yet. We have to assume it's the
		 * latter, and wait.
		 */
		if (insertingat < upto)
		{
			WaitOnSlot(slot, insertingat);
			goto retry;
		}
		else
		{
			/*
			 * We don't need to wait for this insertion, but update the
			 * return value.
			 */
			if (insertingat < finishedUpto)
				finishedUpto = insertingat;
		}
	}
	return finishedUpto;
}

/*
 * Get a pointer to the right location in the WAL buffer containing the
 * given XLogRecPtr.
 *
 * If the page is not initialized yet, it is initialized. That might require
 * evicting an old dirty buffer from the buffer cache, which means I/O.
 *
 * The caller must ensure that the page containing the requested location
 * isn't evicted yet, and won't be evicted. The way to ensure that is to
 * hold onto an XLogInsertSlot with the xlogInsertingAt position set to
 * something <= ptr. GetXLogBuffer() will update xlogInsertingAt if it needs
 * to evict an old page from the buffer. (This means that once you call
 * GetXLogBuffer() with a given 'ptr', you must not access anything before
 * that point anymore, and must not call GetXLogBuffer() with an older 'ptr'
 * later, because older buffers might be recycled already)
 */
static char *
GetXLogBuffer(XLogRecPtr ptr)
{
	int			idx;
	XLogRecPtr	endptr;
	static uint64 cachedPage = 0;
	static char *cachedPos = NULL;
	XLogRecPtr	expectedEndPtr;

	/*
	 * Fast path for the common case that we need to access again the same
	 * page as last time.
	 */
	if (ptr / XLOG_BLCKSZ == cachedPage)
	{
		Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
		Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));
		return cachedPos + ptr % XLOG_BLCKSZ;
	}

	/*
	 * The XLog buffer cache is organized so that a page is always loaded
	 * to a particular buffer.  That way we can easily calculate the buffer
	 * a given page must be loaded into, from the XLogRecPtr alone.
	 */
	idx = XLogRecPtrToBufIdx(ptr);

	/*
	 * See what page is loaded in the buffer at the moment. It could be the
	 * page we're looking for, or something older. It can't be anything newer
	 * - that would imply the page we're looking for has already been written
	 * out to disk and evicted, and the caller is responsible for making sure
	 * that doesn't happen.
	 *
	 * However, we don't hold a lock while we read the value. If someone has
	 * just initialized the page, it's possible that we get a "torn read" of
	 * the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
	 * that case we will see a bogus value. That's ok, we'll grab the mapping
	 * lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
	 * the page we're looking for. But it means that when we do this unlocked
	 * read, we might see a value that appears to be ahead of the page we're
	 * looking for. Don't PANIC on that, until we've verified the value while
	 * holding the lock.
	 */
	expectedEndPtr = ptr;
	expectedEndPtr += XLOG_BLCKSZ - ptr % XLOG_BLCKSZ;

	endptr = XLogCtl->xlblocks[idx];
	if (expectedEndPtr != endptr)
	{
		/*
		 * Let others know that we're finished inserting the record up
		 * to the page boundary.
		 */
		WakeupWaiters(expectedEndPtr - XLOG_BLCKSZ);

		AdvanceXLInsertBuffer(ptr, false);
		endptr = XLogCtl->xlblocks[idx];

		if (expectedEndPtr != endptr)
			elog(PANIC, "could not find WAL buffer for %X/%X",
				 (uint32) (ptr >> 32) , (uint32) ptr);
	}
	else
	{
		/*
		 * Make sure the initialization of the page is visible to us, and
		 * won't arrive later to overwrite the WAL data we write on the page.
		 */
		pg_memory_barrier();
	}

	/*
	 * Found the buffer holding this page. Return a pointer to the right
	 * offset within the page.
	 */
	cachedPage = ptr / XLOG_BLCKSZ;
	cachedPos = XLogCtl->pages + idx * (Size) XLOG_BLCKSZ;

	Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
	Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));

	return cachedPos + ptr % XLOG_BLCKSZ;
}

/*
 * Converts a "usable byte position" to XLogRecPtr. A usable byte position
 * is the position starting from the beginning of WAL, excluding all WAL
 * page headers.
 */
static XLogRecPtr
XLogBytePosToRecPtr(uint64 bytepos)
{
	uint64		fullsegs;
	uint64		fullpages;
	uint64		bytesleft;
	uint32		seg_offset;
	XLogRecPtr	result;

	fullsegs = bytepos / UsableBytesInSegment;
	bytesleft = bytepos % UsableBytesInSegment;

	if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
	{
		/* fits on first page of segment */
		seg_offset = bytesleft + SizeOfXLogLongPHD;
	}
	else
	{
		/* account for the first page on segment with long header */
		seg_offset = XLOG_BLCKSZ;
		bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

		fullpages = bytesleft / UsableBytesInPage;
		bytesleft = bytesleft % UsableBytesInPage;

		seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
	}

	XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, result);

	return result;
}

/*
 * Like XLogBytePosToRecPtr, but if the position is at a page boundary,
 * returns a pointer to the beginning of the page (ie. before page header),
 * not to where the first xlog record on that page would go to. This is used
 * when converting a pointer to the end of a record.
 */
static XLogRecPtr
XLogBytePosToEndRecPtr(uint64 bytepos)
{
	uint64		fullsegs;
	uint64		fullpages;
	uint64		bytesleft;
	uint32		seg_offset;
	XLogRecPtr	result;

	fullsegs = bytepos / UsableBytesInSegment;
	bytesleft = bytepos % UsableBytesInSegment;

	if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
	{
		/* fits on first page of segment */
		if (bytesleft == 0)
			seg_offset = 0;
		else
			seg_offset = bytesleft + SizeOfXLogLongPHD;
	}
	else
	{
		/* account for the first page on segment with long header */
		seg_offset = XLOG_BLCKSZ;
		bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

		fullpages = bytesleft / UsableBytesInPage;
		bytesleft = bytesleft % UsableBytesInPage;

		if (bytesleft == 0)
			seg_offset += fullpages * XLOG_BLCKSZ + bytesleft;
		else
			seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
	}

	XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, result);

	return result;
}

/*
 * Convert an XLogRecPtr to a "usable byte position".
 */
static uint64
XLogRecPtrToBytePos(XLogRecPtr ptr)
{
	uint64		fullsegs;
	uint32		fullpages;
	uint32		offset;
	uint64		result;

	XLByteToSeg(ptr, fullsegs);

	fullpages = (ptr % XLOG_SEG_SIZE) / XLOG_BLCKSZ;
	offset = ptr % XLOG_BLCKSZ;

	if (fullpages == 0)
	{
		result = fullsegs * UsableBytesInSegment;
		if (offset > 0)
		{
			Assert(offset >= SizeOfXLogLongPHD);
			result += offset - SizeOfXLogLongPHD;
		}
	}
	else
	{
		result = fullsegs * UsableBytesInSegment +
			(XLOG_BLCKSZ - SizeOfXLogLongPHD) +  /* account for first page */
			(fullpages - 1) * UsableBytesInPage; /* full pages */
		if (offset > 0)
		{
			Assert(offset >= SizeOfXLogShortPHD);
			result += offset - SizeOfXLogShortPHD;
		}
	}

	return result;
}

/*
 * Determine whether the buffer referenced by an XLogRecData item has to
 * be backed up, and if so fill a BkpBlock struct for it.  In any case
 * save the buffer's LSN at *lsn.
 */
static bool
XLogCheckBuffer(XLogRecData *rdata, bool holdsExclusiveLock,
				XLogRecPtr *lsn, BkpBlock *bkpb)
{
	Page		page;

	page = BufferGetPage(rdata->buffer);

	/*
	 * We assume page LSN is first data on *every* page that can be passed to
	 * XLogInsert, whether it has the standard page layout or not. We don't
	 * need to take the buffer header lock for PageGetLSN if we hold an
	 * exclusive lock on the page and/or the relation.
	 */
	if (holdsExclusiveLock)
		*lsn = PageGetLSN(page);
	else
		*lsn = BufferGetLSNAtomic(rdata->buffer);

	if (*lsn <= RedoRecPtr)
	{
		/*
		 * The page needs to be backed up, so set up *bkpb
		 */
		BufferGetTag(rdata->buffer, &bkpb->node, &bkpb->fork, &bkpb->block);

		if (rdata->buffer_std)
		{
			/* Assume we can omit data between pd_lower and pd_upper */
			uint16		lower = ((PageHeader) page)->pd_lower;
			uint16		upper = ((PageHeader) page)->pd_upper;

			if (lower >= SizeOfPageHeaderData &&
				upper > lower &&
				upper <= BLCKSZ)
			{
				bkpb->hole_offset = lower;
				bkpb->hole_length = upper - lower;
			}
			else
			{
				/* No "hole" to compress out */
				bkpb->hole_offset = 0;
				bkpb->hole_length = 0;
			}
		}
		else
		{
			/* Not a standard page header, don't try to eliminate "hole" */
			bkpb->hole_offset = 0;
			bkpb->hole_length = 0;
		}

		return true;			/* buffer requires backup */
	}

	return false;				/* buffer does not need to be backed up */
}

/*
 * Initialize XLOG buffers, writing out old buffers if they still contain
 * unwritten data, upto the page containing 'upto'. Or if 'opportunistic' is
 * true, initialize as many pages as we can without having to write out
 * unwritten data. Any new pages are initialized to zeros, with pages headers
 * initialized properly.
 */
static void
AdvanceXLInsertBuffer(XLogRecPtr upto, bool opportunistic)
{
	XLogCtlInsert *Insert = &XLogCtl->Insert;
	int			nextidx;
	XLogRecPtr	OldPageRqstPtr;
	XLogwrtRqst WriteRqst;
	XLogRecPtr	NewPageEndPtr = InvalidXLogRecPtr;
	XLogRecPtr	NewPageBeginPtr;
	XLogPageHeader NewPage;
	int			npages = 0;

	LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);

	/*
	 * Now that we have the lock, check if someone initialized the page
	 * already.
	 */
	while (upto >= XLogCtl->InitializedUpTo || opportunistic)
	{
		nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);

		/*
		 * Get ending-offset of the buffer page we need to replace (this may
		 * be zero if the buffer hasn't been used yet).  Fall through if it's
		 * already written out.
		 */
		OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
		if (LogwrtResult.Write < OldPageRqstPtr)
		{
			/*
			 * Nope, got work to do. If we just want to pre-initialize as much
			 * as we can without flushing, give up now.
			 */
			if (opportunistic)
				break;

			/* Before waiting, get info_lck and update LogwrtResult */
			{
				/* use volatile pointer to prevent code rearrangement */
				volatile XLogCtlData *xlogctl = XLogCtl;

				SpinLockAcquire(&xlogctl->info_lck);
				if (xlogctl->LogwrtRqst.Write < OldPageRqstPtr)
					xlogctl->LogwrtRqst.Write = OldPageRqstPtr;
				LogwrtResult = xlogctl->LogwrtResult;
				SpinLockRelease(&xlogctl->info_lck);
			}

			/*
			 * Now that we have an up-to-date LogwrtResult value, see if we
			 * still need to write it or if someone else already did.
			 */
			if (LogwrtResult.Write < OldPageRqstPtr)
			{
				/*
				 * Must acquire write lock. Release WALBufMappingLock first,
				 * to make sure that all insertions that we need to wait for
				 * can finish (up to this same position). Otherwise we risk
				 * deadlock.
				 */
				LWLockRelease(WALBufMappingLock);

				WaitXLogInsertionsToFinish(OldPageRqstPtr);

				LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

				LogwrtResult = XLogCtl->LogwrtResult;
				if (LogwrtResult.Write >= OldPageRqstPtr)
				{
					/* OK, someone wrote it already */
					LWLockRelease(WALWriteLock);
				}
				else
				{
					/* Have to write it ourselves */
					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_START();
					WriteRqst.Write = OldPageRqstPtr;
					WriteRqst.Flush = 0;
					XLogWrite(WriteRqst, false);
					LWLockRelease(WALWriteLock);
					TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
				}
				/* Re-acquire WALBufMappingLock and retry */
				LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
				continue;
			}
		}

		/*
		 * Now the next buffer slot is free and we can set it up to be the next
		 * output page.
		 */
		NewPageBeginPtr = XLogCtl->InitializedUpTo;
		NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;

		Assert(XLogRecPtrToBufIdx(NewPageBeginPtr) == nextidx);

		NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

		/*
		 * Be sure to re-zero the buffer so that bytes beyond what we've
		 * written will look like zeroes and not valid XLOG records...
		 */
		MemSet((char *) NewPage, 0, XLOG_BLCKSZ);

		/*
		 * Fill the new page's header
		 */
		NewPage   ->xlp_magic = XLOG_PAGE_MAGIC;

		/* NewPage->xlp_info = 0; */	/* done by memset */
		NewPage   ->xlp_tli = ThisTimeLineID;
		NewPage   ->xlp_pageaddr = NewPageBeginPtr;
		/* NewPage->xlp_rem_len = 0; */		/* done by memset */

		/*
		 * If online backup is not in progress, mark the header to indicate
		 * that* WAL records beginning in this page have removable backup
		 * blocks.  This allows the WAL archiver to know whether it is safe to
		 * compress archived WAL data by transforming full-block records into
		 * the non-full-block format.  It is sufficient to record this at the
		 * page level because we force a page switch (in fact a segment switch)
		 * when starting a backup, so the flag will be off before any records
		 * can be written during the backup.  At the end of a backup, the last
		 * page will be marked as all unsafe when perhaps only part is unsafe,
		 * but at worst the archiver would miss the opportunity to compress a
		 * few records.
		 */
		if (!Insert->forcePageWrites)
			NewPage   ->xlp_info |= XLP_BKP_REMOVABLE;

		/*
		 * If first page of an XLOG segment file, make it a long header.
		 */
		if ((NewPage->xlp_pageaddr % XLogSegSize) == 0)
		{
			XLogLongPageHeader NewLongPage = (XLogLongPageHeader) NewPage;

			NewLongPage->xlp_sysid = ControlFile->system_identifier;
			NewLongPage->xlp_seg_size = XLogSegSize;
			NewLongPage->xlp_xlog_blcksz = XLOG_BLCKSZ;
			NewPage   ->xlp_info |= XLP_LONG_HEADER;
		}

		/*
		 * Make sure the initialization of the page becomes visible to others
		 * before the xlblocks update. GetXLogBuffer() reads xlblocks without
		 * holding a lock.
		 */
		pg_write_barrier();

		*((volatile XLogRecPtr *) &XLogCtl->xlblocks[nextidx]) = NewPageEndPtr;

		XLogCtl->InitializedUpTo = NewPageEndPtr;

		npages++;
	}
	LWLockRelease(WALBufMappingLock);

#ifdef WAL_DEBUG
	if (npages > 0)
	{
		elog(DEBUG1, "initialized %d pages, upto %X/%X",
			 npages, (uint32) (NewPageEndPtr >> 32), (uint32) NewPageEndPtr);
	}
#endif
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * new_segno indicates a log file that has just been filled up (or read
 * during recovery). We measure the distance from RedoRecPtr to new_segno
 * and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
static bool
XLogCheckpointNeeded(XLogSegNo new_segno)
{
	XLogSegNo	old_segno;

	XLByteToSeg(RedoRecPtr, old_segno);

	if (new_segno >= old_segno + (uint64) (CheckPointSegments - 1))
		return true;
	return false;
}

/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == TRUE, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * Must be called with WALWriteLock held. WaitXLogInsertionsToFinish(WriteRqst)
 * must be called before grabbing the lock, to make sure the data is ready to
 * write.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
{
	bool		ispartialpage;
	bool		last_iteration;
	bool		finishing_seg;
	bool		use_existent;
	int			curridx;
	int			npages;
	int			startidx;
	uint32		startoffset;

	/* We should always be inside a critical section here */
	Assert(CritSectionCount > 0);

	/*
	 * Update local LogwrtResult (caller probably did this already, but...)
	 */
	LogwrtResult = XLogCtl->LogwrtResult;

	/*
	 * Since successive pages in the xlog cache are consecutively allocated,
	 * we can usually gather multiple pages together and issue just one
	 * write() call.  npages is the number of pages we have determined can be
	 * written together; startidx is the cache block index of the first one,
	 * and startoffset is the file offset at which it should go. The latter
	 * two variables are only valid when npages > 0, but we must initialize
	 * all of them to keep the compiler quiet.
	 */
	npages = 0;
	startidx = 0;
	startoffset = 0;

	/*
	 * Within the loop, curridx is the cache block index of the page to
	 * consider writing.  Begin at the buffer containing the next unwritten
	 * page, or last partially written page.
	 */
	curridx = XLogRecPtrToBufIdx(LogwrtResult.Write);

	while (LogwrtResult.Write < WriteRqst.Write)
	{
		/*
		 * Make sure we're not ahead of the insert process.  This could happen
		 * if we're passed a bogus WriteRqst.Write that is past the end of the
		 * last page that's been initialized by AdvanceXLInsertBuffer.
		 */
		XLogRecPtr EndPtr = XLogCtl->xlblocks[curridx];
		if (LogwrtResult.Write >= EndPtr)
			elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
				 (uint32) (LogwrtResult.Write >> 32),
				 (uint32) LogwrtResult.Write,
				 (uint32) (EndPtr >> 32), (uint32) EndPtr);

		/* Advance LogwrtResult.Write to end of current buffer page */
		LogwrtResult.Write = EndPtr;
		ispartialpage = WriteRqst.Write < LogwrtResult.Write;

		if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
		{
			/*
			 * Switch to new logfile segment.  We cannot have any pending
			 * pages here (since we dump what we have at segment end).
			 */
			Assert(npages == 0);
			if (openLogFile >= 0)
				XLogFileClose();
			XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);

			/* create/use new log file */
			use_existent = true;
			openLogFile = XLogFileInit(openLogSegNo, &use_existent, true);
			openLogOff = 0;
		}

		/* Make sure we have the current logfile open */
		if (openLogFile < 0)
		{
			XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
			openLogFile = XLogFileOpen(openLogSegNo);
			openLogOff = 0;
		}

		/* Add current page to the set of pending pages-to-dump */
		if (npages == 0)
		{
			/* first of group */
			startidx = curridx;
			startoffset = (LogwrtResult.Write - XLOG_BLCKSZ) % XLogSegSize;
		}
		npages++;

		/*
		 * Dump the set if this will be the last loop iteration, or if we are
		 * at the last page of the cache area (since the next page won't be
		 * contiguous in memory), or if we are at the end of the logfile
		 * segment.
		 */
		last_iteration = WriteRqst.Write <= LogwrtResult.Write;

		finishing_seg = !ispartialpage &&
			(startoffset + npages * XLOG_BLCKSZ) >= XLogSegSize;

		if (last_iteration ||
			curridx == XLogCtl->XLogCacheBlck ||
			finishing_seg)
		{
			char	   *from;
			Size		nbytes;
			Size		nleft;
			int			written;

			/* Need to seek in the file? */
			if (openLogOff != startoffset)
			{
				if (lseek(openLogFile, (off_t) startoffset, SEEK_SET) < 0)
					ereport(PANIC,
							(errcode_for_file_access(),
					 errmsg("could not seek in log file %s to offset %u: %m",
							XLogFileNameP(ThisTimeLineID, openLogSegNo),
							startoffset)));
				openLogOff = startoffset;
			}

			/* OK to write the page(s) */
			from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
			nbytes = npages * (Size) XLOG_BLCKSZ;
			nleft = nbytes;
			do
			{
				errno = 0;
				written  = write(openLogFile, from, nleft);
				if (written <= 0)
				{
					if (errno == EINTR)
						continue;
					ereport(PANIC,
							(errcode_for_file_access(),
							 errmsg("could not write to log file %s "
									"at offset %u, length %lu: %m",
									XLogFileNameP(ThisTimeLineID, openLogSegNo),
									openLogOff, (unsigned long) nbytes)));
				}
				nleft -= written;
				from += written;
			} while (nleft > 0);

			/* Update state for write */
			openLogOff += nbytes;
			npages = 0;

			/*
			 * If we just wrote the whole last page of a logfile segment,
			 * fsync the segment immediately.  This avoids having to go back
			 * and re-open prior segments when an fsync request comes along
			 * later. Doing it here ensures that one and only one backend will
			 * perform this fsync.
			 *
			 * This is also the right place to notify the Archiver that the
			 * segment is ready to copy to archival storage, and to update the
			 * timer for archive_timeout, and to signal for a checkpoint if
			 * too many logfile segments have been used since the last
			 * checkpoint.
			 */
			if (finishing_seg)
			{
				issue_xlog_fsync(openLogFile, openLogSegNo);

				/* signal that we need to wakeup walsenders later */
				WalSndWakeupRequest();

				LogwrtResult.Flush = LogwrtResult.Write;		/* end of page */

				if (XLogArchivingActive())
					XLogArchiveNotifySeg(openLogSegNo);

				XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);

				/*
				 * Request a checkpoint if we've consumed too much xlog since
				 * the last one.  For speed, we first check using the local
				 * copy of RedoRecPtr, which might be out of date; if it looks
				 * like a checkpoint is needed, forcibly update RedoRecPtr and
				 * recheck.
				 */
				if (IsUnderPostmaster && XLogCheckpointNeeded(openLogSegNo))
				{
					(void) GetRedoRecPtr();
					if (XLogCheckpointNeeded(openLogSegNo))
						RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
				}
			}
		}

		if (ispartialpage)
		{
			/* Only asked to write a partial page */
			LogwrtResult.Write = WriteRqst.Write;
			break;
		}
		curridx = NextBufIdx(curridx);

		/* If flexible, break out of loop as soon as we wrote something */
		if (flexible && npages == 0)
			break;
	}

	Assert(npages == 0);

	/*
	 * If asked to flush, do so
	 */
	if (LogwrtResult.Flush < WriteRqst.Flush &&
		LogwrtResult.Flush < LogwrtResult.Write)

	{
		/*
		 * Could get here without iterating above loop, in which case we might
		 * have no open file or the wrong one.	However, we do not need to
		 * fsync more than one file.
		 */
		if (sync_method != SYNC_METHOD_OPEN &&
			sync_method != SYNC_METHOD_OPEN_DSYNC)
		{
			if (openLogFile >= 0 &&
				!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
				XLogFileClose();
			if (openLogFile < 0)
			{
				XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
				openLogFile = XLogFileOpen(openLogSegNo);
				openLogOff = 0;
			}

			issue_xlog_fsync(openLogFile, openLogSegNo);
		}

		/* signal that we need to wakeup walsenders later */
		WalSndWakeupRequest();

		LogwrtResult.Flush = LogwrtResult.Write;
	}

	/*
	 * Update shared-memory status
	 *
	 * We make sure that the shared 'request' values do not fall behind the
	 * 'result' values.  This is not absolutely essential, but it saves some
	 * code in a couple of places.
	 */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		xlogctl->LogwrtResult = LogwrtResult;
		if (xlogctl->LogwrtRqst.Write < LogwrtResult.Write)
			xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
		if (xlogctl->LogwrtRqst.Flush < LogwrtResult.Flush)
			xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
		SpinLockRelease(&xlogctl->info_lck);
	}
}

/*
 * Record the LSN for an asynchronous transaction commit/abort
 * and nudge the WALWriter if there is work for it to do.
 * (This should not be called for synchronous commits.)
 */
void
XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
{
	XLogRecPtr	WriteRqstPtr = asyncXactLSN;
	bool		sleeping;

	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	SpinLockAcquire(&xlogctl->info_lck);
	LogwrtResult = xlogctl->LogwrtResult;
	sleeping = xlogctl->WalWriterSleeping;
	if (xlogctl->asyncXactLSN < asyncXactLSN)
		xlogctl->asyncXactLSN = asyncXactLSN;
	SpinLockRelease(&xlogctl->info_lck);

	/*
	 * If the WALWriter is sleeping, we should kick it to make it come out of
	 * low-power mode.	Otherwise, determine whether there's a full page of
	 * WAL available to write.
	 */
	if (!sleeping)
	{
		/* back off to last completed page boundary */
		WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

		/* if we have already flushed that far, we're done */
		if (WriteRqstPtr <= LogwrtResult.Flush)
			return;
	}

	/*
	 * Nudge the WALWriter: it has a full page of WAL to write, or we want it
	 * to come out of low-power mode so that this async commit will reach disk
	 * within the expected amount of time.
	 */
	if (ProcGlobal->walwriterLatch)
		SetLatch(ProcGlobal->walwriterLatch);
}

/*
 * Advance minRecoveryPoint in control file.
 *
 * If we crash during recovery, we must reach this point again before the
 * database is consistent.
 *
 * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint
 * is only updated if it's not already greater than or equal to 'lsn'.
 */
static void
UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force)
{
	/* Quick check using our local copy of the variable */
	if (!updateMinRecoveryPoint || (!force && lsn <= minRecoveryPoint))
		return;

	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

	/* update local copy */
	minRecoveryPoint = ControlFile->minRecoveryPoint;
	minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

	/*
	 * An invalid minRecoveryPoint means that we need to recover all the WAL,
	 * i.e., we're doing crash recovery.  We never modify the control file's
	 * value in that case, so we can short-circuit future checks here too.
	 */
	if (minRecoveryPoint == 0)
		updateMinRecoveryPoint = false;
	else if (force || minRecoveryPoint < lsn)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;
		XLogRecPtr	newMinRecoveryPoint;
		TimeLineID	newMinRecoveryPointTLI;

		/*
		 * To avoid having to update the control file too often, we update it
		 * all the way to the last record being replayed, even though 'lsn'
		 * would suffice for correctness.  This also allows the 'force' case
		 * to not need a valid 'lsn' value.
		 *
		 * Another important reason for doing it this way is that the passed
		 * 'lsn' value could be bogus, i.e., past the end of available WAL, if
		 * the caller got it from a corrupted heap page.  Accepting such a
		 * value as the min recovery point would prevent us from coming up at
		 * all.  Instead, we just log a warning and continue with recovery.
		 * (See also the comments about corrupt LSNs in XLogFlush.)
		 */
		SpinLockAcquire(&xlogctl->info_lck);
		newMinRecoveryPoint = xlogctl->replayEndRecPtr;
		newMinRecoveryPointTLI = xlogctl->replayEndTLI;
		SpinLockRelease(&xlogctl->info_lck);

		if (!force && newMinRecoveryPoint < lsn)
			elog(WARNING,
			   "xlog min recovery request %X/%X is past current point %X/%X",
				 (uint32) (lsn >> 32), (uint32) lsn,
				 (uint32) (newMinRecoveryPoint >> 32),
				 (uint32) newMinRecoveryPoint);

		/* update control file */
		if (ControlFile->minRecoveryPoint < newMinRecoveryPoint)
		{
			ControlFile->minRecoveryPoint = newMinRecoveryPoint;
			ControlFile->minRecoveryPointTLI = newMinRecoveryPointTLI;
			UpdateControlFile();
			minRecoveryPoint = newMinRecoveryPoint;
			minRecoveryPointTLI = newMinRecoveryPointTLI;

			ereport(DEBUG2,
				(errmsg("updated min recovery point to %X/%X on timeline %u",
						(uint32) (minRecoveryPoint >> 32),
						(uint32) minRecoveryPoint,
						newMinRecoveryPointTLI)));
		}
	}
	LWLockRelease(ControlFileLock);
}

/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
	XLogRecPtr	WriteRqstPtr;
	XLogwrtRqst WriteRqst;

	/*
	 * During REDO, we are reading not writing WAL.  Therefore, instead of
	 * trying to flush the WAL, we should update minRecoveryPoint instead. We
	 * test XLogInsertAllowed(), not InRecovery, because we need checkpointer
	 * to act this way too, and because when it tries to write the
	 * end-of-recovery checkpoint, it should indeed flush.
	 */
	if (!XLogInsertAllowed())
	{
		UpdateMinRecoveryPoint(record, false);
		return;
	}

	/* Quick exit if already known flushed */
	if (record <= LogwrtResult.Flush)
		return;

#ifdef WAL_DEBUG
	if (XLOG_DEBUG)
		elog(LOG, "xlog flush request %X/%X; write %X/%X; flush %X/%X",
			 (uint32) (record >> 32), (uint32) record,
			 (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
		   (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

	START_CRIT_SECTION();

	/*
	 * Since fsync is usually a horribly expensive operation, we try to
	 * piggyback as much data as we can on each fsync: if we see any more data
	 * entered into the xlog buffer, we'll write and fsync that too, so that
	 * the final value of LogwrtResult.Flush is as large as possible. This
	 * gives us some chance of avoiding another fsync immediately after.
	 */

	/* initialize to given target; may increase below */
	WriteRqstPtr = record;

	/*
	 * Now wait until we get the write lock, or someone else does the flush
	 * for us.
	 */
	for (;;)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;
		XLogRecPtr	insertpos;

		/* read LogwrtResult and update local state */
		SpinLockAcquire(&xlogctl->info_lck);
		if (WriteRqstPtr < xlogctl->LogwrtRqst.Write)
			WriteRqstPtr = xlogctl->LogwrtRqst.Write;
		LogwrtResult = xlogctl->LogwrtResult;
		SpinLockRelease(&xlogctl->info_lck);

		/* done already? */
		if (record <= LogwrtResult.Flush)
			break;

		/*
		 * Before actually performing the write, wait for all in-flight
		 * insertions to the pages we're about to write to finish.
		 */
		insertpos = WaitXLogInsertionsToFinish(WriteRqstPtr);

		/*
		 * Try to get the write lock. If we can't get it immediately, wait
		 * until it's released, and recheck if we still need to do the flush
		 * or if the backend that held the lock did it for us already. This
		 * helps to maintain a good rate of group committing when the system
		 * is bottlenecked by the speed of fsyncing.
		 */
		if (!LWLockAcquireOrWait(WALWriteLock, LW_EXCLUSIVE))
		{
			/*
			 * The lock is now free, but we didn't acquire it yet. Before we
			 * do, loop back to check if someone else flushed the record for
			 * us already.
			 */
			continue;
		}

		/* Got the lock; recheck whether request is satisfied */
		LogwrtResult = XLogCtl->LogwrtResult;
		if (record <= LogwrtResult.Flush)
		{
			LWLockRelease(WALWriteLock);
			break;
		}

		/*
		 * Sleep before flush! By adding a delay here, we may give further
		 * backends the opportunity to join the backlog of group commit
		 * followers; this can significantly improve transaction throughput,
		 * at the risk of increasing transaction latency.
		 *
		 * We do not sleep if enableFsync is not turned on, nor if there are
		 * fewer than CommitSiblings other backends with active transactions.
		 */
		if (CommitDelay > 0 && enableFsync &&
			MinimumActiveBackends(CommitSiblings))
		{
			pg_usleep(CommitDelay);

			/*
			 * Re-check how far we can now flush the WAL. It's generally not
			 * safe to call WaitXLogInsetionsToFinish while holding
			 * WALWriteLock, because an in-progress insertion might need to
			 * also grab WALWriteLock to make progress. But we know that all
			 * the insertions up to insertpos have already finished, because
			 * that's what the earlier WaitXLogInsertionsToFinish() returned.
			 * We're only calling it again to allow insertpos to be moved
			 * further forward, not to actually wait for anyone.
			 */
			insertpos = WaitXLogInsertionsToFinish(insertpos);
		}

		/* try to write/flush later additions to XLOG as well */
		WriteRqst.Write = insertpos;
		WriteRqst.Flush = insertpos;

		XLogWrite(WriteRqst, false);

		LWLockRelease(WALWriteLock);
		/* done */
		break;
	}

	END_CRIT_SECTION();

	/* wake up walsenders now that we've released heavily contended locks */
	WalSndWakeupProcessRequests();

	/*
	 * If we still haven't flushed to the request point then we have a
	 * problem; most likely, the requested flush point is past end of XLOG.
	 * This has been seen to occur when a disk page has a corrupted LSN.
	 *
	 * Formerly we treated this as a PANIC condition, but that hurts the
	 * system's robustness rather than helping it: we do not want to take down
	 * the whole system due to corruption on one data page.  In particular, if
	 * the bad page is encountered again during recovery then we would be
	 * unable to restart the database at all!  (This scenario actually
	 * happened in the field several times with 7.1 releases.)	As of 8.4, bad
	 * LSNs encountered during recovery are UpdateMinRecoveryPoint's problem;
	 * the only time we can reach here during recovery is while flushing the
	 * end-of-recovery checkpoint record, and we don't expect that to have a
	 * bad LSN.
	 *
	 * Note that for calls from xact.c, the ERROR will be promoted to PANIC
	 * since xact.c calls this routine inside a critical section.  However,
	 * calls from bufmgr.c are not within critical sections and so we will not
	 * force a restart for a bad LSN on a data page.
	 */
	if (LogwrtResult.Flush < record)
		elog(ERROR,
		"xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
			 (uint32) (record >> 32), (uint32) record,
		   (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
}

/*
 * Flush xlog, but without specifying exactly where to flush to.
 *
 * We normally flush only completed blocks; but if there is nothing to do on
 * that basis, we check for unflushed async commits in the current incomplete
 * block, and flush through the latest one of those.  Thus, if async commits
 * are not being used, we will flush complete blocks only.	We can guarantee
 * that async commits reach disk after at most three cycles; normally only
 * one or two.	(When flushing complete blocks, we allow XLogWrite to write
 * "flexibly", meaning it can stop at the end of the buffer ring; this makes a
 * difference only with very high load or long wal_writer_delay, but imposes
 * one extra cycle for the worst case for async commits.)
 *
 * This routine is invoked periodically by the background walwriter process.
 *
 * Returns TRUE if we flushed anything.
 */
bool
XLogBackgroundFlush(void)
{
	XLogRecPtr	WriteRqstPtr;
	bool		flexible = true;
	bool		wrote_something = false;

	/* XLOG doesn't need flushing during recovery */
	if (RecoveryInProgress())
		return false;

	/* read LogwrtResult and update local state */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		LogwrtResult = xlogctl->LogwrtResult;
		WriteRqstPtr = xlogctl->LogwrtRqst.Write;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/* back off to last completed page boundary */
	WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

	/* if we have already flushed that far, consider async commit records */
	if (WriteRqstPtr <= LogwrtResult.Flush)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		WriteRqstPtr = xlogctl->asyncXactLSN;
		SpinLockRelease(&xlogctl->info_lck);
		flexible = false;		/* ensure it all gets written */
	}

	/*
	 * If already known flushed, we're done. Just need to check if we are
	 * holding an open file handle to a logfile that's no longer in use,
	 * preventing the file from being deleted.
	 */
	if (WriteRqstPtr <= LogwrtResult.Flush)
	{
		if (openLogFile >= 0)
		{
			if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
			{
				XLogFileClose();
			}
		}
		return false;
	}

#ifdef WAL_DEBUG
	if (XLOG_DEBUG)
		elog(LOG, "xlog bg flush request %X/%X; write %X/%X; flush %X/%X",
			 (uint32) (WriteRqstPtr >> 32), (uint32) WriteRqstPtr,
			 (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
		   (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

	START_CRIT_SECTION();

	/* now wait for any in-progress insertions to finish and get write lock */
	WaitXLogInsertionsToFinish(WriteRqstPtr);
	LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
	LogwrtResult = XLogCtl->LogwrtResult;
	if (WriteRqstPtr > LogwrtResult.Flush)
	{
		XLogwrtRqst WriteRqst;

		WriteRqst.Write = WriteRqstPtr;
		WriteRqst.Flush = WriteRqstPtr;
		XLogWrite(WriteRqst, flexible);
		wrote_something = true;
	}
	LWLockRelease(WALWriteLock);

	END_CRIT_SECTION();

	/* wake up walsenders now that we've released heavily contended locks */
	WalSndWakeupProcessRequests();

	/*
	 * Great, done. To take some work off the critical path, try to initialize
	 * as many of the no-longer-needed WAL buffers for future use as we can.
	 */
	AdvanceXLInsertBuffer(InvalidXLogRecPtr, true);

	return wrote_something;
}

/*
 * Test whether XLOG data has been flushed up to (at least) the given position.
 *
 * Returns true if a flush is still needed.  (It may be that someone else
 * is already in process of flushing that far, however.)
 */
bool
XLogNeedsFlush(XLogRecPtr record)
{
	/*
	 * During recovery, we don't flush WAL but update minRecoveryPoint
	 * instead. So "needs flush" is taken to mean whether minRecoveryPoint
	 * would need to be updated.
	 */
	if (RecoveryInProgress())
	{
		/* Quick exit if already known updated */
		if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
			return false;

		/*
		 * Update local copy of minRecoveryPoint. But if the lock is busy,
		 * just return a conservative guess.
		 */
		if (!LWLockConditionalAcquire(ControlFileLock, LW_SHARED))
			return true;
		minRecoveryPoint = ControlFile->minRecoveryPoint;
		minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
		LWLockRelease(ControlFileLock);

		/*
		 * An invalid minRecoveryPoint means that we need to recover all the
		 * WAL, i.e., we're doing crash recovery.  We never modify the control
		 * file's value in that case, so we can short-circuit future checks
		 * here too.
		 */
		if (minRecoveryPoint == 0)
			updateMinRecoveryPoint = false;

		/* check again */
		if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
			return false;
		else
			return true;
	}

	/* Quick exit if already known flushed */
	if (record <= LogwrtResult.Flush)
		return false;

	/* read LogwrtResult and update local state */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		LogwrtResult = xlogctl->LogwrtResult;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/* check again */
	if (record <= LogwrtResult.Flush)
		return false;

	return true;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * log, seg: identify segment to be created/opened.
 *
 * *use_existent: if TRUE, OK to use a pre-existing file (else, any
 * pre-existing file will be deleted).	On return, TRUE if a pre-existing
 * file was used.
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns FD of opened file.
 *
 * Note: errors here are ERROR not PANIC because we might or might not be
 * inside a critical section (eg, during checkpoint there is no reason to
 * take down the system on failure).  They will promote to PANIC if we are
 * in a critical section.
 */
int
XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
{
	char		path[MAXPGPATH];
	char		tmppath[MAXPGPATH];
	XLogSegNo	installed_segno;
	int			max_advance;
	int			fd;
	bool		zero_fill = true;

	XLogFilePath(path, ThisTimeLineID, logsegno);

	/*
	 * Try to use existent file (checkpoint maker may have created it already)
	 */
	if (*use_existent)
	{
		fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
						   S_IRUSR | S_IWUSR);
		if (fd < 0)
		{
			if (errno != ENOENT)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not open file \"%s\": %m", path)));
		}
		else
			return fd;
	}

	/*
	 * Initialize an empty (all zeroes) segment.  NOTE: it is possible that
	 * another process is doing the same thing.  If so, we will end up
	 * pre-creating an extra log segment.  That seems OK, and better than
	 * holding the lock throughout this lengthy process.
	 */
	elog(DEBUG2, "creating and filling new WAL file");

	snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

	unlink(tmppath);

	/* do not use get_sync_bit() here --- want to fsync only at end of fill */
	fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
					   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not create file \"%s\": %m", tmppath)));

#ifdef HAVE_POSIX_FALLOCATE
	/*
	 * If posix_fallocate() is available and succeeds, then the file is
	 * properly allocated and we don't need to zero-fill it (which is less
	 * efficient).  In case of an error, fall back to writing zeros, because on
	 * some platforms posix_fallocate() is available but will not always
	 * succeed in cases where zero-filling will.
	 */
	if (posix_fallocate(fd, 0, XLogSegSize) == 0)
		zero_fill = false;
#endif /* HAVE_POSIX_FALLOCATE */

	if (zero_fill)
	{
		/*
		 * Allocate a buffer full of zeros. This is done before opening the
		 * file so that we don't leak the file descriptor if palloc fails.
		 *
		 * Note: palloc zbuffer, instead of just using a local char array, to
		 * ensure it is reasonably well-aligned; this may save a few cycles
		 * transferring data to the kernel.
		 */

		char	*zbuffer = (char *) palloc0(XLOG_BLCKSZ);
		int		 nbytes;

		/*
		 * Zero-fill the file. We have to do this the hard way to ensure that
		 * all the file space has really been allocated --- on platforms that
		 * allow "holes" in files, just seeking to the end doesn't allocate
		 * intermediate space.  This way, we know that we have all the space
		 * and (after the fsync below) that all the indirect blocks are down on
		 * disk. Therefore, fdatasync(2) or O_DSYNC will be sufficient to sync
		 * future writes to the log file.
		 */
		for (nbytes = 0; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
		{
			errno = 0;
			if ((int) write(fd, zbuffer, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
			{
				int			save_errno = errno;

				/*
				 * If we fail to make the file, delete it to release disk space
				 */
				unlink(tmppath);

				close(fd);

				/* if write didn't set errno, assume no disk space */
				errno = save_errno ? save_errno : ENOSPC;

				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not write to file \"%s\": %m",
								tmppath)));
			}
		}
		pfree(zbuffer);
	}

	if (pg_fsync(fd) != 0)
	{
		close(fd);
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not fsync file \"%s\": %m", tmppath)));
	}

	if (close(fd))
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not close file \"%s\": %m", tmppath)));

	/*
	 * Now move the segment into place with its final name.
	 *
	 * If caller didn't want to use a pre-existing file, get rid of any
	 * pre-existing file.  Otherwise, cope with possibility that someone else
	 * has created the file while we were filling ours: if so, use ours to
	 * pre-create a future log segment.
	 */
	installed_segno = logsegno;
	max_advance = XLOGfileslop;
	if (!InstallXLogFileSegment(&installed_segno, tmppath,
								*use_existent, &max_advance,
								use_lock))
	{
		/*
		 * No need for any more future segments, or InstallXLogFileSegment()
		 * failed to rename the file into place. If the rename failed, opening
		 * the file below will fail.
		 */
		unlink(tmppath);
	}

	/* Set flag to tell caller there was no existent file */
	*use_existent = false;

	/* Now open original target segment (might not be file I just made) */
	fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
					   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m", path)));

	elog(DEBUG2, "done creating and filling new WAL file");

	return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * destsegno: identify segment to be created.
 *
 * srcTLI, srclog, srcseg: identify segment to be copied (could be from
 *		a different timeline)
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.	But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno)
{
	char		path[MAXPGPATH];
	char		tmppath[MAXPGPATH];
	char		buffer[XLOG_BLCKSZ];
	int			srcfd;
	int			fd;
	int			nbytes;

	/*
	 * Open the source file
	 */
	XLogFilePath(path, srcTLI, srcsegno);
	srcfd = OpenTransientFile(path, O_RDONLY | PG_BINARY, 0);
	if (srcfd < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m", path)));

	/*
	 * Copy into a temp file name.
	 */
	snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

	unlink(tmppath);

	/* do not use get_sync_bit() here --- want to fsync only at end of fill */
	fd = OpenTransientFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
						   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not create file \"%s\": %m", tmppath)));

	/*
	 * Do the data copying.
	 */
	for (nbytes = 0; nbytes < XLogSegSize; nbytes += sizeof(buffer))
	{
		errno = 0;
		if ((int) read(srcfd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
		{
			if (errno != 0)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not read file \"%s\": %m", path)));
			else
				ereport(ERROR,
						(errmsg("not enough data in file \"%s\"", path)));
		}
		errno = 0;
		if ((int) write(fd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
		{
			int			save_errno = errno;

			/*
			 * If we fail to make the file, delete it to release disk space
			 */
			unlink(tmppath);
			/* if write didn't set errno, assume problem is no disk space */
			errno = save_errno ? save_errno : ENOSPC;

			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not write to file \"%s\": %m", tmppath)));
		}
	}

	if (pg_fsync(fd) != 0)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not fsync file \"%s\": %m", tmppath)));

	if (CloseTransientFile(fd))
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not close file \"%s\": %m", tmppath)));

	CloseTransientFile(srcfd);

	/*
	 * Now move the segment into place with its final name.
	 */
	if (!InstallXLogFileSegment(&destsegno, tmppath, false, NULL, false))
		elog(ERROR, "InstallXLogFileSegment should not have failed");
}

/*
 * Install a new XLOG segment file as a current or future log segment.
 *
 * This is used both to install a newly-created segment (which has a temp
 * filename while it's being created) and to recycle an old segment.
 *
 * *segno: identify segment to install as (or first possible target).
 * When find_free is TRUE, this is modified on return to indicate the
 * actual installation location or last segment searched.
 *
 * tmppath: initial name of file to install.  It will be renamed into place.
 *
 * find_free: if TRUE, install the new segment at the first empty segno
 * number at or after the passed numbers.  If FALSE, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * *max_advance: maximum number of segno slots to advance past the starting
 * point.  Fail if no free slot is found in this range.  On return, reduced
 * by the number of slots skipped over.  (Irrelevant, and may be NULL,
 * when find_free is FALSE.)
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns TRUE if the file was installed successfully.  FALSE indicates that
 * max_advance limit was exceeded, or an error occurred while renaming the
 * file into place.
 */
static bool
InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
					   bool find_free, int *max_advance,
					   bool use_lock)
{
	char		path[MAXPGPATH];
	struct stat stat_buf;

	XLogFilePath(path, ThisTimeLineID, *segno);

	/*
	 * We want to be sure that only one process does this at a time.
	 */
	if (use_lock)
		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

	if (!find_free)
	{
		/* Force installation: get rid of any pre-existing segment file */
		unlink(path);
	}
	else
	{
		/* Find a free slot to put it in */
		while (stat(path, &stat_buf) == 0)
		{
			if (*max_advance <= 0)
			{
				/* Failed to find a free slot within specified range */
				if (use_lock)
					LWLockRelease(ControlFileLock);
				return false;
			}
			(*segno)++;
			(*max_advance)--;
			XLogFilePath(path, ThisTimeLineID, *segno);
		}
	}

	/*
	 * Prefer link() to rename() here just to be really sure that we don't
	 * overwrite an existing logfile.  However, there shouldn't be one, so
	 * rename() is an acceptable substitute except for the truly paranoid.
	 */
#if HAVE_WORKING_LINK
	if (link(tmppath, path) < 0)
	{
		if (use_lock)
			LWLockRelease(ControlFileLock);
		ereport(LOG,
				(errcode_for_file_access(),
				 errmsg("could not link file \"%s\" to \"%s\" (initialization of log file): %m",
						tmppath, path)));
		return false;
	}
	unlink(tmppath);
#else
	if (rename(tmppath, path) < 0)
	{
		if (use_lock)
			LWLockRelease(ControlFileLock);
		ereport(LOG,
				(errcode_for_file_access(),
				 errmsg("could not rename file \"%s\" to \"%s\" (initialization of log file): %m",
						tmppath, path)));
		return false;
	}
#endif

	if (use_lock)
		LWLockRelease(ControlFileLock);

	return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
int
XLogFileOpen(XLogSegNo segno)
{
	char		path[MAXPGPATH];
	int			fd;

	XLogFilePath(path, ThisTimeLineID, segno);

	fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
					   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not open xlog file \"%s\": %m", path)));

	return fd;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * If source == XLOG_FROM_ARCHIVE, the segment is retrieved from archive.
 * Otherwise, it's assumed to be already available in pg_xlog.
 */
static int
XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
			 int source, bool notfoundOk)
{
	char		xlogfname[MAXFNAMELEN];
	char		activitymsg[MAXFNAMELEN + 16];
	char		path[MAXPGPATH];
	int			fd;

	XLogFileName(xlogfname, tli, segno);

	switch (source)
	{
		case XLOG_FROM_ARCHIVE:
			/* Report recovery progress in PS display */
			snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
					 xlogfname);
			set_ps_display(activitymsg, false);

			restoredFromArchive = RestoreArchivedFile(path, xlogfname,
													  "RECOVERYXLOG",
													  XLogSegSize,
													  InRedo);
			if (!restoredFromArchive)
				return -1;
			break;

		case XLOG_FROM_PG_XLOG:
		case XLOG_FROM_STREAM:
			XLogFilePath(path, tli, segno);
			restoredFromArchive = false;
			break;

		default:
			elog(ERROR, "invalid XLogFileRead source %d", source);
	}

	/*
	 * If the segment was fetched from archival storage, replace the existing
	 * xlog segment (if any) with the archival version.
	 */
	if (source == XLOG_FROM_ARCHIVE)
	{
		KeepFileRestoredFromArchive(path, xlogfname);

		/*
		 * Set path to point at the new file in pg_xlog.
		 */
		snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
	}

	fd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
	if (fd >= 0)
	{
		/* Success! */
		curFileTLI = tli;

		/* Report recovery progress in PS display */
		snprintf(activitymsg, sizeof(activitymsg), "recovering %s",
				 xlogfname);
		set_ps_display(activitymsg, false);

		/* Track source of data in assorted state variables */
		readSource = source;
		XLogReceiptSource = source;
		/* In FROM_STREAM case, caller tracks receipt time, not me */
		if (source != XLOG_FROM_STREAM)
			XLogReceiptTime = GetCurrentTimestamp();

		return fd;
	}
	if (errno != ENOENT || !notfoundOk) /* unexpected failure? */
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not open file \"%s\": %m", path)));
	return -1;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * This version searches for the segment with any TLI listed in expectedTLEs.
 */
static int
XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source)
{
	char		path[MAXPGPATH];
	ListCell   *cell;
	int			fd;
	List	   *tles;

	/*
	 * Loop looking for a suitable timeline ID: we might need to read any of
	 * the timelines listed in expectedTLEs.
	 *
	 * We expect curFileTLI on entry to be the TLI of the preceding file in
	 * sequence, or 0 if there was no predecessor.	We do not allow curFileTLI
	 * to go backwards; this prevents us from picking up the wrong file when a
	 * parent timeline extends to higher segment numbers than the child we
	 * want to read.
	 *
	 * If we haven't read the timeline history file yet, read it now, so that
	 * we know which TLIs to scan.	We don't save the list in expectedTLEs,
	 * however, unless we actually find a valid segment.  That way if there is
	 * neither a timeline history file nor a WAL segment in the archive, and
	 * streaming replication is set up, we'll read the timeline history file
	 * streamed from the master when we start streaming, instead of recovering
	 * with a dummy history generated here.
	 */
	if (expectedTLEs)
		tles = expectedTLEs;
	else
		tles = readTimeLineHistory(recoveryTargetTLI);

	foreach(cell, tles)
	{
		TimeLineID	tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli;

		if (tli < curFileTLI)
			break;				/* don't bother looking at too-old TLIs */

		if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE)
		{
			fd = XLogFileRead(segno, emode, tli,
							  XLOG_FROM_ARCHIVE, true);
			if (fd != -1)
			{
				elog(DEBUG1, "got WAL segment from archive");
				if (!expectedTLEs)
					expectedTLEs = tles;
				return fd;
			}
		}

		if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_XLOG)
		{
			fd = XLogFileRead(segno, emode, tli,
							  XLOG_FROM_PG_XLOG, true);
			if (fd != -1)
			{
				if (!expectedTLEs)
					expectedTLEs = tles;
				return fd;
			}
		}
	}

	/* Couldn't find it.  For simplicity, complain about front timeline */
	XLogFilePath(path, recoveryTargetTLI, segno);
	errno = ENOENT;
	ereport(emode,
			(errcode_for_file_access(),
			 errmsg("could not open file \"%s\": %m", path)));
	return -1;
}

/*
 * Close the current logfile segment for writing.
 */
static void
XLogFileClose(void)
{
	Assert(openLogFile >= 0);

	/*
	 * WAL segment files will not be re-read in normal operation, so we advise
	 * the OS to release any cached pages.	But do not do so if WAL archiving
	 * or streaming is active, because archiver and walsender process could
	 * use the cache to read the WAL segment.
	 */
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
	if (!XLogIsNeeded())
		(void) posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif

	if (close(openLogFile))
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not close log file %s: %m",
						XLogFileNameP(ThisTimeLineID, openLogSegNo))));
	openLogFile = -1;
}

/*
 * Preallocate log files beyond the specified log endpoint.
 *
 * XXX this is currently extremely conservative, since it forces only one
 * future log segment to exist, and even that only if we are 75% done with
 * the current one.  This is only appropriate for very low-WAL-volume systems.
 * High-volume systems will be OK once they've built up a sufficient set of
 * recycled log segments, but the startup transient is likely to include
 * a lot of segment creations by foreground processes, which is not so good.
 */
static void
PreallocXlogFiles(XLogRecPtr endptr)
{
	XLogSegNo	_logSegNo;
	int			lf;
	bool		use_existent;

	XLByteToPrevSeg(endptr, _logSegNo);
	if ((endptr - 1) % XLogSegSize >= (uint32) (0.75 * XLogSegSize))
	{
		_logSegNo++;
		use_existent = true;
		lf = XLogFileInit(_logSegNo, &use_existent, true);
		close(lf);
		if (!use_existent)
			CheckpointStats.ckpt_segs_added++;
	}
}

/*
 * Throws an error if the given log segment has already been removed or
 * recycled. The caller should only pass a segment that it knows to have
 * existed while the server has been running, as this function always
 * succeeds if no WAL segments have been removed since startup.
 * 'tli' is only used in the error message.
 */
void
CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	XLogSegNo	lastRemovedSegNo;

	SpinLockAcquire(&xlogctl->info_lck);
	lastRemovedSegNo = xlogctl->lastRemovedSegNo;
	SpinLockRelease(&xlogctl->info_lck);

	if (segno <= lastRemovedSegNo)
	{
		char		filename[MAXFNAMELEN];

		XLogFileName(filename, tli, segno);
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("requested WAL segment %s has already been removed",
						filename)));
	}
}

/*
 * Update the last removed segno pointer in shared memory, to reflect
 * that the given XLOG file has been removed.
 */
static void
UpdateLastRemovedPtr(char *filename)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	uint32		tli;
	XLogSegNo	segno;

	XLogFromFileName(filename, &tli, &segno);

	SpinLockAcquire(&xlogctl->info_lck);
	if (segno > xlogctl->lastRemovedSegNo)
		xlogctl->lastRemovedSegNo = segno;
	SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Recycle or remove all log files older or equal to passed segno
 *
 * endptr is current (or recent) end of xlog; this is used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 */
static void
RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr)
{
	XLogSegNo	endlogSegNo;
	int			max_advance;
	DIR		   *xldir;
	struct dirent *xlde;
	char		lastoff[MAXFNAMELEN];
	char		path[MAXPGPATH];

#ifdef WIN32
	char		newpath[MAXPGPATH];
#endif
	struct stat statbuf;

	/*
	 * Initialize info about where to try to recycle to.  We allow recycling
	 * segments up to XLOGfileslop segments beyond the current XLOG location.
	 */
	XLByteToPrevSeg(endptr, endlogSegNo);
	max_advance = XLOGfileslop;

	xldir = AllocateDir(XLOGDIR);
	if (xldir == NULL)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open transaction log directory \"%s\": %m",
						XLOGDIR)));

	/*
	 * Construct a filename of the last segment to be kept. The timeline ID
	 * doesn't matter, we ignore that in the comparison. (During recovery,
	 * ThisTimeLineID isn't set, so we can't use that.)
	 */
	XLogFileName(lastoff, 0, segno);

	elog(DEBUG2, "attempting to remove WAL segments older than log file %s",
		 lastoff);

	while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
	{
		/*
		 * We ignore the timeline part of the XLOG segment identifiers in
		 * deciding whether a segment is still needed.	This ensures that we
		 * won't prematurely remove a segment from a parent timeline. We could
		 * probably be a little more proactive about removing segments of
		 * non-parent timelines, but that would be a whole lot more
		 * complicated.
		 *
		 * We use the alphanumeric sorting property of the filenames to decide
		 * which ones are earlier than the lastoff segment.
		 */
		if (strlen(xlde->d_name) == 24 &&
			strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
			strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
		{
			if (XLogArchiveCheckDone(xlde->d_name))
			{
				snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);

				/* Update the last removed location in shared memory first */
				UpdateLastRemovedPtr(xlde->d_name);

				/*
				 * Before deleting the file, see if it can be recycled as a
				 * future log segment. Only recycle normal files, pg_standby
				 * for example can create symbolic links pointing to a
				 * separate archive directory.
				 */
				if (lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
					InstallXLogFileSegment(&endlogSegNo, path,
										   true, &max_advance, true))
				{
					ereport(DEBUG2,
							(errmsg("recycled transaction log file \"%s\"",
									xlde->d_name)));
					CheckpointStats.ckpt_segs_recycled++;
					/* Needn't recheck that slot on future iterations */
					if (max_advance > 0)
					{
						endlogSegNo++;
						max_advance--;
					}
				}
				else
				{
					/* No need for any more future segments... */
					int			rc;

					ereport(DEBUG2,
							(errmsg("removing transaction log file \"%s\"",
									xlde->d_name)));

#ifdef WIN32

					/*
					 * On Windows, if another process (e.g another backend)
					 * holds the file open in FILE_SHARE_DELETE mode, unlink
					 * will succeed, but the file will still show up in
					 * directory listing until the last handle is closed. To
					 * avoid confusing the lingering deleted file for a live
					 * WAL file that needs to be archived, rename it before
					 * deleting it.
					 *
					 * If another process holds the file open without
					 * FILE_SHARE_DELETE flag, rename will fail. We'll try
					 * again at the next checkpoint.
					 */
					snprintf(newpath, MAXPGPATH, "%s.deleted", path);
					if (rename(path, newpath) != 0)
					{
						ereport(LOG,
								(errcode_for_file_access(),
								 errmsg("could not rename old transaction log file \"%s\": %m",
										path)));
						continue;
					}
					rc = unlink(newpath);
#else
					rc = unlink(path);
#endif
					if (rc != 0)
					{
						ereport(LOG,
								(errcode_for_file_access(),
								 errmsg("could not remove old transaction log file \"%s\": %m",
										path)));
						continue;
					}
					CheckpointStats.ckpt_segs_removed++;
				}

				XLogArchiveCleanup(xlde->d_name);
			}
		}
	}

	FreeDir(xldir);
}

/*
 * Verify whether pg_xlog and pg_xlog/archive_status exist.
 * If the latter does not exist, recreate it.
 *
 * It is not the goal of this function to verify the contents of these
 * directories, but to help in cases where someone has performed a cluster
 * copy for PITR purposes but omitted pg_xlog from the copy.
 *
 * We could also recreate pg_xlog if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_xlog to be
 * a symlink, and if that was the DBA's intent then automatically making a
 * plain directory would result in degraded performance with no notice.
 */
static void
ValidateXLOGDirectoryStructure(void)
{
	char		path[MAXPGPATH];
	struct stat stat_buf;

	/* Check for pg_xlog; if it doesn't exist, error out */
	if (stat(XLOGDIR, &stat_buf) != 0 ||
		!S_ISDIR(stat_buf.st_mode))
		ereport(FATAL,
				(errmsg("required WAL directory \"%s\" does not exist",
						XLOGDIR)));

	/* Check for archive_status */
	snprintf(path, MAXPGPATH, XLOGDIR "/archive_status");
	if (stat(path, &stat_buf) == 0)
	{
		/* Check for weird cases where it exists but isn't a directory */
		if (!S_ISDIR(stat_buf.st_mode))
			ereport(FATAL,
					(errmsg("required WAL directory \"%s\" does not exist",
							path)));
	}
	else
	{
		ereport(LOG,
				(errmsg("creating missing WAL directory \"%s\"", path)));
		if (mkdir(path, S_IRWXU) < 0)
			ereport(FATAL,
					(errmsg("could not create missing directory \"%s\": %m",
							path)));
	}
}

/*
 * Remove previous backup history files.  This also retries creation of
 * .ready files for any backup history files for which XLogArchiveNotify
 * failed earlier.
 */
static void
CleanupBackupHistory(void)
{
	DIR		   *xldir;
	struct dirent *xlde;
	char		path[MAXPGPATH];

	xldir = AllocateDir(XLOGDIR);
	if (xldir == NULL)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open transaction log directory \"%s\": %m",
						XLOGDIR)));

	while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
	{
		if (strlen(xlde->d_name) > 24 &&
			strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
			strcmp(xlde->d_name + strlen(xlde->d_name) - strlen(".backup"),
				   ".backup") == 0)
		{
			if (XLogArchiveCheckDone(xlde->d_name))
			{
				ereport(DEBUG2,
				(errmsg("removing transaction log backup history file \"%s\"",
						xlde->d_name)));
				snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);
				unlink(path);
				XLogArchiveCleanup(xlde->d_name);
			}
		}
	}

	FreeDir(xldir);
}

/*
 * Restore a full-page image from a backup block attached to an XLOG record.
 *
 * lsn: LSN of the XLOG record being replayed
 * record: the complete XLOG record
 * block_index: which backup block to restore (0 .. XLR_MAX_BKP_BLOCKS - 1)
 * get_cleanup_lock: TRUE to get a cleanup rather than plain exclusive lock
 * keep_buffer: TRUE to return the buffer still locked and pinned
 *
 * Returns the buffer number containing the page.  Note this is not terribly
 * useful unless keep_buffer is specified as TRUE.
 *
 * Note: when a backup block is available in XLOG, we restore it
 * unconditionally, even if the page in the database appears newer.
 * This is to protect ourselves against database pages that were partially
 * or incorrectly written during a crash.  We assume that the XLOG data
 * must be good because it has passed a CRC check, while the database
 * page might not be.  This will force us to replay all subsequent
 * modifications of the page that appear in XLOG, rather than possibly
 * ignoring them as already applied, but that's not a huge drawback.
 *
 * If 'get_cleanup_lock' is true, a cleanup lock is obtained on the buffer,
 * else a normal exclusive lock is used.  During crash recovery, that's just
 * pro forma because there can't be any regular backends in the system, but
 * in hot standby mode the distinction is important.
 *
 * If 'keep_buffer' is true, return without releasing the buffer lock and pin;
 * then caller is responsible for doing UnlockReleaseBuffer() later.  This
 * is needed in some cases when replaying XLOG records that touch multiple
 * pages, to prevent inconsistent states from being visible to other backends.
 * (Again, that's only important in hot standby mode.)
 */
Buffer
RestoreBackupBlock(XLogRecPtr lsn, XLogRecord *record, int block_index,
				   bool get_cleanup_lock, bool keep_buffer)
{
	BkpBlock	bkpb;
	char	   *blk;
	int			i;

	/* Locate requested BkpBlock in the record */
	blk = (char *) XLogRecGetData(record) + record->xl_len;
	for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
	{
		if (!(record->xl_info & XLR_BKP_BLOCK(i)))
			continue;

		memcpy(&bkpb, blk, sizeof(BkpBlock));
		blk += sizeof(BkpBlock);

		if (i == block_index)
		{
			/* Found it, apply the update */
			return RestoreBackupBlockContents(lsn, bkpb, blk, get_cleanup_lock,
											  keep_buffer);
		}

		blk += BLCKSZ - bkpb.hole_length;
	}

	/* Caller specified a bogus block_index */
	elog(ERROR, "failed to restore block_index %d", block_index);
	return InvalidBuffer;		/* keep compiler quiet */
}

/*
 * Workhorse for RestoreBackupBlock usable without an xlog record
 *
 * Restores a full-page image from BkpBlock and a data pointer.
 */
static Buffer
RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb, char *blk,
						   bool get_cleanup_lock, bool keep_buffer)
{
	Buffer		buffer;
	Page		page;

	buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
									RBM_ZERO);
	Assert(BufferIsValid(buffer));
	if (get_cleanup_lock)
		LockBufferForCleanup(buffer);
	else
		LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

	page = (Page) BufferGetPage(buffer);

	if (bkpb.hole_length == 0)
	{
		memcpy((char *) page, blk, BLCKSZ);
	}
	else
	{
		memcpy((char *) page, blk, bkpb.hole_offset);
		/* must zero-fill the hole */
		MemSet((char *) page + bkpb.hole_offset, 0, bkpb.hole_length);
		memcpy((char *) page + (bkpb.hole_offset + bkpb.hole_length),
			   blk + bkpb.hole_offset,
			   BLCKSZ - (bkpb.hole_offset + bkpb.hole_length));
	}

	/*
	 * The checksum value on this page is currently invalid. We don't need to
	 * reset it here since it will be set before being written.
	 */

	PageSetLSN(page, lsn);
	MarkBufferDirty(buffer);

	if (!keep_buffer)
		UnlockReleaseBuffer(buffer);

	return buffer;
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is not NULL, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If no valid record is available, returns NULL, or fails if emode is PANIC.
 * (emode must be either PANIC, LOG). In standby mode, retries until a valid
 * record is available.
 *
 * The record is copied into readRecordBuf, so that on successful return,
 * the returned record pointer always points there.
 */
static XLogRecord *
ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,
		   bool fetching_ckpt)
{
	XLogRecord *record;
	XLogPageReadPrivate *private = (XLogPageReadPrivate *) xlogreader->private_data;

	/* Pass through parameters to XLogPageRead */
	private->fetching_ckpt = fetching_ckpt;
	private->emode = emode;
	private->randAccess = (RecPtr != InvalidXLogRecPtr);

	/* This is the first attempt to read this page. */
	lastSourceFailed = false;

	for (;;)
	{
		char	   *errormsg;

		record = XLogReadRecord(xlogreader, RecPtr, &errormsg);
		ReadRecPtr = xlogreader->ReadRecPtr;
		EndRecPtr = xlogreader->EndRecPtr;
		if (record == NULL)
		{
			if (readFile >= 0)
			{
				close(readFile);
				readFile = -1;
			}

			/*
			 * We only end up here without a message when XLogPageRead()
			 * failed - in that case we already logged something. In
			 * StandbyMode that only happens if we have been triggered, so we
			 * shouldn't loop anymore in that case.
			 */
			if (errormsg)
				ereport(emode_for_corrupt_record(emode,
												 RecPtr ? RecPtr : EndRecPtr),
				(errmsg_internal("%s", errormsg) /* already translated */ ));
		}

		/*
		 * Check page TLI is one of the expected values.
		 */
		else if (!tliInHistory(xlogreader->latestPageTLI, expectedTLEs))
		{
			char		fname[MAXFNAMELEN];
			XLogSegNo	segno;
			int32		offset;

			XLByteToSeg(xlogreader->latestPagePtr, segno);
			offset = xlogreader->latestPagePtr % XLogSegSize;
			XLogFileName(fname, xlogreader->readPageTLI, segno);
			ereport(emode_for_corrupt_record(emode,
											 RecPtr ? RecPtr : EndRecPtr),
			(errmsg("unexpected timeline ID %u in log segment %s, offset %u",
					xlogreader->latestPageTLI,
					fname,
					offset)));
			record = NULL;
		}

		if (record)
		{
			/* Great, got a record */
			return record;
		}
		else
		{
			/* No valid record available from this source */
			lastSourceFailed = true;

			/*
			 * If archive recovery was requested, but we were still doing
			 * crash recovery, switch to archive recovery and retry using the
			 * offline archive. We have now replayed all the valid WAL in
			 * pg_xlog, so we are presumably now consistent.
			 *
			 * We require that there's at least some valid WAL present in
			 * pg_xlog, however (!fetch_ckpt). We could recover using the WAL
			 * from the archive, even if pg_xlog is completely empty, but we'd
			 * have no idea how far we'd have to replay to reach consistency.
			 * So err on the safe side and give up.
			 */
			if (!InArchiveRecovery && ArchiveRecoveryRequested &&
				!fetching_ckpt)
			{
				ereport(DEBUG1,
						(errmsg_internal("reached end of WAL in pg_xlog, entering archive recovery")));
				InArchiveRecovery = true;
				if (StandbyModeRequested)
					StandbyMode = true;

				/* initialize minRecoveryPoint to this record */
				LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
				ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
				if (ControlFile->minRecoveryPoint < EndRecPtr)
				{
					ControlFile->minRecoveryPoint = EndRecPtr;
					ControlFile->minRecoveryPointTLI = ThisTimeLineID;
				}
				/* update local copy */
				minRecoveryPoint = ControlFile->minRecoveryPoint;
				minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

				UpdateControlFile();
				LWLockRelease(ControlFileLock);

				CheckRecoveryConsistency();

				/*
				 * Before we retry, reset lastSourceFailed and currentSource
				 * so that we will check the archive next.
				 */
				lastSourceFailed = false;
				currentSource = 0;

				continue;
			}

			/* In standby mode, loop back to retry. Otherwise, give up. */
			if (StandbyMode && !CheckForStandbyTrigger())
				continue;
			else
				return NULL;
		}
	}
}

/*
 * Scan for new timelines that might have appeared in the archive since we
 * started recovery.
 *
 * If there are any, the function changes recovery target TLI to the latest
 * one and returns 'true'.
 */
static bool
rescanLatestTimeLine(void)
{
	List	   *newExpectedTLEs;
	bool		found;
	ListCell   *cell;
	TimeLineID	newtarget;
	TimeLineID	oldtarget = recoveryTargetTLI;
	TimeLineHistoryEntry *currentTle = NULL;

	newtarget = findNewestTimeLine(recoveryTargetTLI);
	if (newtarget == recoveryTargetTLI)
	{
		/* No new timelines found */
		return false;
	}

	/*
	 * Determine the list of expected TLIs for the new TLI
	 */

	newExpectedTLEs = readTimeLineHistory(newtarget);

	/*
	 * If the current timeline is not part of the history of the new timeline,
	 * we cannot proceed to it.
	 */
	found = false;
	foreach(cell, newExpectedTLEs)
	{
		currentTle = (TimeLineHistoryEntry *) lfirst(cell);

		if (currentTle->tli == recoveryTargetTLI)
		{
			found = true;
			break;
		}
	}
	if (!found)
	{
		ereport(LOG,
				(errmsg("new timeline %u is not a child of database system timeline %u",
						newtarget,
						ThisTimeLineID)));
		return false;
	}

	/*
	 * The current timeline was found in the history file, but check that the
	 * next timeline was forked off from it *after* the current recovery
	 * location.
	 */
	if (currentTle->end < EndRecPtr)
	{
		ereport(LOG,
				(errmsg("new timeline %u forked off current database system timeline %u before current recovery point %X/%X",
						newtarget,
						ThisTimeLineID,
						(uint32) (EndRecPtr >> 32), (uint32) EndRecPtr)));
		return false;
	}

	/* The new timeline history seems valid. Switch target */
	recoveryTargetTLI = newtarget;
	list_free_deep(expectedTLEs);
	expectedTLEs = newExpectedTLEs;

	/*
	 * As in StartupXLOG(), try to ensure we have all the history files
	 * between the old target and new target in pg_xlog.
	 */
	restoreTimeLineHistoryFiles(oldtarget + 1, newtarget);

	ereport(LOG,
			(errmsg("new target timeline is %u",
					recoveryTargetTLI)));

	return true;
}

/*
 * I/O routines for pg_control
 *
 * *ControlFile is a buffer in shared memory that holds an image of the
 * contents of pg_control.	WriteControlFile() initializes pg_control
 * given a preloaded buffer, ReadControlFile() loads the buffer from
 * the pg_control file (during postmaster or standalone-backend startup),
 * and UpdateControlFile() rewrites pg_control after we modify xlog state.
 *
 * For simplicity, WriteControlFile() initializes the fields of pg_control
 * that are related to checking backend/database compatibility, and
 * ReadControlFile() verifies they are correct.  We could split out the
 * I/O and compatibility-check functions, but there seems no need currently.
 */
static void
WriteControlFile(void)
{
	int			fd;
	char		buffer[PG_CONTROL_SIZE];		/* need not be aligned */

	/*
	 * Initialize version and compatibility-check fields
	 */
	ControlFile->pg_control_version = PG_CONTROL_VERSION;
	ControlFile->catalog_version_no = CATALOG_VERSION_NO;

	ControlFile->maxAlign = MAXIMUM_ALIGNOF;
	ControlFile->floatFormat = FLOATFORMAT_VALUE;

	ControlFile->blcksz = BLCKSZ;
	ControlFile->relseg_size = RELSEG_SIZE;
	ControlFile->xlog_blcksz = XLOG_BLCKSZ;
	ControlFile->xlog_seg_size = XLOG_SEG_SIZE;

	ControlFile->nameDataLen = NAMEDATALEN;
	ControlFile->indexMaxKeys = INDEX_MAX_KEYS;

	ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;

#ifdef HAVE_INT64_TIMESTAMP
	ControlFile->enableIntTimes = true;
#else
	ControlFile->enableIntTimes = false;
#endif
	ControlFile->float4ByVal = FLOAT4PASSBYVAL;
	ControlFile->float8ByVal = FLOAT8PASSBYVAL;

	/* Contents are protected with a CRC */
	INIT_CRC32(ControlFile->crc);
	COMP_CRC32(ControlFile->crc,
			   (char *) ControlFile,
			   offsetof(ControlFileData, crc));
	FIN_CRC32(ControlFile->crc);

	/*
	 * We write out PG_CONTROL_SIZE bytes into pg_control, zero-padding the
	 * excess over sizeof(ControlFileData).  This reduces the odds of
	 * premature-EOF errors when reading pg_control.  We'll still fail when we
	 * check the contents of the file, but hopefully with a more specific
	 * error than "couldn't read pg_control".
	 */
	if (sizeof(ControlFileData) > PG_CONTROL_SIZE)
		elog(PANIC, "sizeof(ControlFileData) is larger than PG_CONTROL_SIZE; fix either one");

	memset(buffer, 0, PG_CONTROL_SIZE);
	memcpy(buffer, ControlFile, sizeof(ControlFileData));

	fd = BasicOpenFile(XLOG_CONTROL_FILE,
					   O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
					   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not create control file \"%s\": %m",
						XLOG_CONTROL_FILE)));

	errno = 0;
	if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_SIZE)
	{
		/* if write didn't set errno, assume problem is no disk space */
		if (errno == 0)
			errno = ENOSPC;
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not write to control file: %m")));
	}

	if (pg_fsync(fd) != 0)
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not fsync control file: %m")));

	if (close(fd))
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not close control file: %m")));
}

static void
ReadControlFile(void)
{
	pg_crc32	crc;
	int			fd;

	/*
	 * Read data...
	 */
	fd = BasicOpenFile(XLOG_CONTROL_FILE,
					   O_RDWR | PG_BINARY,
					   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not open control file \"%s\": %m",
						XLOG_CONTROL_FILE)));

	if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not read from control file: %m")));

	close(fd);

	/*
	 * Check for expected pg_control format version.  If this is wrong, the
	 * CRC check will likely fail because we'll be checking the wrong number
	 * of bytes.  Complaining about wrong version will probably be more
	 * enlightening than complaining about wrong CRC.
	 */

	if (ControlFile->pg_control_version != PG_CONTROL_VERSION && ControlFile->pg_control_version % 65536 == 0 && ControlFile->pg_control_version / 65536 != 0)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d (0x%08x),"
		 " but the server was compiled with PG_CONTROL_VERSION %d (0x%08x).",
			ControlFile->pg_control_version, ControlFile->pg_control_version,
						   PG_CONTROL_VERSION, PG_CONTROL_VERSION),
				 errhint("This could be a problem of mismatched byte ordering.  It looks like you need to initdb.")));

	if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d,"
				  " but the server was compiled with PG_CONTROL_VERSION %d.",
						ControlFile->pg_control_version, PG_CONTROL_VERSION),
				 errhint("It looks like you need to initdb.")));

	/* Now check the CRC. */
	INIT_CRC32(crc);
	COMP_CRC32(crc,
			   (char *) ControlFile,
			   offsetof(ControlFileData, crc));
	FIN_CRC32(crc);

	if (!EQ_CRC32(crc, ControlFile->crc))
		ereport(FATAL,
				(errmsg("incorrect checksum in control file")));

	/*
	 * Do compatibility checking immediately.  If the database isn't
	 * compatible with the backend executable, we want to abort before we can
	 * possibly do any damage.
	 */
	if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with CATALOG_VERSION_NO %d,"
				  " but the server was compiled with CATALOG_VERSION_NO %d.",
						ControlFile->catalog_version_no, CATALOG_VERSION_NO),
				 errhint("It looks like you need to initdb.")));
	if (ControlFile->maxAlign != MAXIMUM_ALIGNOF)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
		   errdetail("The database cluster was initialized with MAXALIGN %d,"
					 " but the server was compiled with MAXALIGN %d.",
					 ControlFile->maxAlign, MAXIMUM_ALIGNOF),
				 errhint("It looks like you need to initdb.")));
	if (ControlFile->floatFormat != FLOATFORMAT_VALUE)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster appears to use a different floating-point number format than the server executable."),
				 errhint("It looks like you need to initdb.")));
	if (ControlFile->blcksz != BLCKSZ)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
			 errdetail("The database cluster was initialized with BLCKSZ %d,"
					   " but the server was compiled with BLCKSZ %d.",
					   ControlFile->blcksz, BLCKSZ),
				 errhint("It looks like you need to recompile or initdb.")));
	if (ControlFile->relseg_size != RELSEG_SIZE)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
		errdetail("The database cluster was initialized with RELSEG_SIZE %d,"
				  " but the server was compiled with RELSEG_SIZE %d.",
				  ControlFile->relseg_size, RELSEG_SIZE),
				 errhint("It looks like you need to recompile or initdb.")));
	if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
		errdetail("The database cluster was initialized with XLOG_BLCKSZ %d,"
				  " but the server was compiled with XLOG_BLCKSZ %d.",
				  ControlFile->xlog_blcksz, XLOG_BLCKSZ),
				 errhint("It looks like you need to recompile or initdb.")));
	if (ControlFile->xlog_seg_size != XLOG_SEG_SIZE)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with XLOG_SEG_SIZE %d,"
					   " but the server was compiled with XLOG_SEG_SIZE %d.",
						   ControlFile->xlog_seg_size, XLOG_SEG_SIZE),
				 errhint("It looks like you need to recompile or initdb.")));
	if (ControlFile->nameDataLen != NAMEDATALEN)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
		errdetail("The database cluster was initialized with NAMEDATALEN %d,"
				  " but the server was compiled with NAMEDATALEN %d.",
				  ControlFile->nameDataLen, NAMEDATALEN),
				 errhint("It looks like you need to recompile or initdb.")));
	if (ControlFile->indexMaxKeys != INDEX_MAX_KEYS)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with INDEX_MAX_KEYS %d,"
					  " but the server was compiled with INDEX_MAX_KEYS %d.",
						   ControlFile->indexMaxKeys, INDEX_MAX_KEYS),
				 errhint("It looks like you need to recompile or initdb.")));
	if (ControlFile->toast_max_chunk_size != TOAST_MAX_CHUNK_SIZE)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with TOAST_MAX_CHUNK_SIZE %d,"
				" but the server was compiled with TOAST_MAX_CHUNK_SIZE %d.",
			  ControlFile->toast_max_chunk_size, (int) TOAST_MAX_CHUNK_SIZE),
				 errhint("It looks like you need to recompile or initdb.")));

#ifdef HAVE_INT64_TIMESTAMP
	if (ControlFile->enableIntTimes != true)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized without HAVE_INT64_TIMESTAMP"
				  " but the server was compiled with HAVE_INT64_TIMESTAMP."),
				 errhint("It looks like you need to recompile or initdb.")));
#else
	if (ControlFile->enableIntTimes != false)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized with HAVE_INT64_TIMESTAMP"
			   " but the server was compiled without HAVE_INT64_TIMESTAMP."),
				 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT4_BYVAL
	if (ControlFile->float4ByVal != true)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized without USE_FLOAT4_BYVAL"
					  " but the server was compiled with USE_FLOAT4_BYVAL."),
				 errhint("It looks like you need to recompile or initdb.")));
#else
	if (ControlFile->float4ByVal != false)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
		errdetail("The database cluster was initialized with USE_FLOAT4_BYVAL"
				  " but the server was compiled without USE_FLOAT4_BYVAL."),
				 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT8_BYVAL
	if (ControlFile->float8ByVal != true)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
				 errdetail("The database cluster was initialized without USE_FLOAT8_BYVAL"
					  " but the server was compiled with USE_FLOAT8_BYVAL."),
				 errhint("It looks like you need to recompile or initdb.")));
#else
	if (ControlFile->float8ByVal != false)
		ereport(FATAL,
				(errmsg("database files are incompatible with server"),
		errdetail("The database cluster was initialized with USE_FLOAT8_BYVAL"
				  " but the server was compiled without USE_FLOAT8_BYVAL."),
				 errhint("It looks like you need to recompile or initdb.")));
#endif
}

void
UpdateControlFile(void)
{
	int			fd;

	INIT_CRC32(ControlFile->crc);
	COMP_CRC32(ControlFile->crc,
			   (char *) ControlFile,
			   offsetof(ControlFileData, crc));
	FIN_CRC32(ControlFile->crc);

	fd = BasicOpenFile(XLOG_CONTROL_FILE,
					   O_RDWR | PG_BINARY,
					   S_IRUSR | S_IWUSR);
	if (fd < 0)
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not open control file \"%s\": %m",
						XLOG_CONTROL_FILE)));

	errno = 0;
	if (write(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
	{
		/* if write didn't set errno, assume problem is no disk space */
		if (errno == 0)
			errno = ENOSPC;
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not write to control file: %m")));
	}

	if (pg_fsync(fd) != 0)
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not fsync control file: %m")));

	if (close(fd))
		ereport(PANIC,
				(errcode_for_file_access(),
				 errmsg("could not close control file: %m")));
}

/*
 * Returns the unique system identifier from control file.
 */
uint64
GetSystemIdentifier(void)
{
	Assert(ControlFile != NULL);
	return ControlFile->system_identifier;
}

/*
 * Are checksums enabled for data pages?
 */
bool
DataChecksumsEnabled(void)
{
	Assert(ControlFile != NULL);
	return (ControlFile->data_checksum_version > 0);
}

/*
 * Returns a fake LSN for unlogged relations.
 *
 * Each call generates an LSN that is greater than any previous value
 * returned. The current counter value is saved and restored across clean
 * shutdowns, but like unlogged relations, does not survive a crash. This can
 * be used in lieu of real LSN values returned by XLogInsert, if you need an
 * LSN-like increasing sequence of numbers without writing any WAL.
 */
XLogRecPtr
GetFakeLSNForUnloggedRel(void)
{
	XLogRecPtr	nextUnloggedLSN;

	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	/* increment the unloggedLSN counter, need SpinLock */
	SpinLockAcquire(&xlogctl->ulsn_lck);
	nextUnloggedLSN = xlogctl->unloggedLSN++;
	SpinLockRelease(&xlogctl->ulsn_lck);

	return nextUnloggedLSN;
}

/*
 * Auto-tune the number of XLOG buffers.
 *
 * The preferred setting for wal_buffers is about 3% of shared_buffers, with
 * a maximum of one XLOG segment (there is little reason to think that more
 * is helpful, at least so long as we force an fsync when switching log files)
 * and a minimum of 8 blocks (which was the default value prior to PostgreSQL
 * 9.1, when auto-tuning was added).
 *
 * This should not be called until NBuffers has received its final value.
 */
static int
XLOGChooseNumBuffers(void)
{
	int			xbuffers;

	xbuffers = NBuffers / 32;
	if (xbuffers > XLOG_SEG_SIZE / XLOG_BLCKSZ)
		xbuffers = XLOG_SEG_SIZE / XLOG_BLCKSZ;
	if (xbuffers < 8)
		xbuffers = 8;
	return xbuffers;
}

/*
 * GUC check_hook for wal_buffers
 */
bool
check_wal_buffers(int *newval, void **extra, GucSource source)
{
	/*
	 * -1 indicates a request for auto-tune.
	 */
	if (*newval == -1)
	{
		/*
		 * If we haven't yet changed the boot_val default of -1, just let it
		 * be.	We'll fix it when XLOGShmemSize is called.
		 */
		if (XLOGbuffers == -1)
			return true;

		/* Otherwise, substitute the auto-tune value */
		*newval = XLOGChooseNumBuffers();
	}

	/*
	 * We clamp manually-set values to at least 4 blocks.  Prior to PostgreSQL
	 * 9.1, a minimum of 4 was enforced by guc.c, but since that is no longer
	 * the case, we just silently treat such values as a request for the
	 * minimum.  (We could throw an error instead, but that doesn't seem very
	 * helpful.)
	 */
	if (*newval < 4)
		*newval = 4;

	return true;
}

/*
 * Initialization of shared memory for XLOG
 */
Size
XLOGShmemSize(void)
{
	Size		size;

	/*
	 * If the value of wal_buffers is -1, use the preferred auto-tune value.
	 * This isn't an amazingly clean place to do this, but we must wait till
	 * NBuffers has received its final value, and must do it before using the
	 * value of XLOGbuffers to do anything important.
	 */
	if (XLOGbuffers == -1)
	{
		char		buf[32];

		snprintf(buf, sizeof(buf), "%d", XLOGChooseNumBuffers());
		SetConfigOption("wal_buffers", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
	}
	Assert(XLOGbuffers > 0);

	/* XLogCtl */
	size = sizeof(XLogCtlData);

	/* xlog insertion slots, plus alignment */
	size = add_size(size, mul_size(sizeof(XLogInsertSlotPadded), num_xloginsert_slots + 1));
	/* xlblocks array */
	size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
	/* extra alignment padding for XLOG I/O buffers */
	size = add_size(size, XLOG_BLCKSZ);
	/* and the buffers themselves */
	size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));

	/*
	 * Note: we don't count ControlFileData, it comes out of the "slop factor"
	 * added by CreateSharedMemoryAndSemaphores.  This lets us use this
	 * routine again below to compute the actual allocation size.
	 */

	return size;
}

void
XLOGShmemInit(void)
{
	bool		foundCFile,
				foundXLog;
	char	   *allocptr;
	int			i;

	ControlFile = (ControlFileData *)
		ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);
	XLogCtl = (XLogCtlData *)
		ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

	if (foundCFile || foundXLog)
	{
		/* both should be present or neither */
		Assert(foundCFile && foundXLog);
		return;
	}
	memset(XLogCtl, 0, sizeof(XLogCtlData));

	/*
	 * Since XLogCtlData contains XLogRecPtr fields, its sizeof should be a
	 * multiple of the alignment for same, so no extra alignment padding is
	 * needed here.
	 */
	allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
	XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
	memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
	allocptr += sizeof(XLogRecPtr) * XLOGbuffers;

	/* Xlog insertion slots. Ensure they're aligned to the full padded size */
	allocptr += sizeof(XLogInsertSlotPadded) -
		((uintptr_t) allocptr) % sizeof(XLogInsertSlotPadded);
	XLogCtl->Insert.insertSlots = (XLogInsertSlotPadded *) allocptr;
	allocptr += sizeof(XLogInsertSlotPadded) * num_xloginsert_slots;

	/*
	 * Align the start of the page buffers to a full xlog block size boundary.
	 * This simplifies some calculations in XLOG insertion. It is also required
	 * for O_DIRECT.
	 */
	allocptr = (char *) TYPEALIGN(XLOG_BLCKSZ, allocptr);
	XLogCtl->pages = allocptr;
	memset(XLogCtl->pages, 0, (Size) XLOG_BLCKSZ * XLOGbuffers);

	/*
	 * Do basic initialization of XLogCtl shared data. (StartupXLOG will fill
	 * in additional info.)
	 */
	XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
	XLogCtl->SharedRecoveryInProgress = true;
	XLogCtl->SharedHotStandbyActive = false;
	XLogCtl->WalWriterSleeping = false;

	for (i = 0; i < num_xloginsert_slots; i++)
	{
		XLogInsertSlot *slot = &XLogCtl->Insert.insertSlots[i].slot;
		SpinLockInit(&slot->mutex);
		slot->xlogInsertingAt = InvalidXLogRecPtr;
		slot->owner = NULL;

		slot->releaseOK = true;
		slot->exclusive = 0;
		slot->head = NULL;
		slot->tail = NULL;
	}

	SpinLockInit(&XLogCtl->Insert.insertpos_lck);
	SpinLockInit(&XLogCtl->info_lck);
	SpinLockInit(&XLogCtl->ulsn_lck);
	InitSharedLatch(&XLogCtl->recoveryWakeupLatch);

	/*
	 * If we are not in bootstrap mode, pg_control should already exist. Read
	 * and validate it immediately (see comments in ReadControlFile() for the
	 * reasons why).
	 */
	if (!IsBootstrapProcessingMode())
		ReadControlFile();
}

/*
 * This func must be called ONCE on system install.  It creates pg_control
 * and the initial XLOG segment.
 */
void
BootStrapXLOG(void)
{
	CheckPoint	checkPoint;
	char	   *buffer;
	XLogPageHeader page;
	XLogLongPageHeader longpage;
	XLogRecord *record;
	bool		use_existent;
	uint64		sysidentifier;
	struct timeval tv;
	pg_crc32	crc;

	/*
	 * Select a hopefully-unique system identifier code for this installation.
	 * We use the result of gettimeofday(), including the fractional seconds
	 * field, as being about as unique as we can easily get.  (Think not to
	 * use random(), since it hasn't been seeded and there's no portable way
	 * to seed it other than the system clock value...)  The upper half of the
	 * uint64 value is just the tv_sec part, while the lower half is the XOR
	 * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness
	 * unnecessarily if "uint64" is really only 32 bits wide.  A person
	 * knowing this encoding can determine the initialization time of the
	 * installation, which could perhaps be useful sometimes.
	 */
	gettimeofday(&tv, NULL);
	sysidentifier = ((uint64) tv.tv_sec) << 32;
	sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);

	/* First timeline ID is always 1 */
	ThisTimeLineID = 1;

	/* page buffer must be aligned suitably for O_DIRECT */
	buffer = (char *) palloc(XLOG_BLCKSZ + XLOG_BLCKSZ);
	page = (XLogPageHeader) TYPEALIGN(XLOG_BLCKSZ, buffer);
	memset(page, 0, XLOG_BLCKSZ);

	/*
	 * Set up information for the initial checkpoint record
	 *
	 * The initial checkpoint record is written to the beginning of the WAL
	 * segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not
	 * used, so that we can use 0/0 to mean "before any valid WAL segment".
	 */
	checkPoint.redo = XLogSegSize + SizeOfXLogLongPHD;
	checkPoint.ThisTimeLineID = ThisTimeLineID;
	checkPoint.PrevTimeLineID = ThisTimeLineID;
	checkPoint.fullPageWrites = fullPageWrites;
	checkPoint.nextXidEpoch = 0;
	checkPoint.nextXid = FirstNormalTransactionId;
	checkPoint.nextOid = FirstBootstrapObjectId;
	checkPoint.nextMulti = FirstMultiXactId;
	checkPoint.nextMultiOffset = 0;
	checkPoint.oldestXid = FirstNormalTransactionId;
	checkPoint.oldestXidDB = TemplateDbOid;
	checkPoint.oldestMulti = FirstMultiXactId;
	checkPoint.oldestMultiDB = TemplateDbOid;
	checkPoint.time = (pg_time_t) time(NULL);
	checkPoint.oldestActiveXid = InvalidTransactionId;

	ShmemVariableCache->nextXid = checkPoint.nextXid;
	ShmemVariableCache->nextOid = checkPoint.nextOid;
	ShmemVariableCache->oidCount = 0;
	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
	SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
	SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);

	/* Set up the XLOG page header */
	page->xlp_magic = XLOG_PAGE_MAGIC;
	page->xlp_info = XLP_LONG_HEADER;
	page->xlp_tli = ThisTimeLineID;
	page->xlp_pageaddr = XLogSegSize;
	longpage = (XLogLongPageHeader) page;
	longpage->xlp_sysid = sysidentifier;
	longpage->xlp_seg_size = XLogSegSize;
	longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

	/* Insert the initial checkpoint record */
	record = (XLogRecord *) ((char *) page + SizeOfXLogLongPHD);
	record->xl_prev = 0;
	record->xl_xid = InvalidTransactionId;
	record->xl_tot_len = SizeOfXLogRecord + sizeof(checkPoint);
	record->xl_len = sizeof(checkPoint);
	record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
	record->xl_rmid = RM_XLOG_ID;
	memcpy(XLogRecGetData(record), &checkPoint, sizeof(checkPoint));

	INIT_CRC32(crc);
	COMP_CRC32(crc, &checkPoint, sizeof(checkPoint));
	COMP_CRC32(crc, (char *) record, offsetof(XLogRecord, xl_crc));
	FIN_CRC32(crc);
	record->xl_crc = crc;

	/* Create first XLOG segment file */
	use_existent = false;
	openLogFile = XLogFileInit(1, &use_existent, false);

	/* Write the first page with the initial record */
	errno = 0;
	if (write(openLogFile, page, XLOG_BLCKSZ) != XLOG_BLCKSZ)
	{
		/* if write didn't set errno, assume problem is no disk space */
		if (errno == 0)
			errno = ENOSPC;
		ereport(PANIC,
				(errcode_for_file_access(),
			  errmsg("could not write bootstrap transaction log file: %m")));
	}

	if (pg_fsync(openLogFile) != 0)
		ereport(PANIC,
				(errcode_for_file_access(),
			  errmsg("could not fsync bootstrap transaction log file: %m")));

	if (close(openLogFile))
		ereport(PANIC,
				(errcode_for_file_access(),
			  errmsg("could not close bootstrap transaction log file: %m")));

	openLogFile = -1;

	/* Now create pg_control */

	memset(ControlFile, 0, sizeof(ControlFileData));
	/* Initialize pg_control status fields */
	ControlFile->system_identifier = sysidentifier;
	ControlFile->state = DB_SHUTDOWNED;
	ControlFile->time = checkPoint.time;
	ControlFile->checkPoint = checkPoint.redo;
	ControlFile->checkPointCopy = checkPoint;
	ControlFile->unloggedLSN = 1;

	/* Set important parameter values for use when replaying WAL */
	ControlFile->MaxConnections = MaxConnections;
	ControlFile->max_worker_processes = max_worker_processes;
	ControlFile->max_prepared_xacts = max_prepared_xacts;
	ControlFile->max_locks_per_xact = max_locks_per_xact;
	ControlFile->wal_level = wal_level;
	ControlFile->data_checksum_version = bootstrap_data_checksum_version;

	/* some additional ControlFile fields are set in WriteControlFile() */

	WriteControlFile();

	/* Bootstrap the commit log, too */
	BootStrapCLOG();
	BootStrapSUBTRANS();
	BootStrapMultiXact();

	pfree(buffer);
}

static char *
str_time(pg_time_t tnow)
{
	static char buf[128];

	pg_strftime(buf, sizeof(buf),
				"%Y-%m-%d %H:%M:%S %Z",
				pg_localtime(&tnow, log_timezone));

	return buf;
}

/*
 * See if there is a recovery command file (recovery.conf), and if so
 * read in parameters for archive recovery and XLOG streaming.
 *
 * The file is parsed using the main configuration parser.
 */
static void
readRecoveryCommandFile(void)
{
	FILE	   *fd;
	TimeLineID	rtli = 0;
	bool		rtliGiven = false;
	ConfigVariable *item,
			   *head = NULL,
			   *tail = NULL;

	fd = AllocateFile(RECOVERY_COMMAND_FILE, "r");
	if (fd == NULL)
	{
		if (errno == ENOENT)
			return;				/* not there, so no archive recovery */
		ereport(FATAL,
				(errcode_for_file_access(),
				 errmsg("could not open recovery command file \"%s\": %m",
						RECOVERY_COMMAND_FILE)));
	}

	/*
	 * Since we're asking ParseConfigFp() to report errors as FATAL, there's
	 * no need to check the return value.
	 */
	(void) ParseConfigFp(fd, RECOVERY_COMMAND_FILE, 0, FATAL, &head, &tail);

	FreeFile(fd);

	for (item = head; item; item = item->next)
	{
		if (strcmp(item->name, "restore_command") == 0)
		{
			recoveryRestoreCommand = pstrdup(item->value);
			ereport(DEBUG2,
					(errmsg_internal("restore_command = '%s'",
									 recoveryRestoreCommand)));
		}
		else if (strcmp(item->name, "recovery_end_command") == 0)
		{
			recoveryEndCommand = pstrdup(item->value);
			ereport(DEBUG2,
					(errmsg_internal("recovery_end_command = '%s'",
									 recoveryEndCommand)));
		}
		else if (strcmp(item->name, "archive_cleanup_command") == 0)
		{
			archiveCleanupCommand = pstrdup(item->value);
			ereport(DEBUG2,
					(errmsg_internal("archive_cleanup_command = '%s'",
									 archiveCleanupCommand)));
		}
		else if (strcmp(item->name, "pause_at_recovery_target") == 0)
		{
			if (!parse_bool(item->value, &recoveryPauseAtTarget))
				ereport(ERROR,
						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
						 errmsg("parameter \"%s\" requires a Boolean value", "pause_at_recovery_target")));
			ereport(DEBUG2,
					(errmsg_internal("pause_at_recovery_target = '%s'",
									 item->value)));
		}
		else if (strcmp(item->name, "recovery_target_timeline") == 0)
		{
			rtliGiven = true;
			if (strcmp(item->value, "latest") == 0)
				rtli = 0;
			else
			{
				errno = 0;
				rtli = (TimeLineID) strtoul(item->value, NULL, 0);
				if (errno == EINVAL || errno == ERANGE)
					ereport(FATAL,
							(errmsg("recovery_target_timeline is not a valid number: \"%s\"",
									item->value)));
			}
			if (rtli)
				ereport(DEBUG2,
				   (errmsg_internal("recovery_target_timeline = %u", rtli)));
			else
				ereport(DEBUG2,
					 (errmsg_internal("recovery_target_timeline = latest")));
		}
		else if (strcmp(item->name, "recovery_target_xid") == 0)
		{
			errno = 0;
			recoveryTargetXid = (TransactionId) strtoul(item->value, NULL, 0);
			if (errno == EINVAL || errno == ERANGE)
				ereport(FATAL,
				 (errmsg("recovery_target_xid is not a valid number: \"%s\"",
						 item->value)));
			ereport(DEBUG2,
					(errmsg_internal("recovery_target_xid = %u",
									 recoveryTargetXid)));
			recoveryTarget = RECOVERY_TARGET_XID;
		}
		else if (strcmp(item->name, "recovery_target_time") == 0)
		{
			/*
			 * if recovery_target_xid or recovery_target_name specified, then
			 * this overrides recovery_target_time
			 */
			if (recoveryTarget == RECOVERY_TARGET_XID ||
				recoveryTarget == RECOVERY_TARGET_NAME)
				continue;
			recoveryTarget = RECOVERY_TARGET_TIME;

			/*
			 * Convert the time string given by the user to TimestampTz form.
			 */
			recoveryTargetTime =
				DatumGetTimestampTz(DirectFunctionCall3(timestamptz_in,
												CStringGetDatum(item->value),
												ObjectIdGetDatum(InvalidOid),
														Int32GetDatum(-1)));
			ereport(DEBUG2,
					(errmsg_internal("recovery_target_time = '%s'",
								   timestamptz_to_str(recoveryTargetTime))));
		}
		else if (strcmp(item->name, "recovery_target_name") == 0)
		{
			/*
			 * if recovery_target_xid specified, then this overrides
			 * recovery_target_name
			 */
			if (recoveryTarget == RECOVERY_TARGET_XID)
				continue;
			recoveryTarget = RECOVERY_TARGET_NAME;

			recoveryTargetName = pstrdup(item->value);
			if (strlen(recoveryTargetName) >= MAXFNAMELEN)
				ereport(FATAL,
						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
						 errmsg("recovery_target_name is too long (maximum %d characters)",
								MAXFNAMELEN - 1)));

			ereport(DEBUG2,
					(errmsg_internal("recovery_target_name = '%s'",
									 recoveryTargetName)));
		}
		else if (strcmp(item->name, "recovery_target_inclusive") == 0)
		{
			/*
			 * does nothing if a recovery_target is not also set
			 */
			if (!parse_bool(item->value, &recoveryTargetInclusive))
				ereport(ERROR,
						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
						 errmsg("parameter \"%s\" requires a Boolean value",
								"recovery_target_inclusive")));
			ereport(DEBUG2,
					(errmsg_internal("recovery_target_inclusive = %s",
									 item->value)));
		}
		else if (strcmp(item->name, "standby_mode") == 0)
		{
			if (!parse_bool(item->value, &StandbyModeRequested))
				ereport(ERROR,
						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
						 errmsg("parameter \"%s\" requires a Boolean value",
								"standby_mode")));
			ereport(DEBUG2,
					(errmsg_internal("standby_mode = '%s'", item->value)));
		}
		else if (strcmp(item->name, "primary_conninfo") == 0)
		{
			PrimaryConnInfo = pstrdup(item->value);
			ereport(DEBUG2,
					(errmsg_internal("primary_conninfo = '%s'",
									 PrimaryConnInfo)));
		}
		else if (strcmp(item->name, "trigger_file") == 0)
		{
			TriggerFile = pstrdup(item->value);
			ereport(DEBUG2,
					(errmsg_internal("trigger_file = '%s'",
									 TriggerFile)));
		}
		else
			ereport(FATAL,
					(errmsg("unrecognized recovery parameter \"%s\"",
							item->name)));
	}

	/*
	 * Check for compulsory parameters
	 */
	if (StandbyModeRequested)
	{
		if (PrimaryConnInfo == NULL && recoveryRestoreCommand == NULL)
			ereport(WARNING,
					(errmsg("recovery command file \"%s\" specified neither primary_conninfo nor restore_command",
							RECOVERY_COMMAND_FILE),
					 errhint("The database server will regularly poll the pg_xlog subdirectory to check for files placed there.")));
	}
	else
	{
		if (recoveryRestoreCommand == NULL)
			ereport(FATAL,
					(errmsg("recovery command file \"%s\" must specify restore_command when standby mode is not enabled",
							RECOVERY_COMMAND_FILE)));
	}

	/* Enable fetching from archive recovery area */
	ArchiveRecoveryRequested = true;

	/*
	 * If user specified recovery_target_timeline, validate it or compute the
	 * "latest" value.	We can't do this until after we've gotten the restore
	 * command and set InArchiveRecovery, because we need to fetch timeline
	 * history files from the archive.
	 */
	if (rtliGiven)
	{
		if (rtli)
		{
			/* Timeline 1 does not have a history file, all else should */
			if (rtli != 1 && !existsTimeLineHistory(rtli))
				ereport(FATAL,
						(errmsg("recovery target timeline %u does not exist",
								rtli)));
			recoveryTargetTLI = rtli;
			recoveryTargetIsLatest = false;
		}
		else
		{
			/* We start the "latest" search from pg_control's timeline */
			recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);
			recoveryTargetIsLatest = true;
		}
	}

	FreeConfigVariables(head);
}

/*
 * Exit archive-recovery state
 */
static void
exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo)
{
	char		recoveryPath[MAXPGPATH];
	char		xlogpath[MAXPGPATH];

	/*
	 * We are no longer in archive recovery state.
	 */
	InArchiveRecovery = false;

	/*
	 * Update min recovery point one last time.
	 */
	UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);

	/*
	 * If the ending log segment is still open, close it (to avoid problems on
	 * Windows with trying to rename or delete an open file).
	 */
	if (readFile >= 0)
	{
		close(readFile);
		readFile = -1;
	}

	/*
	 * If we are establishing a new timeline, we have to copy data from the
	 * last WAL segment of the old timeline to create a starting WAL segment
	 * for the new timeline.
	 *
	 * Notify the archiver that the last WAL segment of the old timeline is
	 * ready to copy to archival storage. Otherwise, it is not archived for a
	 * while.
	 */
	if (endTLI != ThisTimeLineID)
	{
		XLogFileCopy(endLogSegNo, endTLI, endLogSegNo);

		if (XLogArchivingActive())
		{
			XLogFileName(xlogpath, endTLI, endLogSegNo);
			XLogArchiveNotify(xlogpath);
		}
	}

	/*
	 * Let's just make real sure there are not .ready or .done flags posted
	 * for the new segment.
	 */
	XLogFileName(xlogpath, ThisTimeLineID, endLogSegNo);
	XLogArchiveCleanup(xlogpath);

	/*
	 * Since there might be a partial WAL segment named RECOVERYXLOG, get rid
	 * of it.
	 */
	snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYXLOG");
	unlink(recoveryPath);		/* ignore any error */

	/* Get rid of any remaining recovered timeline-history file, too */
	snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYHISTORY");
	unlink(recoveryPath);		/* ignore any error */

	/*
	 * Rename the config file out of the way, so that we don't accidentally
	 * re-enter archive recovery mode in a subsequent crash.
	 */
	unlink(RECOVERY_COMMAND_DONE);
	if (rename(RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE) != 0)
		ereport(FATAL,
				(errcode_for_file_access(),
				 errmsg("could not rename file \"%s\" to \"%s\": %m",
						RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE)));

	ereport(LOG,
			(errmsg("archive recovery complete")));
}

/*
 * For point-in-time recovery, this function decides whether we want to
 * stop applying the XLOG at or after the current record.
 *
 * Returns TRUE if we are stopping, FALSE otherwise.  On TRUE return,
 * *includeThis is set TRUE if we should apply this record before stopping.
 *
 * We also track the timestamp of the latest applied COMMIT/ABORT
 * record in XLogCtl->recoveryLastXTime, for logging purposes.
 * Also, some information is saved in recoveryStopXid et al for use in
 * annotating the new timeline's history file.
 */
static bool
recoveryStopsHere(XLogRecord *record, bool *includeThis)
{
	bool		stopsHere;
	uint8		record_info;
	TimestampTz recordXtime;
	char		recordRPName[MAXFNAMELEN];

	/* We only consider stopping at COMMIT, ABORT or RESTORE POINT records */
	if (record->xl_rmid != RM_XACT_ID && record->xl_rmid != RM_XLOG_ID)
		return false;
	record_info = record->xl_info & ~XLR_INFO_MASK;
	if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_COMMIT_COMPACT)
	{
		xl_xact_commit_compact *recordXactCommitData;

		recordXactCommitData = (xl_xact_commit_compact *) XLogRecGetData(record);
		recordXtime = recordXactCommitData->xact_time;
	}
	else if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_COMMIT)
	{
		xl_xact_commit *recordXactCommitData;

		recordXactCommitData = (xl_xact_commit *) XLogRecGetData(record);
		recordXtime = recordXactCommitData->xact_time;
	}
	else if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_ABORT)
	{
		xl_xact_abort *recordXactAbortData;

		recordXactAbortData = (xl_xact_abort *) XLogRecGetData(record);
		recordXtime = recordXactAbortData->xact_time;
	}
	else if (record->xl_rmid == RM_XLOG_ID && record_info == XLOG_RESTORE_POINT)
	{
		xl_restore_point *recordRestorePointData;

		recordRestorePointData = (xl_restore_point *) XLogRecGetData(record);
		recordXtime = recordRestorePointData->rp_time;
		strncpy(recordRPName, recordRestorePointData->rp_name, MAXFNAMELEN);
	}
	else
		return false;

	/* Do we have a PITR target at all? */
	if (recoveryTarget == RECOVERY_TARGET_UNSET)
	{
		/*
		 * Save timestamp of latest transaction commit/abort if this is a
		 * transaction record
		 */
		if (record->xl_rmid == RM_XACT_ID)
			SetLatestXTime(recordXtime);
		return false;
	}

	if (recoveryTarget == RECOVERY_TARGET_XID)
	{
		/*
		 * There can be only one transaction end record with this exact
		 * transactionid
		 *
		 * when testing for an xid, we MUST test for equality only, since
		 * transactions are numbered in the order they start, not the order
		 * they complete. A higher numbered xid will complete before you about
		 * 50% of the time...
		 */
		stopsHere = (record->xl_xid == recoveryTargetXid);
		if (stopsHere)
			*includeThis = recoveryTargetInclusive;
	}
	else if (recoveryTarget == RECOVERY_TARGET_NAME)
	{
		/*
		 * There can be many restore points that share the same name, so we
		 * stop at the first one
		 */
		stopsHere = (strcmp(recordRPName, recoveryTargetName) == 0);

		/*
		 * Ignore recoveryTargetInclusive because this is not a transaction
		 * record
		 */
		*includeThis = false;
	}
	else
	{
		/*
		 * There can be many transactions that share the same commit time, so
		 * we stop after the last one, if we are inclusive, or stop at the
		 * first one if we are exclusive
		 */
		if (recoveryTargetInclusive)
			stopsHere = (recordXtime > recoveryTargetTime);
		else
			stopsHere = (recordXtime >= recoveryTargetTime);
		if (stopsHere)
			*includeThis = false;
	}

	if (stopsHere)
	{
		recoveryStopXid = record->xl_xid;
		recoveryStopTime = recordXtime;
		recoveryStopAfter = *includeThis;

		if (record_info == XLOG_XACT_COMMIT_COMPACT || record_info == XLOG_XACT_COMMIT)
		{
			if (recoveryStopAfter)
				ereport(LOG,
						(errmsg("recovery stopping after commit of transaction %u, time %s",
								recoveryStopXid,
								timestamptz_to_str(recoveryStopTime))));
			else
				ereport(LOG,
						(errmsg("recovery stopping before commit of transaction %u, time %s",
								recoveryStopXid,
								timestamptz_to_str(recoveryStopTime))));
		}
		else if (record_info == XLOG_XACT_ABORT)
		{
			if (recoveryStopAfter)
				ereport(LOG,
						(errmsg("recovery stopping after abort of transaction %u, time %s",
								recoveryStopXid,
								timestamptz_to_str(recoveryStopTime))));
			else
				ereport(LOG,
						(errmsg("recovery stopping before abort of transaction %u, time %s",
								recoveryStopXid,
								timestamptz_to_str(recoveryStopTime))));
		}
		else
		{
			strncpy(recoveryStopName, recordRPName, MAXFNAMELEN);

			ereport(LOG,
				(errmsg("recovery stopping at restore point \"%s\", time %s",
						recoveryStopName,
						timestamptz_to_str(recoveryStopTime))));
		}

		/*
		 * Note that if we use a RECOVERY_TARGET_TIME then we can stop at a
		 * restore point since they are timestamped, though the latest
		 * transaction time is not updated.
		 */
		if (record->xl_rmid == RM_XACT_ID && recoveryStopAfter)
			SetLatestXTime(recordXtime);
	}
	else if (record->xl_rmid == RM_XACT_ID)
		SetLatestXTime(recordXtime);

	return stopsHere;
}

/*
 * Wait until shared recoveryPause flag is cleared.
 *
 * XXX Could also be done with shared latch, avoiding the pg_usleep loop.
 * Probably not worth the trouble though.  This state shouldn't be one that
 * anyone cares about server power consumption in.
 */
static void
recoveryPausesHere(void)
{
	/* Don't pause unless users can connect! */
	if (!LocalHotStandbyActive)
		return;

	ereport(LOG,
			(errmsg("recovery has paused"),
			 errhint("Execute pg_xlog_replay_resume() to continue.")));

	while (RecoveryIsPaused())
	{
		pg_usleep(1000000L);	/* 1000 ms */
		HandleStartupProcInterrupts();
	}
}

bool
RecoveryIsPaused(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	bool		recoveryPause;

	SpinLockAcquire(&xlogctl->info_lck);
	recoveryPause = xlogctl->recoveryPause;
	SpinLockRelease(&xlogctl->info_lck);

	return recoveryPause;
}

void
SetRecoveryPause(bool recoveryPause)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->recoveryPause = recoveryPause;
	SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Save timestamp of latest processed commit/abort record.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by processes other than the startup process.  Note in particular
 * that CreateRestartPoint is executed in the checkpointer.
 */
static void
SetLatestXTime(TimestampTz xtime)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->recoveryLastXTime = xtime;
	SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 */
TimestampTz
GetLatestXTime(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	TimestampTz xtime;

	SpinLockAcquire(&xlogctl->info_lck);
	xtime = xlogctl->recoveryLastXTime;
	SpinLockRelease(&xlogctl->info_lck);

	return xtime;
}

/*
 * Save timestamp of the next chunk of WAL records to apply.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by all backends.
 */
static void
SetCurrentChunkStartTime(TimestampTz xtime)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->currentChunkStartTime = xtime;
	SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 * Startup process maintains an accurate local copy in XLogReceiptTime
 */
TimestampTz
GetCurrentChunkReplayStartTime(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	TimestampTz xtime;

	SpinLockAcquire(&xlogctl->info_lck);
	xtime = xlogctl->currentChunkStartTime;
	SpinLockRelease(&xlogctl->info_lck);

	return xtime;
}

/*
 * Returns time of receipt of current chunk of XLOG data, as well as
 * whether it was received from streaming replication or from archives.
 */
void
GetXLogReceiptTime(TimestampTz *rtime, bool *fromStream)
{
	/*
	 * This must be executed in the startup process, since we don't export the
	 * relevant state to shared memory.
	 */
	Assert(InRecovery);

	*rtime = XLogReceiptTime;
	*fromStream = (XLogReceiptSource == XLOG_FROM_STREAM);
}

/*
 * Note that text field supplied is a parameter name and does not require
 * translation
 */
#define RecoveryRequiresIntParameter(param_name, currValue, minValue) \
do { \
	if ((currValue) < (minValue)) \
		ereport(ERROR, \
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE), \
				 errmsg("hot standby is not possible because " \
						"%s = %d is a lower setting than on the master server " \
						"(its value was %d)", \
						param_name, \
						currValue, \
						minValue))); \
} while(0)

/*
 * Check to see if required parameters are set high enough on this server
 * for various aspects of recovery operation.
 */
static void
CheckRequiredParameterValues(void)
{
	/*
	 * For archive recovery, the WAL must be generated with at least 'archive'
	 * wal_level.
	 */
	if (InArchiveRecovery && ControlFile->wal_level == WAL_LEVEL_MINIMAL)
	{
		ereport(WARNING,
				(errmsg("WAL was generated with wal_level=minimal, data may be missing"),
				 errhint("This happens if you temporarily set wal_level=minimal without taking a new base backup.")));
	}

	/*
	 * For Hot Standby, the WAL must be generated with 'hot_standby' mode, and
	 * we must have at least as many backend slots as the primary.
	 */
	if (InArchiveRecovery && EnableHotStandby)
	{
		if (ControlFile->wal_level < WAL_LEVEL_HOT_STANDBY)
			ereport(ERROR,
					(errmsg("hot standby is not possible because wal_level was not set to \"hot_standby\" on the master server"),
					 errhint("Either set wal_level to \"hot_standby\" on the master, or turn off hot_standby here.")));

		/* We ignore autovacuum_max_workers when we make this test. */
		RecoveryRequiresIntParameter("max_connections",
									 MaxConnections,
									 ControlFile->MaxConnections);
		RecoveryRequiresIntParameter("max_worker_processes",
									 max_worker_processes,
									 ControlFile->max_worker_processes);
		RecoveryRequiresIntParameter("max_prepared_transactions",
									 max_prepared_xacts,
									 ControlFile->max_prepared_xacts);
		RecoveryRequiresIntParameter("max_locks_per_transaction",
									 max_locks_per_xact,
									 ControlFile->max_locks_per_xact);
	}
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
	XLogCtlInsert *Insert;
	CheckPoint	checkPoint;
	bool		wasShutdown;
	bool		reachedStopPoint = false;
	bool		haveBackupLabel = false;
	XLogRecPtr	RecPtr,
				checkPointLoc,
				EndOfLog;
	XLogSegNo	endLogSegNo;
	TimeLineID	PrevTimeLineID;
	XLogRecord *record;
	TransactionId oldestActiveXID;
	bool		backupEndRequired = false;
	bool		backupFromStandby = false;
	DBState		dbstate_at_startup;
	XLogReaderState *xlogreader;
	XLogPageReadPrivate private;
	bool		fast_promoted = false;

	/*
	 * Read control file and check XLOG status looks valid.
	 *
	 * Note: in most control paths, *ControlFile is already valid and we need
	 * not do ReadControlFile() here, but might as well do it to be sure.
	 */
	ReadControlFile();

	if (ControlFile->state < DB_SHUTDOWNED ||
		ControlFile->state > DB_IN_PRODUCTION ||
		!XRecOffIsValid(ControlFile->checkPoint))
		ereport(FATAL,
				(errmsg("control file contains invalid data")));

	if (ControlFile->state == DB_SHUTDOWNED)
	{
		/* This is the expected case, so don't be chatty in standalone mode */
		ereport(IsPostmasterEnvironment ? LOG : NOTICE,
				(errmsg("database system was shut down at %s",
						str_time(ControlFile->time))));
	}
	else if (ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)
		ereport(LOG,
				(errmsg("database system was shut down in recovery at %s",
						str_time(ControlFile->time))));
	else if (ControlFile->state == DB_SHUTDOWNING)
		ereport(LOG,
				(errmsg("database system shutdown was interrupted; last known up at %s",
						str_time(ControlFile->time))));
	else if (ControlFile->state == DB_IN_CRASH_RECOVERY)
		ereport(LOG,
		   (errmsg("database system was interrupted while in recovery at %s",
				   str_time(ControlFile->time)),
			errhint("This probably means that some data is corrupted and"
					" you will have to use the last backup for recovery.")));
	else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)
		ereport(LOG,
				(errmsg("database system was interrupted while in recovery at log time %s",
						str_time(ControlFile->checkPointCopy.time)),
				 errhint("If this has occurred more than once some data might be corrupted"
			  " and you might need to choose an earlier recovery target.")));
	else if (ControlFile->state == DB_IN_PRODUCTION)
		ereport(LOG,
			  (errmsg("database system was interrupted; last known up at %s",
					  str_time(ControlFile->time))));

	/* This is just to allow attaching to startup process with a debugger */
#ifdef XLOG_REPLAY_DELAY
	if (ControlFile->state != DB_SHUTDOWNED)
		pg_usleep(60000000L);
#endif

	/*
	 * Verify that pg_xlog and pg_xlog/archive_status exist.  In cases where
	 * someone has performed a copy for PITR, these directories may have been
	 * excluded and need to be re-created.
	 */
	ValidateXLOGDirectoryStructure();

	/*
	 * Clear out any old relcache cache files.	This is *necessary* if we do
	 * any WAL replay, since that would probably result in the cache files
	 * being out of sync with database reality.  In theory we could leave them
	 * in place if the database had been cleanly shut down, but it seems
	 * safest to just remove them always and let them be rebuilt during the
	 * first backend startup.
	 */
	RelationCacheInitFileRemove();

	/*
	 * Initialize on the assumption we want to recover to the latest timeline
	 * that's active according to pg_control.
	 */
	if (ControlFile->minRecoveryPointTLI >
		ControlFile->checkPointCopy.ThisTimeLineID)
		recoveryTargetTLI = ControlFile->minRecoveryPointTLI;
	else
		recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;

	/*
	 * Check for recovery control file, and if so set up state for offline
	 * recovery
	 */
	readRecoveryCommandFile();

	/*
	 * Save archive_cleanup_command in shared memory so that other processes
	 * can see it.
	 */
	strncpy(XLogCtl->archiveCleanupCommand,
			archiveCleanupCommand ? archiveCleanupCommand : "",
			sizeof(XLogCtl->archiveCleanupCommand));

	if (ArchiveRecoveryRequested)
	{
		if (StandbyModeRequested)
			ereport(LOG,
					(errmsg("entering standby mode")));
		else if (recoveryTarget == RECOVERY_TARGET_XID)
			ereport(LOG,
					(errmsg("starting point-in-time recovery to XID %u",
							recoveryTargetXid)));
		else if (recoveryTarget == RECOVERY_TARGET_TIME)
			ereport(LOG,
					(errmsg("starting point-in-time recovery to %s",
							timestamptz_to_str(recoveryTargetTime))));
		else if (recoveryTarget == RECOVERY_TARGET_NAME)
			ereport(LOG,
					(errmsg("starting point-in-time recovery to \"%s\"",
							recoveryTargetName)));
		else
			ereport(LOG,
					(errmsg("starting archive recovery")));
	}

	/*
	 * Take ownership of the wakeup latch if we're going to sleep during
	 * recovery.
	 */
	if (StandbyModeRequested)
		OwnLatch(&XLogCtl->recoveryWakeupLatch);

	/* Set up XLOG reader facility */
	MemSet(&private, 0, sizeof(XLogPageReadPrivate));
	xlogreader = XLogReaderAllocate(&XLogPageRead, &private);
	if (!xlogreader)
		ereport(ERROR,
				(errcode(ERRCODE_OUT_OF_MEMORY),
				 errmsg("out of memory"),
			errdetail("Failed while allocating an XLog reading processor")));
	xlogreader->system_identifier = ControlFile->system_identifier;

	if (read_backup_label(&checkPointLoc, &backupEndRequired,
						  &backupFromStandby))
	{
		/*
		 * Archive recovery was requested, and thanks to the backup label
		 * file, we know how far we need to replay to reach consistency. Enter
		 * archive recovery directly.
		 */
		InArchiveRecovery = true;
		if (StandbyModeRequested)
			StandbyMode = true;

		/*
		 * When a backup_label file is present, we want to roll forward from
		 * the checkpoint it identifies, rather than using pg_control.
		 */
		record = ReadCheckpointRecord(xlogreader, checkPointLoc, 0, true);
		if (record != NULL)
		{
			memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
			wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
			ereport(DEBUG1,
					(errmsg("checkpoint record is at %X/%X",
				   (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
			InRecovery = true;	/* force recovery even if SHUTDOWNED */

			/*
			 * Make sure that REDO location exists. This may not be the case
			 * if there was a crash during an online backup, which left a
			 * backup_label around that references a WAL segment that's
			 * already been archived.
			 */
			if (checkPoint.redo < checkPointLoc)
			{
				if (!ReadRecord(xlogreader, checkPoint.redo, LOG, false))
					ereport(FATAL,
							(errmsg("could not find redo location referenced by checkpoint record"),
							 errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
			}
		}
		else
		{
			ereport(FATAL,
					(errmsg("could not locate required checkpoint record"),
					 errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
			wasShutdown = false;	/* keep compiler quiet */
		}
		/* set flag to delete it later */
		haveBackupLabel = true;
	}
	else
	{
		/*
		 * It's possible that archive recovery was requested, but we don't
		 * know how far we need to replay the WAL before we reach consistency.
		 * This can happen for example if a base backup is taken from a
		 * running server using an atomic filesystem snapshot, without calling
		 * pg_start/stop_backup. Or if you just kill a running master server
		 * and put it into archive recovery by creating a recovery.conf file.
		 *
		 * Our strategy in that case is to perform crash recovery first,
		 * replaying all the WAL present in pg_xlog, and only enter archive
		 * recovery after that.
		 *
		 * But usually we already know how far we need to replay the WAL (up
		 * to minRecoveryPoint, up to backupEndPoint, or until we see an
		 * end-of-backup record), and we can enter archive recovery directly.
		 */
		if (ArchiveRecoveryRequested &&
			(ControlFile->minRecoveryPoint != InvalidXLogRecPtr ||
			 ControlFile->backupEndRequired ||
			 ControlFile->backupEndPoint != InvalidXLogRecPtr ||
			 ControlFile->state == DB_SHUTDOWNED))
		{
			InArchiveRecovery = true;
			if (StandbyModeRequested)
				StandbyMode = true;
		}

		/*
		 * Get the last valid checkpoint record.  If the latest one according
		 * to pg_control is broken, try the next-to-last one.
		 */
		checkPointLoc = ControlFile->checkPoint;
		RedoStartLSN = ControlFile->checkPointCopy.redo;
		record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, true);
		if (record != NULL)
		{
			ereport(DEBUG1,
					(errmsg("checkpoint record is at %X/%X",
				   (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
		}
		else if (StandbyMode)
		{
			/*
			 * The last valid checkpoint record required for a streaming
			 * recovery exists in neither standby nor the primary.
			 */
			ereport(PANIC,
					(errmsg("could not locate a valid checkpoint record")));
		}
		else
		{
			checkPointLoc = ControlFile->prevCheckPoint;
			record = ReadCheckpointRecord(xlogreader, checkPointLoc, 2, true);
			if (record != NULL)
			{
				ereport(LOG,
						(errmsg("using previous checkpoint record at %X/%X",
				   (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
				InRecovery = true;		/* force recovery even if SHUTDOWNED */
			}
			else
				ereport(PANIC,
					 (errmsg("could not locate a valid checkpoint record")));
		}
		memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
		wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
	}

	/*
	 * If the location of the checkpoint record is not on the expected
	 * timeline in the history of the requested timeline, we cannot proceed:
	 * the backup is not part of the history of the requested timeline.
	 */
	Assert(expectedTLEs);		/* was initialized by reading checkpoint
								 * record */
	if (tliOfPointInHistory(checkPointLoc, expectedTLEs) !=
		checkPoint.ThisTimeLineID)
	{
		XLogRecPtr	switchpoint;

		/*
		 * tliSwitchPoint will throw an error if the checkpoint's timeline is
		 * not in expectedTLEs at all.
		 */
		switchpoint = tliSwitchPoint(ControlFile->checkPointCopy.ThisTimeLineID, expectedTLEs, NULL);
		ereport(FATAL,
				(errmsg("requested timeline %u is not a child of this server's history",
						recoveryTargetTLI),
				 errdetail("Latest checkpoint is at %X/%X on timeline %u, but in the history of the requested timeline, the server forked off from that timeline at %X/%X",
						   (uint32) (ControlFile->checkPoint >> 32),
						   (uint32) ControlFile->checkPoint,
						   ControlFile->checkPointCopy.ThisTimeLineID,
						   (uint32) (switchpoint >> 32),
						   (uint32) switchpoint)));
	}

	/*
	 * The min recovery point should be part of the requested timeline's
	 * history, too.
	 */
	if (!XLogRecPtrIsInvalid(ControlFile->minRecoveryPoint) &&
	  tliOfPointInHistory(ControlFile->minRecoveryPoint - 1, expectedTLEs) !=
		ControlFile->minRecoveryPointTLI)
		ereport(FATAL,
				(errmsg("requested timeline %u does not contain minimum recovery point %X/%X on timeline %u",
						recoveryTargetTLI,
						(uint32) (ControlFile->minRecoveryPoint >> 32),
						(uint32) ControlFile->minRecoveryPoint,
						ControlFile->minRecoveryPointTLI)));

	LastRec = RecPtr = checkPointLoc;

	ereport(DEBUG1,
			(errmsg("redo record is at %X/%X; shutdown %s",
				  (uint32) (checkPoint.redo >> 32), (uint32) checkPoint.redo,
					wasShutdown ? "TRUE" : "FALSE")));
	ereport(DEBUG1,
			(errmsg("next transaction ID: %u/%u; next OID: %u",
					checkPoint.nextXidEpoch, checkPoint.nextXid,
					checkPoint.nextOid)));
	ereport(DEBUG1,
			(errmsg("next MultiXactId: %u; next MultiXactOffset: %u",
					checkPoint.nextMulti, checkPoint.nextMultiOffset)));
	ereport(DEBUG1,
			(errmsg("oldest unfrozen transaction ID: %u, in database %u",
					checkPoint.oldestXid, checkPoint.oldestXidDB)));
	ereport(DEBUG1,
			(errmsg("oldest MultiXactId: %u, in database %u",
					checkPoint.oldestMulti, checkPoint.oldestMultiDB)));
	if (!TransactionIdIsNormal(checkPoint.nextXid))
		ereport(PANIC,
				(errmsg("invalid next transaction ID")));

	/* initialize shared memory variables from the checkpoint record */
	ShmemVariableCache->nextXid = checkPoint.nextXid;
	ShmemVariableCache->nextOid = checkPoint.nextOid;
	ShmemVariableCache->oidCount = 0;
	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
	SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
	SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);
	XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
	XLogCtl->ckptXid = checkPoint.nextXid;

	/*
	 * Initialize unlogged LSN. On a clean shutdown, it's restored from the
	 * control file. On recovery, all unlogged relations are blown away, so
	 * the unlogged LSN counter can be reset too.
	 */
	if (ControlFile->state == DB_SHUTDOWNED)
		XLogCtl->unloggedLSN = ControlFile->unloggedLSN;
	else
		XLogCtl->unloggedLSN = 1;

	/*
	 * We must replay WAL entries using the same TimeLineID they were created
	 * under, so temporarily adopt the TLI indicated by the checkpoint (see
	 * also xlog_redo()).
	 */
	ThisTimeLineID = checkPoint.ThisTimeLineID;

	/*
	 * Copy any missing timeline history files between 'now' and the recovery
	 * target timeline from archive to pg_xlog. While we don't need those
	 * files ourselves - the history file of the recovery target timeline
	 * covers all the previous timelines in the history too - a cascading
	 * standby server might be interested in them. Or, if you archive the WAL
	 * from this server to a different archive than the master, it'd be good
	 * for all the history files to get archived there after failover, so that
	 * you can use one of the old timelines as a PITR target. Timeline history
	 * files are small, so it's better to copy them unnecessarily than not
	 * copy them and regret later.
	 */
	restoreTimeLineHistoryFiles(ThisTimeLineID, recoveryTargetTLI);

	lastFullPageWrites = checkPoint.fullPageWrites;

	RedoRecPtr = XLogCtl->RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

	if (RecPtr < checkPoint.redo)
		ereport(PANIC,
				(errmsg("invalid redo in checkpoint record")));

	/*
	 * Check whether we need to force recovery from WAL.  If it appears to
	 * have been a clean shutdown and we did not have a recovery.conf file,
	 * then assume no recovery needed.
	 */
	if (checkPoint.redo < RecPtr)
	{
		if (wasShutdown)
			ereport(PANIC,
					(errmsg("invalid redo record in shutdown checkpoint")));
		InRecovery = true;
	}
	else if (ControlFile->state != DB_SHUTDOWNED)
		InRecovery = true;
	else if (ArchiveRecoveryRequested)
	{
		/* force recovery due to presence of recovery.conf */
		InRecovery = true;
	}

	/* REDO */
	if (InRecovery)
	{
		int			rmid;

		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		/*
		 * Update pg_control to show that we are recovering and to show the
		 * selected checkpoint as the place we are starting from. We also mark
		 * pg_control with any minimum recovery stop point obtained from a
		 * backup history file.
		 */
		dbstate_at_startup = ControlFile->state;
		if (InArchiveRecovery)
			ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
		else
		{
			ereport(LOG,
					(errmsg("database system was not properly shut down; "
							"automatic recovery in progress")));
			if (recoveryTargetTLI > ControlFile->checkPointCopy.ThisTimeLineID)
				ereport(LOG,
						(errmsg("crash recovery starts in timeline %u "
								"and has target timeline %u",
								ControlFile->checkPointCopy.ThisTimeLineID,
								recoveryTargetTLI)));
			ControlFile->state = DB_IN_CRASH_RECOVERY;
		}
		ControlFile->prevCheckPoint = ControlFile->checkPoint;
		ControlFile->checkPoint = checkPointLoc;
		ControlFile->checkPointCopy = checkPoint;
		if (InArchiveRecovery)
		{
			/* initialize minRecoveryPoint if not set yet */
			if (ControlFile->minRecoveryPoint < checkPoint.redo)
			{
				ControlFile->minRecoveryPoint = checkPoint.redo;
				ControlFile->minRecoveryPointTLI = checkPoint.ThisTimeLineID;
			}
		}

		/*
		 * Set backupStartPoint if we're starting recovery from a base backup.
		 *
		 * Set backupEndPoint and use minRecoveryPoint as the backup end
		 * location if we're starting recovery from a base backup which was
		 * taken from the standby. In this case, the database system status in
		 * pg_control must indicate DB_IN_ARCHIVE_RECOVERY. If not, which
		 * means that backup is corrupted, so we cancel recovery.
		 */
		if (haveBackupLabel)
		{
			ControlFile->backupStartPoint = checkPoint.redo;
			ControlFile->backupEndRequired = backupEndRequired;

			if (backupFromStandby)
			{
				if (dbstate_at_startup != DB_IN_ARCHIVE_RECOVERY)
					ereport(FATAL,
							(errmsg("backup_label contains data inconsistent with control file"),
							 errhint("This means that the backup is corrupted and you will "
							   "have to use another backup for recovery.")));
				ControlFile->backupEndPoint = ControlFile->minRecoveryPoint;
			}
		}
		ControlFile->time = (pg_time_t) time(NULL);
		/* No need to hold ControlFileLock yet, we aren't up far enough */
		UpdateControlFile();

		/* initialize our local copy of minRecoveryPoint */
		minRecoveryPoint = ControlFile->minRecoveryPoint;
		minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

		/*
		 * Reset pgstat data, because it may be invalid after recovery.
		 */
		pgstat_reset_all();

		/*
		 * If there was a backup label file, it's done its job and the info
		 * has now been propagated into pg_control.  We must get rid of the
		 * label file so that if we crash during recovery, we'll pick up at
		 * the latest recovery restartpoint instead of going all the way back
		 * to the backup start point.  It seems prudent though to just rename
		 * the file out of the way rather than delete it completely.
		 */
		if (haveBackupLabel)
		{
			unlink(BACKUP_LABEL_OLD);
			if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) != 0)
				ereport(FATAL,
						(errcode_for_file_access(),
						 errmsg("could not rename file \"%s\" to \"%s\": %m",
								BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
		}

		/* Check that the GUCs used to generate the WAL allow recovery */
		CheckRequiredParameterValues();

		/*
		 * We're in recovery, so unlogged relations may be trashed and must be
		 * reset.  This should be done BEFORE allowing Hot Standby
		 * connections, so that read-only backends don't try to read whatever
		 * garbage is left over from before.
		 */
		ResetUnloggedRelations(UNLOGGED_RELATION_CLEANUP);

		/*
		 * Likewise, delete any saved transaction snapshot files that got left
		 * behind by crashed backends.
		 */
		DeleteAllExportedSnapshotFiles();

		/*
		 * Initialize for Hot Standby, if enabled. We won't let backends in
		 * yet, not until we've reached the min recovery point specified in
		 * control file and we've established a recovery snapshot from a
		 * running-xacts WAL record.
		 */
		if (ArchiveRecoveryRequested && EnableHotStandby)
		{
			TransactionId *xids;
			int			nxids;

			ereport(DEBUG1,
					(errmsg("initializing for hot standby")));

			InitRecoveryTransactionEnvironment();

			if (wasShutdown)
				oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);
			else
				oldestActiveXID = checkPoint.oldestActiveXid;
			Assert(TransactionIdIsValid(oldestActiveXID));

			/* Tell procarray about the range of xids it has to deal with */
			ProcArrayInitRecovery(ShmemVariableCache->nextXid);

			/*
			 * Startup commit log and subtrans only. Other SLRUs are not
			 * maintained during recovery and need not be started yet.
			 */
			StartupCLOG();
			StartupSUBTRANS(oldestActiveXID);

			/*
			 * If we're beginning at a shutdown checkpoint, we know that
			 * nothing was running on the master at this point. So fake-up an
			 * empty running-xacts record and use that here and now. Recover
			 * additional standby state for prepared transactions.
			 */
			if (wasShutdown)
			{
				RunningTransactionsData running;
				TransactionId latestCompletedXid;

				/*
				 * Construct a RunningTransactions snapshot representing a
				 * shut down server, with only prepared transactions still
				 * alive. We're never overflowed at this point because all
				 * subxids are listed with their parent prepared transactions.
				 */
				running.xcnt = nxids;
				running.subxcnt = 0;
				running.subxid_overflow = false;
				running.nextXid = checkPoint.nextXid;
				running.oldestRunningXid = oldestActiveXID;
				latestCompletedXid = checkPoint.nextXid;
				TransactionIdRetreat(latestCompletedXid);
				Assert(TransactionIdIsNormal(latestCompletedXid));
				running.latestCompletedXid = latestCompletedXid;
				running.xids = xids;

				ProcArrayApplyRecoveryInfo(&running);

				StandbyRecoverPreparedTransactions(false);
			}
		}

		/* Initialize resource managers */
		for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
		{
			if (RmgrTable[rmid].rm_startup != NULL)
				RmgrTable[rmid].rm_startup();
		}

		/*
		 * Initialize shared replayEndRecPtr, lastReplayedEndRecPtr, and
		 * recoveryLastXTime.
		 *
		 * This is slightly confusing if we're starting from an online
		 * checkpoint; we've just read and replayed the checkpoint record, but
		 * we're going to start replay from its redo pointer, which precedes
		 * the location of the checkpoint record itself. So even though the
		 * last record we've replayed is indeed ReadRecPtr, we haven't
		 * replayed all the preceding records yet. That's OK for the current
		 * use of these variables.
		 */
		SpinLockAcquire(&xlogctl->info_lck);
		xlogctl->replayEndRecPtr = ReadRecPtr;
		xlogctl->replayEndTLI = ThisTimeLineID;
		xlogctl->lastReplayedEndRecPtr = EndRecPtr;
		xlogctl->lastReplayedTLI = ThisTimeLineID;
		xlogctl->recoveryLastXTime = 0;
		xlogctl->currentChunkStartTime = 0;
		xlogctl->recoveryPause = false;
		SpinLockRelease(&xlogctl->info_lck);

		/* Also ensure XLogReceiptTime has a sane value */
		XLogReceiptTime = GetCurrentTimestamp();

		/*
		 * Let postmaster know we've started redo now, so that it can launch
		 * checkpointer to perform restartpoints.  We don't bother during
		 * crash recovery as restartpoints can only be performed during
		 * archive recovery.  And we'd like to keep crash recovery simple, to
		 * avoid introducing bugs that could affect you when recovering after
		 * crash.
		 *
		 * After this point, we can no longer assume that we're the only
		 * process in addition to postmaster!  Also, fsync requests are
		 * subsequently to be handled by the checkpointer, not locally.
		 */
		if (ArchiveRecoveryRequested && IsUnderPostmaster)
		{
			PublishStartupProcessInformation();
			SetForwardFsyncRequests();
			SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
			bgwriterLaunched = true;
		}

		/*
		 * Allow read-only connections immediately if we're consistent
		 * already.
		 */
		CheckRecoveryConsistency();

		/*
		 * Find the first record that logically follows the checkpoint --- it
		 * might physically precede it, though.
		 */
		if (checkPoint.redo < RecPtr)
		{
			/* back up to find the record */
			record = ReadRecord(xlogreader, checkPoint.redo, PANIC, false);
		}
		else
		{
			/* just have to read next record after CheckPoint */
			record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
		}

		if (record != NULL)
		{
			bool		recoveryContinue = true;
			bool		recoveryApply = true;
			ErrorContextCallback errcallback;
			TimestampTz xtime;

			InRedo = true;

			ereport(LOG,
					(errmsg("redo starts at %X/%X",
						 (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));

			/*
			 * main redo apply loop
			 */
			do
			{
				bool		switchedTLI = false;

#ifdef WAL_DEBUG
				if (XLOG_DEBUG ||
				 (rmid == RM_XACT_ID && trace_recovery_messages <= DEBUG2) ||
					(rmid != RM_XACT_ID && trace_recovery_messages <= DEBUG3))
				{
					StringInfoData buf;

					initStringInfo(&buf);
					appendStringInfo(&buf, "REDO @ %X/%X; LSN %X/%X: ",
							(uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr,
							 (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr);
					xlog_outrec(&buf, record);
					appendStringInfo(&buf, " - ");
					RmgrTable[record->xl_rmid].rm_desc(&buf,
													   record->xl_info,
													 XLogRecGetData(record));
					elog(LOG, "%s", buf.data);
					pfree(buf.data);
				}
#endif

				/* Handle interrupt signals of startup process */
				HandleStartupProcInterrupts();

				/*
				 * Pause WAL replay, if requested by a hot-standby session via
				 * SetRecoveryPause().
				 *
				 * Note that we intentionally don't take the info_lck spinlock
				 * here.  We might therefore read a slightly stale value of
				 * the recoveryPause flag, but it can't be very stale (no
				 * worse than the last spinlock we did acquire).  Since a
				 * pause request is a pretty asynchronous thing anyway,
				 * possibly responding to it one WAL record later than we
				 * otherwise would is a minor issue, so it doesn't seem worth
				 * adding another spinlock cycle to prevent that.
				 */
				if (xlogctl->recoveryPause)
					recoveryPausesHere();

				/*
				 * Have we reached our recovery target?
				 */
				if (recoveryStopsHere(record, &recoveryApply))
				{
					if (recoveryPauseAtTarget)
					{
						SetRecoveryPause(true);
						recoveryPausesHere();
					}
					reachedStopPoint = true;	/* see below */
					recoveryContinue = false;

					/* Exit loop if we reached non-inclusive recovery target */
					if (!recoveryApply)
						break;
				}

				/* Setup error traceback support for ereport() */
				errcallback.callback = rm_redo_error_callback;
				errcallback.arg = (void *) record;
				errcallback.previous = error_context_stack;
				error_context_stack = &errcallback;

				/*
				 * ShmemVariableCache->nextXid must be beyond record's xid.
				 *
				 * We don't expect anyone else to modify nextXid, hence we
				 * don't need to hold a lock while examining it.  We still
				 * acquire the lock to modify it, though.
				 */
				if (TransactionIdFollowsOrEquals(record->xl_xid,
												 ShmemVariableCache->nextXid))
				{
					LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
					ShmemVariableCache->nextXid = record->xl_xid;
					TransactionIdAdvance(ShmemVariableCache->nextXid);
					LWLockRelease(XidGenLock);
				}

				/*
				 * Before replaying this record, check if this record causes
				 * the current timeline to change. The record is already
				 * considered to be part of the new timeline, so we update
				 * ThisTimeLineID before replaying it. That's important so
				 * that replayEndTLI, which is recorded as the minimum
				 * recovery point's TLI if recovery stops after this record,
				 * is set correctly.
				 */
				if (record->xl_rmid == RM_XLOG_ID)
				{
					TimeLineID	newTLI = ThisTimeLineID;
					TimeLineID	prevTLI = ThisTimeLineID;
					uint8		info = record->xl_info & ~XLR_INFO_MASK;

					if (info == XLOG_CHECKPOINT_SHUTDOWN)
					{
						CheckPoint	checkPoint;

						memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
						newTLI = checkPoint.ThisTimeLineID;
						prevTLI = checkPoint.PrevTimeLineID;
					}
					else if (info == XLOG_END_OF_RECOVERY)
					{
						xl_end_of_recovery xlrec;

						memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));
						newTLI = xlrec.ThisTimeLineID;
						prevTLI = xlrec.PrevTimeLineID;
					}

					if (newTLI != ThisTimeLineID)
					{
						/* Check that it's OK to switch to this TLI */
						checkTimeLineSwitch(EndRecPtr, newTLI, prevTLI);

						/* Following WAL records should be run with new TLI */
						ThisTimeLineID = newTLI;
						switchedTLI = true;
					}
				}

				/*
				 * Update shared replayEndRecPtr before replaying this record,
				 * so that XLogFlush will update minRecoveryPoint correctly.
				 */
				SpinLockAcquire(&xlogctl->info_lck);
				xlogctl->replayEndRecPtr = EndRecPtr;
				xlogctl->replayEndTLI = ThisTimeLineID;
				SpinLockRelease(&xlogctl->info_lck);

				/*
				 * If we are attempting to enter Hot Standby mode, process
				 * XIDs we see
				 */
				if (standbyState >= STANDBY_INITIALIZED &&
					TransactionIdIsValid(record->xl_xid))
					RecordKnownAssignedTransactionIds(record->xl_xid);

				/* Now apply the WAL record itself */
				RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record);

				/* Pop the error context stack */
				error_context_stack = errcallback.previous;

				/*
				 * Update lastReplayedEndRecPtr after this record has been
				 * successfully replayed.
				 */
				SpinLockAcquire(&xlogctl->info_lck);
				xlogctl->lastReplayedEndRecPtr = EndRecPtr;
				xlogctl->lastReplayedTLI = ThisTimeLineID;
				SpinLockRelease(&xlogctl->info_lck);

				/* Remember this record as the last-applied one */
				LastRec = ReadRecPtr;

				/* Allow read-only connections if we're consistent now */
				CheckRecoveryConsistency();

				/*
				 * If this record was a timeline switch, wake up any
				 * walsenders to notice that we are on a new timeline.
				 */
				if (switchedTLI && AllowCascadeReplication())
					WalSndWakeup();

				/* Exit loop if we reached inclusive recovery target */
				if (!recoveryContinue)
					break;

				/* Else, try to fetch the next WAL record */
				record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
			} while (record != NULL);

			/*
			 * end of main redo apply loop
			 */

			ereport(LOG,
					(errmsg("redo done at %X/%X",
						 (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));
			xtime = GetLatestXTime();
			if (xtime)
				ereport(LOG,
					 (errmsg("last completed transaction was at log time %s",
							 timestamptz_to_str(xtime))));
			InRedo = false;
		}
		else
		{
			/* there are no WAL records following the checkpoint */
			ereport(LOG,
					(errmsg("redo is not required")));
		}
	}

	/*
	 * Kill WAL receiver, if it's still running, before we continue to write
	 * the startup checkpoint record. It will trump over the checkpoint and
	 * subsequent records if it's still alive when we start writing WAL.
	 */
	ShutdownWalRcv();

	/*
	 * We don't need the latch anymore. It's not strictly necessary to disown
	 * it, but let's do it for the sake of tidiness.
	 */
	if (StandbyModeRequested)
		DisownLatch(&XLogCtl->recoveryWakeupLatch);

	/*
	 * We are now done reading the xlog from stream. Turn off streaming
	 * recovery to force fetching the files (which would be required at end of
	 * recovery, e.g., timeline history file) from archive or pg_xlog.
	 */
	StandbyMode = false;

	/*
	 * Re-fetch the last valid or last applied record, so we can identify the
	 * exact endpoint of what we consider the valid portion of WAL.
	 */
	record = ReadRecord(xlogreader, LastRec, PANIC, false);
	EndOfLog = EndRecPtr;
	XLByteToPrevSeg(EndOfLog, endLogSegNo);

	/*
	 * Complain if we did not roll forward far enough to render the backup
	 * dump consistent.  Note: it is indeed okay to look at the local variable
	 * minRecoveryPoint here, even though ControlFile->minRecoveryPoint might
	 * be further ahead --- ControlFile->minRecoveryPoint cannot have been
	 * advanced beyond the WAL we processed.
	 */
	if (InRecovery &&
		(EndOfLog < minRecoveryPoint ||
		 !XLogRecPtrIsInvalid(ControlFile->backupStartPoint)))
	{
		if (reachedStopPoint)
		{
			/* stopped because of stop request */
			ereport(FATAL,
					(errmsg("requested recovery stop point is before consistent recovery point")));
		}

		/*
		 * Ran off end of WAL before reaching end-of-backup WAL record, or
		 * minRecoveryPoint. That's usually a bad sign, indicating that you
		 * tried to recover from an online backup but never called
		 * pg_stop_backup(), or you didn't archive all the WAL up to that
		 * point. However, this also happens in crash recovery, if the system
		 * crashes while an online backup is in progress. We must not treat
		 * that as an error, or the database will refuse to start up.
		 */
		if (ArchiveRecoveryRequested || ControlFile->backupEndRequired)
		{
			if (ControlFile->backupEndRequired)
				ereport(FATAL,
						(errmsg("WAL ends before end of online backup"),
						 errhint("All WAL generated while online backup was taken must be available at recovery.")));
			else if (!XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
				ereport(FATAL,
						(errmsg("WAL ends before end of online backup"),
						 errhint("Online backup started with pg_start_backup() must be ended with pg_stop_backup(), and all WAL up to that point must be available at recovery.")));
			else
				ereport(FATAL,
					  (errmsg("WAL ends before consistent recovery point")));
		}
	}

	/*
	 * Consider whether we need to assign a new timeline ID.
	 *
	 * If we are doing an archive recovery, we always assign a new ID.	This
	 * handles a couple of issues.	If we stopped short of the end of WAL
	 * during recovery, then we are clearly generating a new timeline and must
	 * assign it a unique new ID.  Even if we ran to the end, modifying the
	 * current last segment is problematic because it may result in trying to
	 * overwrite an already-archived copy of that segment, and we encourage
	 * DBAs to make their archive_commands reject that.  We can dodge the
	 * problem by making the new active segment have a new timeline ID.
	 *
	 * In a normal crash recovery, we can just extend the timeline we were in.
	 */
	PrevTimeLineID = ThisTimeLineID;
	if (ArchiveRecoveryRequested)
	{
		char		reason[200];

		Assert(InArchiveRecovery);

		ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
		ereport(LOG,
				(errmsg("selected new timeline ID: %u", ThisTimeLineID)));

		/*
		 * Create a comment for the history file to explain why and where
		 * timeline changed.
		 */
		if (recoveryTarget == RECOVERY_TARGET_XID)
			snprintf(reason, sizeof(reason),
					 "%s transaction %u",
					 recoveryStopAfter ? "after" : "before",
					 recoveryStopXid);
		else if (recoveryTarget == RECOVERY_TARGET_TIME)
			snprintf(reason, sizeof(reason),
					 "%s %s\n",
					 recoveryStopAfter ? "after" : "before",
					 timestamptz_to_str(recoveryStopTime));
		else if (recoveryTarget == RECOVERY_TARGET_NAME)
			snprintf(reason, sizeof(reason),
					 "at restore point \"%s\"",
					 recoveryStopName);
		else
			snprintf(reason, sizeof(reason), "no recovery target specified");

		writeTimeLineHistory(ThisTimeLineID, recoveryTargetTLI,
							 EndRecPtr, reason);
	}

	/* Save the selected TimeLineID in shared memory, too */
	XLogCtl->ThisTimeLineID = ThisTimeLineID;
	XLogCtl->PrevTimeLineID = PrevTimeLineID;

	/*
	 * We are now done reading the old WAL.  Turn off archive fetching if it
	 * was active, and make a writable copy of the last WAL segment. (Note
	 * that we also have a copy of the last block of the old WAL in readBuf;
	 * we will use that below.)
	 */
	if (ArchiveRecoveryRequested)
		exitArchiveRecovery(xlogreader->readPageTLI, endLogSegNo);

	/*
	 * Prepare to write WAL starting at EndOfLog position, and init xlog
	 * buffer cache using the block containing the last record from the
	 * previous incarnation.
	 */
	openLogSegNo = endLogSegNo;
	openLogFile = XLogFileOpen(openLogSegNo);
	openLogOff = 0;
	Insert = &XLogCtl->Insert;
	Insert->PrevBytePos = XLogRecPtrToBytePos(LastRec);
	Insert->CurrBytePos = XLogRecPtrToBytePos(EndOfLog);

	/*
	 * Tricky point here: readBuf contains the *last* block that the LastRec
	 * record spans, not the one it starts in.	The last block is indeed the
	 * one we want to use.
	 */
	if (EndOfLog % XLOG_BLCKSZ != 0)
	{
		char	   *page;
		int			len;
		int			firstIdx;
		XLogRecPtr	pageBeginPtr;

		pageBeginPtr = EndOfLog - (EndOfLog % XLOG_BLCKSZ);
		Assert(readOff == pageBeginPtr % XLogSegSize);

		firstIdx = XLogRecPtrToBufIdx(EndOfLog);

		/* Copy the valid part of the last block, and zero the rest */
		page = &XLogCtl->pages[firstIdx * XLOG_BLCKSZ];
		len = EndOfLog % XLOG_BLCKSZ;
		memcpy(page, xlogreader->readBuf, len);
		memset(page + len, 0, XLOG_BLCKSZ - len);

		XLogCtl->xlblocks[firstIdx] = pageBeginPtr + XLOG_BLCKSZ;
		XLogCtl->InitializedUpTo = pageBeginPtr + XLOG_BLCKSZ;
	}
	else
	{
		/*
		 * There is no partial block to copy. Just set InitializedUpTo,
		 * and let the first attempt to insert a log record to initialize
		 * the next buffer.
		 */
		XLogCtl->InitializedUpTo = EndOfLog;
	}

	LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

	XLogCtl->LogwrtResult = LogwrtResult;

	XLogCtl->LogwrtRqst.Write = EndOfLog;
	XLogCtl->LogwrtRqst.Flush = EndOfLog;

	/* Pre-scan prepared transactions to find out the range of XIDs present */
	oldestActiveXID = PrescanPreparedTransactions(NULL, NULL);

	/*
	 * Update full_page_writes in shared memory and write an XLOG_FPW_CHANGE
	 * record before resource manager writes cleanup WAL records or checkpoint
	 * record is written.
	 */
	Insert->fullPageWrites = lastFullPageWrites;
	LocalSetXLogInsertAllowed();
	UpdateFullPageWrites();
	LocalXLogInsertAllowed = -1;

	if (InRecovery)
	{
		int			rmid;

		/*
		 * Resource managers might need to write WAL records, eg, to record
		 * index cleanup actions.  So temporarily enable XLogInsertAllowed in
		 * this process only.
		 */
		LocalSetXLogInsertAllowed();

		/*
		 * Allow resource managers to do any required cleanup.
		 */
		for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
		{
			if (RmgrTable[rmid].rm_cleanup != NULL)
				RmgrTable[rmid].rm_cleanup();
		}

		/* Disallow XLogInsert again */
		LocalXLogInsertAllowed = -1;

		/*
		 * Perform a checkpoint to update all our recovery activity to disk.
		 *
		 * Note that we write a shutdown checkpoint rather than an on-line
		 * one. This is not particularly critical, but since we may be
		 * assigning a new TLI, using a shutdown checkpoint allows us to have
		 * the rule that TLI only changes in shutdown checkpoints, which
		 * allows some extra error checking in xlog_redo.
		 *
		 * In fast promotion, only create a lightweight end-of-recovery record
		 * instead of a full checkpoint. A checkpoint is requested later,
		 * after we're fully out of recovery mode and already accepting
		 * queries.
		 */
		if (bgwriterLaunched)
		{
			if (fast_promote)
			{
				checkPointLoc = ControlFile->prevCheckPoint;

				/*
				 * Confirm the last checkpoint is available for us to recover
				 * from if we fail. Note that we don't check for the secondary
				 * checkpoint since that isn't available in most base backups.
				 */
				record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, false);
				if (record != NULL)
				{
					fast_promoted = true;

					/*
					 * Insert a special WAL record to mark the end of
					 * recovery, since we aren't doing a checkpoint. That
					 * means that the checkpointer process may likely be in
					 * the middle of a time-smoothed restartpoint and could
					 * continue to be for minutes after this. That sounds
					 * strange, but the effect is roughly the same and it
					 * would be stranger to try to come out of the
					 * restartpoint and then checkpoint. We request a
					 * checkpoint later anyway, just for safety.
					 */
					CreateEndOfRecoveryRecord();
				}
			}

			if (!fast_promoted)
				RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
								  CHECKPOINT_IMMEDIATE |
								  CHECKPOINT_WAIT);
		}
		else
			CreateCheckPoint(CHECKPOINT_END_OF_RECOVERY | CHECKPOINT_IMMEDIATE);

		/*
		 * And finally, execute the recovery_end_command, if any.
		 */
		if (recoveryEndCommand)
			ExecuteRecoveryCommand(recoveryEndCommand,
								   "recovery_end_command",
								   true);
	}

	/*
	 * Preallocate additional log files, if wanted.
	 */
	PreallocXlogFiles(EndOfLog);

	/*
	 * Reset initial contents of unlogged relations.  This has to be done
	 * AFTER recovery is complete so that any unlogged relations created
	 * during recovery also get picked up.
	 */
	if (InRecovery)
		ResetUnloggedRelations(UNLOGGED_RELATION_INIT);

	/*
	 * Okay, we're officially UP.
	 */
	InRecovery = false;

	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
	ControlFile->state = DB_IN_PRODUCTION;
	ControlFile->time = (pg_time_t) time(NULL);
	UpdateControlFile();
	LWLockRelease(ControlFileLock);

	/* start the archive_timeout timer running */
	XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);

	/* also initialize latestCompletedXid, to nextXid - 1 */
	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
	ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
	TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);
	LWLockRelease(ProcArrayLock);

	/*
	 * Start up the commit log and subtrans, if not already done for hot
	 * standby.
	 */
	if (standbyState == STANDBY_DISABLED)
	{
		StartupCLOG();
		StartupSUBTRANS(oldestActiveXID);
	}

	/*
	 * Perform end of recovery actions for any SLRUs that need it.
	 */
	StartupMultiXact();
	TrimCLOG();

	/* Reload shared-memory state for prepared transactions */
	RecoverPreparedTransactions();

	/*
	 * Shutdown the recovery environment. This must occur after
	 * RecoverPreparedTransactions(), see notes for lock_twophase_recover()
	 */
	if (standbyState != STANDBY_DISABLED)
		ShutdownRecoveryTransactionEnvironment();

	/* Shut down xlogreader */
	if (readFile >= 0)
	{
		close(readFile);
		readFile = -1;
	}
	XLogReaderFree(xlogreader);

	/*
	 * If any of the critical GUCs have changed, log them before we allow
	 * backends to write WAL.
	 */
	LocalSetXLogInsertAllowed();
	XLogReportParameters();

	/*
	 * All done.  Allow backends to write WAL.	(Although the bool flag is
	 * probably atomic in itself, we use the info_lck here to ensure that
	 * there are no race conditions concerning visibility of other recent
	 * updates to shared memory.)
	 */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		xlogctl->SharedRecoveryInProgress = false;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/*
	 * If there were cascading standby servers connected to us, nudge any wal
	 * sender processes to notice that we've been promoted.
	 */
	WalSndWakeup();

	/*
	 * If this was a fast promotion, request an (online) checkpoint now. This
	 * isn't required for consistency, but the last restartpoint might be far
	 * back, and in case of a crash, recovering from it might take a longer
	 * than is appropriate now that we're not in standby mode anymore.
	 */
	if (fast_promoted)
		RequestCheckpoint(CHECKPOINT_FORCE);
}

/*
 * Checks if recovery has reached a consistent state. When consistency is
 * reached and we have a valid starting standby snapshot, tell postmaster
 * that it can start accepting read-only connections.
 */
static void
CheckRecoveryConsistency(void)
{
	/*
	 * During crash recovery, we don't reach a consistent state until we've
	 * replayed all the WAL.
	 */
	if (XLogRecPtrIsInvalid(minRecoveryPoint))
		return;

	/*
	 * Have we reached the point where our base backup was completed?
	 */
	if (!XLogRecPtrIsInvalid(ControlFile->backupEndPoint) &&
		ControlFile->backupEndPoint <= EndRecPtr)
	{
		/*
		 * We have reached the end of base backup, as indicated by pg_control.
		 * The data on disk is now consistent. Reset backupStartPoint and
		 * backupEndPoint, and update minRecoveryPoint to make sure we don't
		 * allow starting up at an earlier point even if recovery is stopped
		 * and restarted soon after this.
		 */
		elog(DEBUG1, "end of backup reached");

		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

		if (ControlFile->minRecoveryPoint < EndRecPtr)
			ControlFile->minRecoveryPoint = EndRecPtr;

		ControlFile->backupStartPoint = InvalidXLogRecPtr;
		ControlFile->backupEndPoint = InvalidXLogRecPtr;
		ControlFile->backupEndRequired = false;
		UpdateControlFile();

		LWLockRelease(ControlFileLock);
	}

	/*
	 * Have we passed our safe starting point? Note that minRecoveryPoint is
	 * known to be incorrectly set if ControlFile->backupEndRequired, until
	 * the XLOG_BACKUP_RECORD arrives to advise us of the correct
	 * minRecoveryPoint. All we know prior to that is that we're not
	 * consistent yet.
	 */
	if (!reachedConsistency && !ControlFile->backupEndRequired &&
		minRecoveryPoint <= XLogCtl->lastReplayedEndRecPtr &&
		XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
	{
		/*
		 * Check to see if the XLOG sequence contained any unresolved
		 * references to uninitialized pages.
		 */
		XLogCheckInvalidPages();

		reachedConsistency = true;
		ereport(LOG,
				(errmsg("consistent recovery state reached at %X/%X",
						(uint32) (XLogCtl->lastReplayedEndRecPtr >> 32),
						(uint32) XLogCtl->lastReplayedEndRecPtr)));
	}

	/*
	 * Have we got a valid starting snapshot that will allow queries to be
	 * run? If so, we can tell postmaster that the database is consistent now,
	 * enabling connections.
	 */
	if (standbyState == STANDBY_SNAPSHOT_READY &&
		!LocalHotStandbyActive &&
		reachedConsistency &&
		IsUnderPostmaster)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		xlogctl->SharedHotStandbyActive = true;
		SpinLockRelease(&xlogctl->info_lck);

		LocalHotStandbyActive = true;

		SendPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY);
	}
}

/*
 * Is the system still in recovery?
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 *
 * As a side-effect, we initialize the local TimeLineID and RedoRecPtr
 * variables the first time we see that recovery is finished.
 */
bool
RecoveryInProgress(void)
{
	/*
	 * We check shared state each time only until we leave recovery mode. We
	 * can't re-enter recovery, so there's no need to keep checking after the
	 * shared variable has once been seen false.
	 */
	if (!LocalRecoveryInProgress)
		return false;
	else
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		/* spinlock is essential on machines with weak memory ordering! */
		SpinLockAcquire(&xlogctl->info_lck);
		LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;
		SpinLockRelease(&xlogctl->info_lck);

		/*
		 * Initialize TimeLineID and RedoRecPtr when we discover that recovery
		 * is finished. InitPostgres() relies upon this behaviour to ensure
		 * that InitXLOGAccess() is called at backend startup.	(If you change
		 * this, see also LocalSetXLogInsertAllowed.)
		 */
		if (!LocalRecoveryInProgress)
			InitXLOGAccess();

		return LocalRecoveryInProgress;
	}
}

/*
 * Is HotStandby active yet? This is only important in special backends
 * since normal backends won't ever be able to connect until this returns
 * true. Postmaster knows this by way of signal, not via shared memory.
 *
 * Unlike testing standbyState, this works in any process that's connected to
 * shared memory.
 */
bool
HotStandbyActive(void)
{
	/*
	 * We check shared state each time only until Hot Standby is active. We
	 * can't de-activate Hot Standby, so there's no need to keep checking
	 * after the shared variable has once been seen true.
	 */
	if (LocalHotStandbyActive)
		return true;
	else
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		/* spinlock is essential on machines with weak memory ordering! */
		SpinLockAcquire(&xlogctl->info_lck);
		LocalHotStandbyActive = xlogctl->SharedHotStandbyActive;
		SpinLockRelease(&xlogctl->info_lck);

		return LocalHotStandbyActive;
	}
}

/*
 * Is this process allowed to insert new WAL records?
 *
 * Ordinarily this is essentially equivalent to !RecoveryInProgress().
 * But we also have provisions for forcing the result "true" or "false"
 * within specific processes regardless of the global state.
 */
bool
XLogInsertAllowed(void)
{
	/*
	 * If value is "unconditionally true" or "unconditionally false", just
	 * return it.  This provides the normal fast path once recovery is known
	 * done.
	 */
	if (LocalXLogInsertAllowed >= 0)
		return (bool) LocalXLogInsertAllowed;

	/*
	 * Else, must check to see if we're still in recovery.
	 */
	if (RecoveryInProgress())
		return false;

	/*
	 * On exit from recovery, reset to "unconditionally true", since there is
	 * no need to keep checking.
	 */
	LocalXLogInsertAllowed = 1;
	return true;
}

/*
 * Make XLogInsertAllowed() return true in the current process only.
 *
 * Note: it is allowed to switch LocalXLogInsertAllowed back to -1 later,
 * and even call LocalSetXLogInsertAllowed() again after that.
 */
static void
LocalSetXLogInsertAllowed(void)
{
	Assert(LocalXLogInsertAllowed == -1);
	LocalXLogInsertAllowed = 1;

	/* Initialize as RecoveryInProgress() would do when switching state */
	InitXLOGAccess();
}

/*
 * Subroutine to try to fetch and validate a prior checkpoint record.
 *
 * whichChkpt identifies the checkpoint (merely for reporting purposes).
 * 1 for "primary", 2 for "secondary", 0 for "other" (backup_label)
 */
static XLogRecord *
ReadCheckpointRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
					 int whichChkpt, bool report)
{
	XLogRecord *record;

	if (!XRecOffIsValid(RecPtr))
	{
		if (!report)
			return NULL;

		switch (whichChkpt)
		{
			case 1:
				ereport(LOG,
				(errmsg("invalid primary checkpoint link in control file")));
				break;
			case 2:
				ereport(LOG,
						(errmsg("invalid secondary checkpoint link in control file")));
				break;
			default:
				ereport(LOG,
				   (errmsg("invalid checkpoint link in backup_label file")));
				break;
		}
		return NULL;
	}

	record = ReadRecord(xlogreader, RecPtr, LOG, true);

	if (record == NULL)
	{
		if (!report)
			return NULL;

		switch (whichChkpt)
		{
			case 1:
				ereport(LOG,
						(errmsg("invalid primary checkpoint record")));
				break;
			case 2:
				ereport(LOG,
						(errmsg("invalid secondary checkpoint record")));
				break;
			default:
				ereport(LOG,
						(errmsg("invalid checkpoint record")));
				break;
		}
		return NULL;
	}
	if (record->xl_rmid != RM_XLOG_ID)
	{
		switch (whichChkpt)
		{
			case 1:
				ereport(LOG,
						(errmsg("invalid resource manager ID in primary checkpoint record")));
				break;
			case 2:
				ereport(LOG,
						(errmsg("invalid resource manager ID in secondary checkpoint record")));
				break;
			default:
				ereport(LOG,
				(errmsg("invalid resource manager ID in checkpoint record")));
				break;
		}
		return NULL;
	}
	if (record->xl_info != XLOG_CHECKPOINT_SHUTDOWN &&
		record->xl_info != XLOG_CHECKPOINT_ONLINE)
	{
		switch (whichChkpt)
		{
			case 1:
				ereport(LOG,
				   (errmsg("invalid xl_info in primary checkpoint record")));
				break;
			case 2:
				ereport(LOG,
				 (errmsg("invalid xl_info in secondary checkpoint record")));
				break;
			default:
				ereport(LOG,
						(errmsg("invalid xl_info in checkpoint record")));
				break;
		}
		return NULL;
	}
	if (record->xl_len != sizeof(CheckPoint) ||
		record->xl_tot_len != SizeOfXLogRecord + sizeof(CheckPoint))
	{
		switch (whichChkpt)
		{
			case 1:
				ereport(LOG,
					(errmsg("invalid length of primary checkpoint record")));
				break;
			case 2:
				ereport(LOG,
				  (errmsg("invalid length of secondary checkpoint record")));
				break;
			default:
				ereport(LOG,
						(errmsg("invalid length of checkpoint record")));
				break;
		}
		return NULL;
	}
	return record;
}

/*
 * This must be called during startup of a backend process, except that
 * it need not be called in a standalone backend (which does StartupXLOG
 * instead).  We need to initialize the local copies of ThisTimeLineID and
 * RedoRecPtr.
 *
 * Note: before Postgres 8.0, we went to some effort to keep the postmaster
 * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
 * unnecessary however, since the postmaster itself never touches XLOG anyway.
 */
void
InitXLOGAccess(void)
{
	/* ThisTimeLineID doesn't change so we need no lock to copy it */
	ThisTimeLineID = XLogCtl->ThisTimeLineID;
	Assert(ThisTimeLineID != 0 || IsBootstrapProcessingMode());

	/* Use GetRedoRecPtr to copy the RedoRecPtr safely */
	(void) GetRedoRecPtr();
}

/*
 * Return the current Redo pointer from shared memory.
 *
 * As a side-effect, the local RedoRecPtr copy is updated.
 */
XLogRecPtr
GetRedoRecPtr(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	XLogRecPtr ptr;

	/*
	 * The possibly not up-to-date copy in XlogCtl is enough. Even if we
	 * grabbed a WAL insertion slot to read the master copy, someone might
	 * update it just after we've released the lock.
	 */
	SpinLockAcquire(&xlogctl->info_lck);
	ptr = xlogctl->RedoRecPtr;
	SpinLockRelease(&xlogctl->info_lck);

	if (RedoRecPtr < ptr)
		RedoRecPtr = ptr;

	return RedoRecPtr;
}

/*
 * GetInsertRecPtr -- Returns the current insert position.
 *
 * NOTE: The value *actually* returned is the position of the last full
 * xlog page. It lags behind the real insert position by at most 1 page.
 * For that, we don't need to scan through WAL insertion slots, and an
 * approximation is enough for the current usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	XLogRecPtr	recptr;

	SpinLockAcquire(&xlogctl->info_lck);
	recptr = xlogctl->LogwrtRqst.Write;
	SpinLockRelease(&xlogctl->info_lck);

	return recptr;
}

/*
 * GetFlushRecPtr -- Returns the current flush position, ie, the last WAL
 * position known to be fsync'd to disk.
 */
XLogRecPtr
GetFlushRecPtr(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	XLogRecPtr	recptr;

	SpinLockAcquire(&xlogctl->info_lck);
	recptr = xlogctl->LogwrtResult.Flush;
	SpinLockRelease(&xlogctl->info_lck);

	return recptr;
}

/*
 * Get the time of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchTime(void)
{
	pg_time_t	result;

	/* Need WALWriteLock, but shared lock is sufficient */
	LWLockAcquire(WALWriteLock, LW_SHARED);
	result = XLogCtl->lastSegSwitchTime;
	LWLockRelease(WALWriteLock);

	return result;
}

/*
 * GetNextXidAndEpoch - get the current nextXid value and associated epoch
 *
 * This is exported for use by code that would like to have 64-bit XIDs.
 * We don't really support such things, but all XIDs within the system
 * can be presumed "close to" the result, and thus the epoch associated
 * with them can be determined.
 */
void
GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch)
{
	uint32		ckptXidEpoch;
	TransactionId ckptXid;
	TransactionId nextXid;

	/* Must read checkpoint info first, else have race condition */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		ckptXidEpoch = xlogctl->ckptXidEpoch;
		ckptXid = xlogctl->ckptXid;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/* Now fetch current nextXid */
	nextXid = ReadNewTransactionId();

	/*
	 * nextXid is certainly logically later than ckptXid.  So if it's
	 * numerically less, it must have wrapped into the next epoch.
	 */
	if (nextXid < ckptXid)
		ckptXidEpoch++;

	*xid = nextXid;
	*epoch = ckptXidEpoch;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
	/* Don't be chatty in standalone mode */
	ereport(IsPostmasterEnvironment ? LOG : NOTICE,
			(errmsg("shutting down")));

	if (RecoveryInProgress())
		CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
	else
	{
		/*
		 * If archiving is enabled, rotate the last XLOG file so that all the
		 * remaining records are archived (postmaster wakes up the archiver
		 * process one more time at the end of shutdown). The checkpoint
		 * record will go to the next XLOG file and won't be archived (yet).
		 */
		if (XLogArchivingActive() && XLogArchiveCommandSet())
			RequestXLogSwitch();

		CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
	}
	ShutdownCLOG();
	ShutdownSUBTRANS();
	ShutdownMultiXact();

	/* Don't be chatty in standalone mode */
	ereport(IsPostmasterEnvironment ? LOG : NOTICE,
			(errmsg("database system is shut down")));
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags, bool restartpoint)
{
	const char *msg;

	/*
	 * XXX: This is hopelessly untranslatable. We could call gettext_noop for
	 * the main message, but what about all the flags?
	 */
	if (restartpoint)
		msg = "restartpoint starting:%s%s%s%s%s%s%s";
	else
		msg = "checkpoint starting:%s%s%s%s%s%s%s";

	elog(LOG, msg,
		 (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
		 (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
		 (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
		 (flags & CHECKPOINT_FORCE) ? " force" : "",
		 (flags & CHECKPOINT_WAIT) ? " wait" : "",
		 (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
		 (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "");
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(bool restartpoint)
{
	long		write_secs,
				sync_secs,
				total_secs,
				longest_secs,
				average_secs;
	int			write_usecs,
				sync_usecs,
				total_usecs,
				longest_usecs,
				average_usecs;
	uint64		average_sync_time;

	CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

	TimestampDifference(CheckpointStats.ckpt_write_t,
						CheckpointStats.ckpt_sync_t,
						&write_secs, &write_usecs);

	TimestampDifference(CheckpointStats.ckpt_sync_t,
						CheckpointStats.ckpt_sync_end_t,
						&sync_secs, &sync_usecs);

	/* Accumulate checkpoint timing summary data, in milliseconds. */
	BgWriterStats.m_checkpoint_write_time +=
		write_secs * 1000 + write_usecs / 1000;
	BgWriterStats.m_checkpoint_sync_time +=
		sync_secs * 1000 + sync_usecs / 1000;

	/*
	 * All of the published timing statistics are accounted for.  Only
	 * continue if a log message is to be written.
	 */
	if (!log_checkpoints)
		return;

	TimestampDifference(CheckpointStats.ckpt_start_t,
						CheckpointStats.ckpt_end_t,
						&total_secs, &total_usecs);

	/*
	 * Timing values returned from CheckpointStats are in microseconds.
	 * Convert to the second plus microsecond form that TimestampDifference
	 * returns for homogeneous printing.
	 */
	longest_secs = (long) (CheckpointStats.ckpt_longest_sync / 1000000);
	longest_usecs = CheckpointStats.ckpt_longest_sync -
		(uint64) longest_secs *1000000;

	average_sync_time = 0;
	if (CheckpointStats.ckpt_sync_rels > 0)
		average_sync_time = CheckpointStats.ckpt_agg_sync_time /
			CheckpointStats.ckpt_sync_rels;
	average_secs = (long) (average_sync_time / 1000000);
	average_usecs = average_sync_time - (uint64) average_secs *1000000;

	if (restartpoint)
		elog(LOG, "restartpoint complete: wrote %d buffers (%.1f%%); "
			 "%d transaction log file(s) added, %d removed, %d recycled; "
			 "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
			 "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
			 CheckpointStats.ckpt_bufs_written,
			 (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
			 CheckpointStats.ckpt_segs_added,
			 CheckpointStats.ckpt_segs_removed,
			 CheckpointStats.ckpt_segs_recycled,
			 write_secs, write_usecs / 1000,
			 sync_secs, sync_usecs / 1000,
			 total_secs, total_usecs / 1000,
			 CheckpointStats.ckpt_sync_rels,
			 longest_secs, longest_usecs / 1000,
			 average_secs, average_usecs / 1000);
	else
		elog(LOG, "checkpoint complete: wrote %d buffers (%.1f%%); "
			 "%d transaction log file(s) added, %d removed, %d recycled; "
			 "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
			 "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
			 CheckpointStats.ckpt_bufs_written,
			 (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
			 CheckpointStats.ckpt_segs_added,
			 CheckpointStats.ckpt_segs_removed,
			 CheckpointStats.ckpt_segs_recycled,
			 write_secs, write_usecs / 1000,
			 sync_secs, sync_usecs / 1000,
			 total_secs, total_usecs / 1000,
			 CheckpointStats.ckpt_sync_rels,
			 longest_secs, longest_usecs / 1000,
			 average_secs, average_usecs / 1000);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * flags is a bitwise OR of the following:
 *	CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *	CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *	CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *		ignoring checkpoint_completion_target parameter.
 *	CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
 *		since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *		CHECKPOINT_END_OF_RECOVERY).
 *
 * Note: flags contains other bits, of interest here only for logging purposes.
 * In particular note that this routine is synchronous and does not pay
 * attention to CHECKPOINT_WAIT.
 *
 * If !shutdown then we are writing an online checkpoint. This is a very special
 * kind of operation and WAL record because the checkpoint action occurs over
 * a period of time yet logically occurs at just a single LSN. The logical
 * position of the WAL record (redo ptr) is the same or earlier than the
 * physical position. When we replay WAL we locate the checkpoint via its
 * physical position then read the redo ptr and actually start replay at the
 * earlier logical position. Note that we don't write *anything* to WAL at
 * the logical position, so that location could be any other kind of WAL record.
 * All of this mechanism allows us to continue working while we checkpoint.
 * As a result, timing of actions is critical here and be careful to note that
 * this function will likely take minutes to execute on a busy system.
 */
void
CreateCheckPoint(int flags)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	bool		shutdown;
	CheckPoint	checkPoint;
	XLogRecPtr	recptr;
	XLogCtlInsert *Insert = &XLogCtl->Insert;
	XLogRecData rdata;
	uint32		freespace;
	XLogSegNo	_logSegNo;
	XLogRecPtr	curInsert;
	VirtualTransactionId *vxids;
	int			nvxids;

	/*
	 * An end-of-recovery checkpoint is really a shutdown checkpoint, just
	 * issued at a different time.
	 */
	if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
		shutdown = true;
	else
		shutdown = false;

	/* sanity check */
	if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
		elog(ERROR, "can't create a checkpoint during recovery");

	/*
	 * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
	 * (This is just pro forma, since in the present system structure there is
	 * only one process that is allowed to issue checkpoints at any given
	 * time.)
	 */
	LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

	/*
	 * Prepare to accumulate statistics.
	 *
	 * Note: because it is possible for log_checkpoints to change while a
	 * checkpoint proceeds, we always accumulate stats, even if
	 * log_checkpoints is currently off.
	 */
	MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
	CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

	/*
	 * Use a critical section to force system panic if we have trouble.
	 */
	START_CRIT_SECTION();

	if (shutdown)
	{
		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
		ControlFile->state = DB_SHUTDOWNING;
		ControlFile->time = (pg_time_t) time(NULL);
		UpdateControlFile();
		LWLockRelease(ControlFileLock);
	}

	/*
	 * Let smgr prepare for checkpoint; this has to happen before we determine
	 * the REDO pointer.  Note that smgr must not do anything that'd have to
	 * be undone if we decide no checkpoint is needed.
	 */
	smgrpreckpt();

	/* Begin filling in the checkpoint WAL record */
	MemSet(&checkPoint, 0, sizeof(checkPoint));
	checkPoint.time = (pg_time_t) time(NULL);

	/*
	 * For Hot Standby, derive the oldestActiveXid before we fix the redo
	 * pointer. This allows us to begin accumulating changes to assemble our
	 * starting snapshot of locks and transactions.
	 */
	if (!shutdown && XLogStandbyInfoActive())
		checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
	else
		checkPoint.oldestActiveXid = InvalidTransactionId;

	/*
	 * We must block concurrent insertions while examining insert state to
	 * determine the checkpoint REDO pointer.
	 */
	WALInsertSlotAcquire(true);
	curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);

	/*
	 * If this isn't a shutdown or forced checkpoint, and we have not inserted
	 * any XLOG records since the start of the last checkpoint, skip the
	 * checkpoint.	The idea here is to avoid inserting duplicate checkpoints
	 * when the system is idle. That wastes log space, and more importantly it
	 * exposes us to possible loss of both current and previous checkpoint
	 * records if the machine crashes just as we're writing the update.
	 * (Perhaps it'd make even more sense to checkpoint only when the previous
	 * checkpoint record is in a different xlog page?)
	 *
	 * We have to make two tests to determine that nothing has happened since
	 * the start of the last checkpoint: current insertion point must match
	 * the end of the last checkpoint record, and its redo pointer must point
	 * to itself.
	 */
	if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
				  CHECKPOINT_FORCE)) == 0)
	{
		if (curInsert == ControlFile->checkPoint +
			MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
			ControlFile->checkPoint == ControlFile->checkPointCopy.redo)
		{
			WALInsertSlotRelease();
			LWLockRelease(CheckpointLock);
			END_CRIT_SECTION();
			return;
		}
	}

	/*
	 * An end-of-recovery checkpoint is created before anyone is allowed to
	 * write WAL. To allow us to write the checkpoint record, temporarily
	 * enable XLogInsertAllowed.  (This also ensures ThisTimeLineID is
	 * initialized, which we need here and in AdvanceXLInsertBuffer.)
	 */
	if (flags & CHECKPOINT_END_OF_RECOVERY)
		LocalSetXLogInsertAllowed();

	checkPoint.ThisTimeLineID = ThisTimeLineID;
	if (flags & CHECKPOINT_END_OF_RECOVERY)
		checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
	else
		checkPoint.PrevTimeLineID = ThisTimeLineID;

	checkPoint.fullPageWrites = Insert->fullPageWrites;

	/*
	 * Compute new REDO record ptr = location of next XLOG record.
	 *
	 * NB: this is NOT necessarily where the checkpoint record itself will be,
	 * since other backends may insert more XLOG records while we're off doing
	 * the buffer flush work.  Those XLOG records are logically after the
	 * checkpoint, even though physically before it.  Got that?
	 */
	freespace = INSERT_FREESPACE(curInsert);
	if (freespace == 0)
	{
		if (curInsert % XLogSegSize == 0)
			curInsert += SizeOfXLogLongPHD;
		else
			curInsert += SizeOfXLogShortPHD;
	}
	checkPoint.redo = curInsert;

	/*
	 * Here we update the shared RedoRecPtr for future XLogInsert calls; this
	 * must be done while holding the insertion slots.
	 *
	 * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
	 * pointing past where it really needs to point.  This is okay; the only
	 * consequence is that XLogInsert might back up whole buffers that it
	 * didn't really need to.  We can't postpone advancing RedoRecPtr because
	 * XLogInserts that happen while we are dumping buffers must assume that
	 * their buffer changes are not included in the checkpoint.
	 */
	RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;

	/*
	 * Now we can release the WAL insertion slots, allowing other xacts to
	 * proceed while we are flushing disk buffers.
	 */
	WALInsertSlotRelease();

	/* Update the info_lck-protected copy of RedoRecPtr as well */
	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->RedoRecPtr = checkPoint.redo;
	SpinLockRelease(&xlogctl->info_lck);

	/*
	 * If enabled, log checkpoint start.  We postpone this until now so as not
	 * to log anything if we decided to skip the checkpoint.
	 */
	if (log_checkpoints)
		LogCheckpointStart(flags, false);

	TRACE_POSTGRESQL_CHECKPOINT_START(flags);

	/*
	 * In some cases there are groups of actions that must all occur on one
	 * side or the other of a checkpoint record. Before flushing the
	 * checkpoint record we must explicitly wait for any backend currently
	 * performing those groups of actions.
	 *
	 * One example is end of transaction, so we must wait for any transactions
	 * that are currently in commit critical sections.	If an xact inserted
	 * its commit record into XLOG just before the REDO point, then a crash
	 * restart from the REDO point would not replay that record, which means
	 * that our flushing had better include the xact's update of pg_clog.  So
	 * we wait till he's out of his commit critical section before proceeding.
	 * See notes in RecordTransactionCommit().
	 *
	 * Because we've already released the insertion slots, this test is a bit
	 * fuzzy: it is possible that we will wait for xacts we didn't really need
	 * to wait for.  But the delay should be short and it seems better to make
	 * checkpoint take a bit longer than to hold off insertions longer than
	 * necessary.
	 * (In fact, the whole reason we have this issue is that xact.c does
	 * commit record XLOG insertion and clog update as two separate steps
	 * protected by different locks, but again that seems best on grounds of
	 * minimizing lock contention.)
	 *
	 * A transaction that has not yet set delayChkpt when we look cannot be at
	 * risk, since he's not inserted his commit record yet; and one that's
	 * already cleared it is not at risk either, since he's done fixing clog
	 * and we will correctly flush the update below.  So we cannot miss any
	 * xacts we need to wait for.
	 */
	vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
	if (nvxids > 0)
	{
		do
		{
			pg_usleep(10000L);	/* wait for 10 msec */
		} while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
	}
	pfree(vxids);

	/*
	 * Get the other info we need for the checkpoint record.
	 */
	LWLockAcquire(XidGenLock, LW_SHARED);
	checkPoint.nextXid = ShmemVariableCache->nextXid;
	checkPoint.oldestXid = ShmemVariableCache->oldestXid;
	checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
	LWLockRelease(XidGenLock);

	/* Increase XID epoch if we've wrapped around since last checkpoint */
	checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
	if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
		checkPoint.nextXidEpoch++;

	LWLockAcquire(OidGenLock, LW_SHARED);
	checkPoint.nextOid = ShmemVariableCache->nextOid;
	if (!shutdown)
		checkPoint.nextOid += ShmemVariableCache->oidCount;
	LWLockRelease(OidGenLock);

	MultiXactGetCheckptMulti(shutdown,
							 &checkPoint.nextMulti,
							 &checkPoint.nextMultiOffset,
							 &checkPoint.oldestMulti,
							 &checkPoint.oldestMultiDB);

	/*
	 * Having constructed the checkpoint record, ensure all shmem disk buffers
	 * and commit-log buffers are flushed to disk.
	 *
	 * This I/O could fail for various reasons.  If so, we will fail to
	 * complete the checkpoint, but there is no reason to force a system
	 * panic. Accordingly, exit critical section while doing it.
	 */
	END_CRIT_SECTION();

	CheckPointGuts(checkPoint.redo, flags);

	/*
	 * Take a snapshot of running transactions and write this to WAL. This
	 * allows us to reconstruct the state of running transactions during
	 * archive recovery, if required. Skip, if this info disabled.
	 *
	 * If we are shutting down, or Startup process is completing crash
	 * recovery we don't need to write running xact data.
	 */
	if (!shutdown && XLogStandbyInfoActive())
		LogStandbySnapshot();

	START_CRIT_SECTION();

	/*
	 * Now insert the checkpoint record into XLOG.
	 */
	rdata.data = (char *) (&checkPoint);
	rdata.len = sizeof(checkPoint);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;

	recptr = XLogInsert(RM_XLOG_ID,
						shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
						XLOG_CHECKPOINT_ONLINE,
						&rdata);

	XLogFlush(recptr);

	/*
	 * We mustn't write any new WAL after a shutdown checkpoint, or it will be
	 * overwritten at next startup.  No-one should even try, this just allows
	 * sanity-checking.  In the case of an end-of-recovery checkpoint, we want
	 * to just temporarily disable writing until the system has exited
	 * recovery.
	 */
	if (shutdown)
	{
		if (flags & CHECKPOINT_END_OF_RECOVERY)
			LocalXLogInsertAllowed = -1;		/* return to "check" state */
		else
			LocalXLogInsertAllowed = 0; /* never again write WAL */
	}

	/*
	 * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
	 * = end of actual checkpoint record.
	 */
	if (shutdown && checkPoint.redo != ProcLastRecPtr)
		ereport(PANIC,
				(errmsg("concurrent transaction log activity while database system is shutting down")));

	/*
	 * Select point at which we can truncate the log, which we base on the
	 * prior checkpoint's earliest info.
	 */
	XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

	/*
	 * Update the control file.
	 */
	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
	if (shutdown)
		ControlFile->state = DB_SHUTDOWNED;
	ControlFile->prevCheckPoint = ControlFile->checkPoint;
	ControlFile->checkPoint = ProcLastRecPtr;
	ControlFile->checkPointCopy = checkPoint;
	ControlFile->time = (pg_time_t) time(NULL);
	/* crash recovery should always recover to the end of WAL */
	ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
	ControlFile->minRecoveryPointTLI = 0;

	/*
	 * Persist unloggedLSN value. It's reset on crash recovery, so this goes
	 * unused on non-shutdown checkpoints, but seems useful to store it always
	 * for debugging purposes.
	 */
	SpinLockAcquire(&XLogCtl->ulsn_lck);
	ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
	SpinLockRelease(&XLogCtl->ulsn_lck);

	UpdateControlFile();
	LWLockRelease(ControlFileLock);

	/* Update shared-memory copy of checkpoint XID/epoch */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
		xlogctl->ckptXid = checkPoint.nextXid;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/*
	 * We are now done with critical updates; no need for system panic if we
	 * have trouble while fooling with old log segments.
	 */
	END_CRIT_SECTION();

	/*
	 * Let smgr do post-checkpoint cleanup (eg, deleting old files).
	 */
	smgrpostckpt();

	/*
	 * Delete old log files (those no longer needed even for previous
	 * checkpoint or the standbys in XLOG streaming).
	 */
	if (_logSegNo)
	{
		KeepLogSeg(recptr, &_logSegNo);
		_logSegNo--;
		RemoveOldXlogFiles(_logSegNo, recptr);
	}

	/*
	 * Make more log segments if needed.  (Do this after recycling old log
	 * segments, since that may supply some of the needed files.)
	 */
	if (!shutdown)
		PreallocXlogFiles(recptr);

	/*
	 * Truncate pg_subtrans if possible.  We can throw away all data before
	 * the oldest XMIN of any running transaction.	No future transaction will
	 * attempt to reference any pg_subtrans entry older than that (see Asserts
	 * in subtrans.c).	During recovery, though, we mustn't do this because
	 * StartupSUBTRANS hasn't been called yet.
	 */
	if (!RecoveryInProgress())
		TruncateSUBTRANS(GetOldestXmin(true, false));

	/* Real work is done, but log and update stats before releasing lock. */
	LogCheckpointEnd(false);

	TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
									 NBuffers,
									 CheckpointStats.ckpt_segs_added,
									 CheckpointStats.ckpt_segs_removed,
									 CheckpointStats.ckpt_segs_recycled);

	LWLockRelease(CheckpointLock);
}

/*
 * Mark the end of recovery in WAL though without running a full checkpoint.
 * We can expect that a restartpoint is likely to be in progress as we
 * do this, though we are unwilling to wait for it to complete. So be
 * careful to avoid taking the CheckpointLock anywhere here.
 *
 * CreateRestartPoint() allows for the case where recovery may end before
 * the restartpoint completes so there is no concern of concurrent behaviour.
 */
void
CreateEndOfRecoveryRecord(void)
{
	xl_end_of_recovery xlrec;
	XLogRecData rdata;
	XLogRecPtr	recptr;

	/* sanity check */
	if (!RecoveryInProgress())
		elog(ERROR, "can only be used to end recovery");

	xlrec.end_time = time(NULL);

	WALInsertSlotAcquire(true);
	xlrec.ThisTimeLineID = ThisTimeLineID;
	xlrec.PrevTimeLineID = XLogCtl->PrevTimeLineID;
	WALInsertSlotRelease();

	LocalSetXLogInsertAllowed();

	START_CRIT_SECTION();

	rdata.data = (char *) &xlrec;
	rdata.len = sizeof(xl_end_of_recovery);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;

	recptr = XLogInsert(RM_XLOG_ID, XLOG_END_OF_RECOVERY, &rdata);

	XLogFlush(recptr);

	/*
	 * Update the control file so that crash recovery can follow the timeline
	 * changes to this point.
	 */
	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
	ControlFile->time = (pg_time_t) xlrec.end_time;
	ControlFile->minRecoveryPoint = recptr;
	ControlFile->minRecoveryPointTLI = ThisTimeLineID;
	UpdateControlFile();
	LWLockRelease(ControlFileLock);

	END_CRIT_SECTION();

	LocalXLogInsertAllowed = -1;	/* return to "check" state */
}

/*
 * Flush all data in shared memory to disk, and fsync
 *
 * This is the common code shared between regular checkpoints and
 * recovery restartpoints.
 */
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
	CheckPointCLOG();
	CheckPointSUBTRANS();
	CheckPointMultiXact();
	CheckPointPredicate();
	CheckPointRelationMap();
	CheckPointBuffers(flags);	/* performs all required fsyncs */
	/* We deliberately delay 2PC checkpointing as long as possible */
	CheckPointTwoPhase(checkPointRedo);
}

/*
 * Save a checkpoint for recovery restart if appropriate
 *
 * This function is called each time a checkpoint record is read from XLOG.
 * It must determine whether the checkpoint represents a safe restartpoint or
 * not.  If so, the checkpoint record is stashed in shared memory so that
 * CreateRestartPoint can consult it.  (Note that the latter function is
 * executed by the checkpointer, while this one will be executed by the
 * startup process.)
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
	int			rmid;

	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	/*
	 * Is it safe to restartpoint?	We must ask each of the resource managers
	 * whether they have any partial state information that might prevent a
	 * correct restart from this point.  If so, we skip this opportunity, but
	 * return at the next checkpoint record for another try.
	 */
	for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
	{
		if (RmgrTable[rmid].rm_safe_restartpoint != NULL)
			if (!(RmgrTable[rmid].rm_safe_restartpoint()))
			{
				elog(trace_recovery(DEBUG2),
					 "RM %d not safe to record restart point at %X/%X",
					 rmid,
					 (uint32) (checkPoint->redo >> 32),
					 (uint32) checkPoint->redo);
				return;
			}
	}

	/*
	 * Also refrain from creating a restartpoint if we have seen any
	 * references to non-existent pages. Restarting recovery from the
	 * restartpoint would not see the references, so we would lose the
	 * cross-check that the pages belonged to a relation that was dropped
	 * later.
	 */
	if (XLogHaveInvalidPages())
	{
		elog(trace_recovery(DEBUG2),
			 "could not record restart point at %X/%X because there "
			 "are unresolved references to invalid pages",
			 (uint32) (checkPoint->redo >> 32),
			 (uint32) checkPoint->redo);
		return;
	}

	/*
	 * Copy the checkpoint record to shared memory, so that checkpointer can
	 * work out the next time it wants to perform a restartpoint.
	 */
	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->lastCheckPointRecPtr = ReadRecPtr;
	xlogctl->lastCheckPoint = *checkPoint;
	SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Establish a restartpoint if possible.
 *
 * This is similar to CreateCheckPoint, but is used during WAL recovery
 * to establish a point from which recovery can roll forward without
 * replaying the entire recovery log.
 *
 * Returns true if a new restartpoint was established. We can only establish
 * a restartpoint if we have replayed a safe checkpoint record since last
 * restartpoint.
 */
bool
CreateRestartPoint(int flags)
{
	XLogRecPtr	lastCheckPointRecPtr;
	CheckPoint	lastCheckPoint;
	XLogSegNo	_logSegNo;
	TimestampTz xtime;

	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	/*
	 * Acquire CheckpointLock to ensure only one restartpoint or checkpoint
	 * happens at a time.
	 */
	LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

	/* Get a local copy of the last safe checkpoint record. */
	SpinLockAcquire(&xlogctl->info_lck);
	lastCheckPointRecPtr = xlogctl->lastCheckPointRecPtr;
	lastCheckPoint = xlogctl->lastCheckPoint;
	SpinLockRelease(&xlogctl->info_lck);

	/*
	 * Check that we're still in recovery mode. It's ok if we exit recovery
	 * mode after this check, the restart point is valid anyway.
	 */
	if (!RecoveryInProgress())
	{
		ereport(DEBUG2,
			  (errmsg("skipping restartpoint, recovery has already ended")));
		LWLockRelease(CheckpointLock);
		return false;
	}

	/*
	 * If the last checkpoint record we've replayed is already our last
	 * restartpoint, we can't perform a new restart point. We still update
	 * minRecoveryPoint in that case, so that if this is a shutdown restart
	 * point, we won't start up earlier than before. That's not strictly
	 * necessary, but when hot standby is enabled, it would be rather weird if
	 * the database opened up for read-only connections at a point-in-time
	 * before the last shutdown. Such time travel is still possible in case of
	 * immediate shutdown, though.
	 *
	 * We don't explicitly advance minRecoveryPoint when we do create a
	 * restartpoint. It's assumed that flushing the buffers will do that as a
	 * side-effect.
	 */
	if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||
		lastCheckPoint.redo <= ControlFile->checkPointCopy.redo)
	{
		ereport(DEBUG2,
				(errmsg("skipping restartpoint, already performed at %X/%X",
						(uint32) (lastCheckPoint.redo >> 32),
						(uint32) lastCheckPoint.redo)));

		UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
		if (flags & CHECKPOINT_IS_SHUTDOWN)
		{
			LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
			ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
			ControlFile->time = (pg_time_t) time(NULL);
			UpdateControlFile();
			LWLockRelease(ControlFileLock);
		}
		LWLockRelease(CheckpointLock);
		return false;
	}

	/*
	 * Update the shared RedoRecPtr so that the startup process can calculate
	 * the number of segments replayed since last restartpoint, and request a
	 * restartpoint if it exceeds checkpoint_segments.
	 *
	 * Like in CreateCheckPoint(), hold off insertions to update it, although
	 * during recovery this is just pro forma, because no WAL insertions are
	 * happening.
	 */
	WALInsertSlotAcquire(true);
	xlogctl->Insert.RedoRecPtr = lastCheckPoint.redo;
	WALInsertSlotRelease();

	/* Also update the info_lck-protected copy */
	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->RedoRecPtr = lastCheckPoint.redo;
	SpinLockRelease(&xlogctl->info_lck);

	/*
	 * Prepare to accumulate statistics.
	 *
	 * Note: because it is possible for log_checkpoints to change while a
	 * checkpoint proceeds, we always accumulate stats, even if
	 * log_checkpoints is currently off.
	 */
	MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
	CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

	if (log_checkpoints)
		LogCheckpointStart(flags, true);

	CheckPointGuts(lastCheckPoint.redo, flags);

	/*
	 * Select point at which we can truncate the xlog, which we base on the
	 * prior checkpoint's earliest info.
	 */
	XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

	/*
	 * Update pg_control, using current time.  Check that it still shows
	 * IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
	 * this is a quick hack to make sure nothing really bad happens if somehow
	 * we get here after the end-of-recovery checkpoint.
	 */
	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
	if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
		ControlFile->checkPointCopy.redo < lastCheckPoint.redo)
	{
		ControlFile->prevCheckPoint = ControlFile->checkPoint;
		ControlFile->checkPoint = lastCheckPointRecPtr;
		ControlFile->checkPointCopy = lastCheckPoint;
		ControlFile->time = (pg_time_t) time(NULL);
		if (flags & CHECKPOINT_IS_SHUTDOWN)
			ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
		UpdateControlFile();
	}
	LWLockRelease(ControlFileLock);

	/*
	 * Delete old log files (those no longer needed even for previous
	 * checkpoint/restartpoint) to prevent the disk holding the xlog from
	 * growing full.
	 */
	if (_logSegNo)
	{
		XLogRecPtr	receivePtr;
		XLogRecPtr	replayPtr;
		TimeLineID	replayTLI;
		XLogRecPtr	endptr;

		/*
		 * Get the current end of xlog replayed or received, whichever is
		 * later.
		 */
		receivePtr = GetWalRcvWriteRecPtr(NULL, NULL);
		replayPtr = GetXLogReplayRecPtr(&replayTLI);
		endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr;

		KeepLogSeg(endptr, &_logSegNo);
		_logSegNo--;

		/*
		 * Try to recycle segments on a useful timeline. If we've been promoted
		 * since the beginning of this restartpoint, use the new timeline
		 * chosen at end of recovery (RecoveryInProgress() sets ThisTimeLineID
		 * in that case). If we're still in recovery, use the timeline we're
		 * currently replaying.
		 *
		 * There is no guarantee that the WAL segments will be useful on the
		 * current timeline; if recovery proceeds to a new timeline right
		 * after this, the pre-allocated WAL segments on this timeline will
		 * not be used, and will go wasted until recycled on the next
		 * restartpoint. We'll live with that.
		 */
		if (RecoveryInProgress())
			ThisTimeLineID = replayTLI;

		RemoveOldXlogFiles(_logSegNo, endptr);

		/*
		 * Make more log segments if needed.  (Do this after recycling old log
		 * segments, since that may supply some of the needed files.)
		 */
		PreallocXlogFiles(endptr);

		/*
		 * ThisTimeLineID is normally not set when we're still in recovery.
		 * However, recycling/preallocating segments above needed
		 * ThisTimeLineID to determine which timeline to install the segments
		 * on. Reset it now, to restore the normal state of affairs for
		 * debugging purposes.
		 */
		if (RecoveryInProgress())
			ThisTimeLineID = 0;
	}

	/*
	 * Truncate pg_subtrans if possible.  We can throw away all data before
	 * the oldest XMIN of any running transaction.	No future transaction will
	 * attempt to reference any pg_subtrans entry older than that (see Asserts
	 * in subtrans.c).	When hot standby is disabled, though, we mustn't do
	 * this because StartupSUBTRANS hasn't been called yet.
	 */
	if (EnableHotStandby)
		TruncateSUBTRANS(GetOldestXmin(true, false));

	/* Real work is done, but log and update before releasing lock. */
	LogCheckpointEnd(true);

	xtime = GetLatestXTime();
	ereport((log_checkpoints ? LOG : DEBUG2),
			(errmsg("recovery restart point at %X/%X",
		 (uint32) (lastCheckPoint.redo >> 32), (uint32) lastCheckPoint.redo),
		   xtime ? errdetail("last completed transaction was at log time %s",
							 timestamptz_to_str(xtime)) : 0));

	LWLockRelease(CheckpointLock);

	/*
	 * Finally, execute archive_cleanup_command, if any.
	 */
	if (XLogCtl->archiveCleanupCommand[0])
		ExecuteRecoveryCommand(XLogCtl->archiveCleanupCommand,
							   "archive_cleanup_command",
							   false);

	return true;
}

/*
 * Retreat *logSegNo to the last segment that we need to retain because of
 * wal_keep_segments. This is calculated by subtracting wal_keep_segments
 * from the given xlog location, recptr.
 */
static void
KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
{
	XLogSegNo	segno;

	if (wal_keep_segments == 0)
		return;

	XLByteToSeg(recptr, segno);

	/* avoid underflow, don't go below 1 */
	if (segno <= wal_keep_segments)
		segno = 1;
	else
		segno = segno - wal_keep_segments;

	/* don't delete WAL segments newer than the calculated segment */
	if (segno < *logSegNo)
		*logSegNo = segno;
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
	XLogRecData rdata;

	rdata.data = (char *) (&nextOid);
	rdata.len = sizeof(Oid);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;
	(void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID, &rdata);

	/*
	 * We need not flush the NEXTOID record immediately, because any of the
	 * just-allocated OIDs could only reach disk as part of a tuple insert or
	 * update that would have its own XLOG record that must follow the NEXTOID
	 * record.	Therefore, the standard buffer LSN interlock applied to those
	 * records will ensure no such OID reaches disk before the NEXTOID record
	 * does.
	 *
	 * Note, however, that the above statement only covers state "within" the
	 * database.  When we use a generated OID as a file or directory name, we
	 * are in a sense violating the basic WAL rule, because that filesystem
	 * change may reach disk before the NEXTOID WAL record does.  The impact
	 * of this is that if a database crash occurs immediately afterward, we
	 * might after restart re-generate the same OID and find that it conflicts
	 * with the leftover file or directory.  But since for safety's sake we
	 * always loop until finding a nonconflicting filename, this poses no real
	 * problem in practice. See pgsql-hackers discussion 27-Sep-2006.
	 */
}

/*
 * Write an XLOG SWITCH record.
 *
 * Here we just blindly issue an XLogInsert request for the record.
 * All the magic happens inside XLogInsert.
 *
 * The return value is either the end+1 address of the switch record,
 * or the end+1 address of the prior segment if we did not need to
 * write a switch record because we are already at segment start.
 */
XLogRecPtr
RequestXLogSwitch(void)
{
	XLogRecPtr	RecPtr;
	XLogRecData rdata;

	/* XLOG SWITCH, alone among xlog record types, has no data */
	rdata.buffer = InvalidBuffer;
	rdata.data = NULL;
	rdata.len = 0;
	rdata.next = NULL;

	RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH, &rdata);

	return RecPtr;
}

/*
 * Write a RESTORE POINT record
 */
XLogRecPtr
XLogRestorePoint(const char *rpName)
{
	XLogRecPtr	RecPtr;
	XLogRecData rdata;
	xl_restore_point xlrec;

	xlrec.rp_time = GetCurrentTimestamp();
	strncpy(xlrec.rp_name, rpName, MAXFNAMELEN);

	rdata.buffer = InvalidBuffer;
	rdata.data = (char *) &xlrec;
	rdata.len = sizeof(xl_restore_point);
	rdata.next = NULL;

	RecPtr = XLogInsert(RM_XLOG_ID, XLOG_RESTORE_POINT, &rdata);

	ereport(LOG,
			(errmsg("restore point \"%s\" created at %X/%X",
					rpName, (uint32) (RecPtr >> 32), (uint32) RecPtr)));

	return RecPtr;
}

/*
 * Write a backup block if needed when we are setting a hint. Note that
 * this may be called for a variety of page types, not just heaps.
 *
 * Callable while holding just share lock on the buffer content.
 *
 * We can't use the plain backup block mechanism since that relies on the
 * Buffer being exclusively locked. Since some modifications (setting LSN, hint
 * bits) are allowed in a sharelocked buffer that can lead to wal checksum
 * failures. So instead we copy the page and insert the copied data as normal
 * record data.
 *
 * We only need to do something if page has not yet been full page written in
 * this checkpoint round. The LSN of the inserted wal record is returned if we
 * had to write, InvalidXLogRecPtr otherwise.
 *
 * It is possible that multiple concurrent backends could attempt to write WAL
 * records. In that case, multiple copies of the same block would be recorded
 * in separate WAL records by different backends, though that is still OK from
 * a correctness perspective.
 */
XLogRecPtr
XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
{
	XLogRecPtr	recptr = InvalidXLogRecPtr;
	XLogRecPtr	lsn;
	XLogRecData rdata[2];
	BkpBlock	bkpb;

	/*
	 * Ensure no checkpoint can change our view of RedoRecPtr.
	 */
	Assert(MyPgXact->delayChkpt);

	/*
	 * Update RedoRecPtr so XLogCheckBuffer can make the right decision
	 */
	GetRedoRecPtr();

	/*
	 * Setup phony rdata element for use within XLogCheckBuffer only. We reuse
	 * and reset rdata for any actual WAL record insert.
	 */
	rdata[0].buffer = buffer;
	rdata[0].buffer_std = buffer_std;

	/*
	 * Check buffer while not holding an exclusive lock.
	 */
	if (XLogCheckBuffer(rdata, false, &lsn, &bkpb))
	{
		char		copied_buffer[BLCKSZ];
		char	   *origdata = (char *) BufferGetBlock(buffer);

		/*
		 * Copy buffer so we don't have to worry about concurrent hint bit or
		 * lsn updates. We assume pd_lower/upper cannot be changed without an
		 * exclusive lock, so the contents bkp are not racy.
		 *
		 * With buffer_std set to false, XLogCheckBuffer() sets hole_length and
		 * hole_offset to 0; so the following code is safe for either case.
		 */
		memcpy(copied_buffer, origdata, bkpb.hole_offset);
		memcpy(copied_buffer + bkpb.hole_offset,
			   origdata + bkpb.hole_offset + bkpb.hole_length,
			   BLCKSZ - bkpb.hole_offset - bkpb.hole_length);

		/*
		 * Header for backup block.
		 */
		rdata[0].data = (char *) &bkpb;
		rdata[0].len = sizeof(BkpBlock);
		rdata[0].buffer = InvalidBuffer;
		rdata[0].next = &(rdata[1]);

		/*
		 * Save copy of the buffer.
		 */
		rdata[1].data = copied_buffer;
		rdata[1].len = BLCKSZ - bkpb.hole_length;
		rdata[1].buffer = InvalidBuffer;
		rdata[1].next = NULL;

		recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI, rdata);
	}

	return recptr;
}

/*
 * Check if any of the GUC parameters that are critical for hot standby
 * have changed, and update the value in pg_control file if necessary.
 */
static void
XLogReportParameters(void)
{
	if (wal_level != ControlFile->wal_level ||
		MaxConnections != ControlFile->MaxConnections ||
		max_worker_processes != ControlFile->max_worker_processes ||
		max_prepared_xacts != ControlFile->max_prepared_xacts ||
		max_locks_per_xact != ControlFile->max_locks_per_xact)
	{
		/*
		 * The change in number of backend slots doesn't need to be WAL-logged
		 * if archiving is not enabled, as you can't start archive recovery
		 * with wal_level=minimal anyway. We don't really care about the
		 * values in pg_control either if wal_level=minimal, but seems better
		 * to keep them up-to-date to avoid confusion.
		 */
		if (wal_level != ControlFile->wal_level || XLogIsNeeded())
		{
			XLogRecData rdata;
			xl_parameter_change xlrec;

			xlrec.MaxConnections = MaxConnections;
			xlrec.max_worker_processes = max_worker_processes;
			xlrec.max_prepared_xacts = max_prepared_xacts;
			xlrec.max_locks_per_xact = max_locks_per_xact;
			xlrec.wal_level = wal_level;

			rdata.buffer = InvalidBuffer;
			rdata.data = (char *) &xlrec;
			rdata.len = sizeof(xlrec);
			rdata.next = NULL;

			XLogInsert(RM_XLOG_ID, XLOG_PARAMETER_CHANGE, &rdata);
		}

		ControlFile->MaxConnections = MaxConnections;
		ControlFile->max_worker_processes = max_worker_processes;
		ControlFile->max_prepared_xacts = max_prepared_xacts;
		ControlFile->max_locks_per_xact = max_locks_per_xact;
		ControlFile->wal_level = wal_level;
		UpdateControlFile();
	}
}

/*
 * Update full_page_writes in shared memory, and write an
 * XLOG_FPW_CHANGE record if necessary.
 *
 * Note: this function assumes there is no other process running
 * concurrently that could update it.
 */
void
UpdateFullPageWrites(void)
{
	XLogCtlInsert *Insert = &XLogCtl->Insert;

	/*
	 * Do nothing if full_page_writes has not been changed.
	 *
	 * It's safe to check the shared full_page_writes without the lock,
	 * because we assume that there is no concurrently running process which
	 * can update it.
	 */
	if (fullPageWrites == Insert->fullPageWrites)
		return;

	START_CRIT_SECTION();

	/*
	 * It's always safe to take full page images, even when not strictly
	 * required, but not the other round. So if we're setting full_page_writes
	 * to true, first set it true and then write the WAL record. If we're
	 * setting it to false, first write the WAL record and then set the global
	 * flag.
	 */
	if (fullPageWrites)
	{
		WALInsertSlotAcquire(true);
		Insert->fullPageWrites = true;
		WALInsertSlotRelease();
	}

	/*
	 * Write an XLOG_FPW_CHANGE record. This allows us to keep track of
	 * full_page_writes during archive recovery, if required.
	 */
	if (XLogStandbyInfoActive() && !RecoveryInProgress())
	{
		XLogRecData rdata;

		rdata.data = (char *) (&fullPageWrites);
		rdata.len = sizeof(bool);
		rdata.buffer = InvalidBuffer;
		rdata.next = NULL;

		XLogInsert(RM_XLOG_ID, XLOG_FPW_CHANGE, &rdata);
	}

	if (!fullPageWrites)
	{
		WALInsertSlotAcquire(true);
		Insert->fullPageWrites = false;
		WALInsertSlotRelease();
	}
	END_CRIT_SECTION();
}

/*
 * Check that it's OK to switch to new timeline during recovery.
 *
 * 'lsn' is the address of the shutdown checkpoint record we're about to
 * replay. (Currently, timeline can only change at a shutdown checkpoint).
 */
static void
checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI, TimeLineID prevTLI)
{
	/* Check that the record agrees on what the current (old) timeline is */
	if (prevTLI != ThisTimeLineID)
		ereport(PANIC,
				(errmsg("unexpected prev timeline ID %u (current timeline ID %u) in checkpoint record",
						prevTLI, ThisTimeLineID)));

	/*
	 * The new timeline better be in the list of timelines we expect to see,
	 * according to the timeline history. It should also not decrease.
	 */
	if (newTLI < ThisTimeLineID || !tliInHistory(newTLI, expectedTLEs))
		ereport(PANIC,
		 (errmsg("unexpected timeline ID %u (after %u) in checkpoint record",
				 newTLI, ThisTimeLineID)));

	/*
	 * If we have not yet reached min recovery point, and we're about to
	 * switch to a timeline greater than the timeline of the min recovery
	 * point: trouble. After switching to the new timeline, we could not
	 * possibly visit the min recovery point on the correct timeline anymore.
	 * This can happen if there is a newer timeline in the archive that
	 * branched before the timeline the min recovery point is on, and you
	 * attempt to do PITR to the new timeline.
	 */
	if (!XLogRecPtrIsInvalid(minRecoveryPoint) &&
		lsn < minRecoveryPoint &&
		newTLI > minRecoveryPointTLI)
		ereport(PANIC,
				(errmsg("unexpected timeline ID %u in checkpoint record, before reaching minimum recovery point %X/%X on timeline %u",
						newTLI,
						(uint32) (minRecoveryPoint >> 32),
						(uint32) minRecoveryPoint,
						minRecoveryPointTLI)));

	/* Looks good */
}

/*
 * XLOG resource manager's routines
 *
 * Definitions of info values are in include/catalog/pg_control.h, though
 * not all record types are related to control file updates.
 */
void
xlog_redo(XLogRecPtr lsn, XLogRecord *record)
{
	uint8		info = record->xl_info & ~XLR_INFO_MASK;

	/* Backup blocks are not used by XLOG rmgr */
	Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

	if (info == XLOG_NEXTOID)
	{
		Oid			nextOid;

		/*
		 * We used to try to take the maximum of ShmemVariableCache->nextOid
		 * and the recorded nextOid, but that fails if the OID counter wraps
		 * around.	Since no OID allocation should be happening during replay
		 * anyway, better to just believe the record exactly.  We still take
		 * OidGenLock while setting the variable, just in case.
		 */
		memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
		LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
		ShmemVariableCache->nextOid = nextOid;
		ShmemVariableCache->oidCount = 0;
		LWLockRelease(OidGenLock);
	}
	else if (info == XLOG_CHECKPOINT_SHUTDOWN)
	{
		CheckPoint	checkPoint;

		memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
		/* In a SHUTDOWN checkpoint, believe the counters exactly */
		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
		ShmemVariableCache->nextXid = checkPoint.nextXid;
		LWLockRelease(XidGenLock);
		LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
		ShmemVariableCache->nextOid = checkPoint.nextOid;
		ShmemVariableCache->oidCount = 0;
		LWLockRelease(OidGenLock);
		MultiXactSetNextMXact(checkPoint.nextMulti,
							  checkPoint.nextMultiOffset);
		SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
		SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);

		/*
		 * If we see a shutdown checkpoint while waiting for an end-of-backup
		 * record, the backup was canceled and the end-of-backup record will
		 * never arrive.
		 */
		if (ArchiveRecoveryRequested &&
			!XLogRecPtrIsInvalid(ControlFile->backupStartPoint) &&
			XLogRecPtrIsInvalid(ControlFile->backupEndPoint))
			ereport(PANIC,
			(errmsg("online backup was canceled, recovery cannot continue")));

		/*
		 * If we see a shutdown checkpoint, we know that nothing was running
		 * on the master at this point. So fake-up an empty running-xacts
		 * record and use that here and now. Recover additional standby state
		 * for prepared transactions.
		 */
		if (standbyState >= STANDBY_INITIALIZED)
		{
			TransactionId *xids;
			int			nxids;
			TransactionId oldestActiveXID;
			TransactionId latestCompletedXid;
			RunningTransactionsData running;

			oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);

			/*
			 * Construct a RunningTransactions snapshot representing a shut
			 * down server, with only prepared transactions still alive. We're
			 * never overflowed at this point because all subxids are listed
			 * with their parent prepared transactions.
			 */
			running.xcnt = nxids;
			running.subxcnt = 0;
			running.subxid_overflow = false;
			running.nextXid = checkPoint.nextXid;
			running.oldestRunningXid = oldestActiveXID;
			latestCompletedXid = checkPoint.nextXid;
			TransactionIdRetreat(latestCompletedXid);
			Assert(TransactionIdIsNormal(latestCompletedXid));
			running.latestCompletedXid = latestCompletedXid;
			running.xids = xids;

			ProcArrayApplyRecoveryInfo(&running);

			StandbyRecoverPreparedTransactions(true);
		}

		/* ControlFile->checkPointCopy always tracks the latest ckpt XID */
		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
		ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

		/* Update shared-memory copy of checkpoint XID/epoch */
		{
			/* use volatile pointer to prevent code rearrangement */
			volatile XLogCtlData *xlogctl = XLogCtl;

			SpinLockAcquire(&xlogctl->info_lck);
			xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
			xlogctl->ckptXid = checkPoint.nextXid;
			SpinLockRelease(&xlogctl->info_lck);
		}

		/*
		 * We should've already switched to the new TLI before replaying this
		 * record.
		 */
		if (checkPoint.ThisTimeLineID != ThisTimeLineID)
			ereport(PANIC,
					(errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
							checkPoint.ThisTimeLineID, ThisTimeLineID)));

		RecoveryRestartPoint(&checkPoint);
	}
	else if (info == XLOG_CHECKPOINT_ONLINE)
	{
		CheckPoint	checkPoint;

		memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
		/* In an ONLINE checkpoint, treat the XID counter as a minimum */
		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
		if (TransactionIdPrecedes(ShmemVariableCache->nextXid,
								  checkPoint.nextXid))
			ShmemVariableCache->nextXid = checkPoint.nextXid;
		LWLockRelease(XidGenLock);
		/* ... but still treat OID counter as exact */
		LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
		ShmemVariableCache->nextOid = checkPoint.nextOid;
		ShmemVariableCache->oidCount = 0;
		LWLockRelease(OidGenLock);
		MultiXactAdvanceNextMXact(checkPoint.nextMulti,
								  checkPoint.nextMultiOffset);
		if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
								  checkPoint.oldestXid))
			SetTransactionIdLimit(checkPoint.oldestXid,
								  checkPoint.oldestXidDB);
		MultiXactAdvanceOldest(checkPoint.oldestMulti,
							   checkPoint.oldestMultiDB);

		/* ControlFile->checkPointCopy always tracks the latest ckpt XID */
		ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
		ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

		/* Update shared-memory copy of checkpoint XID/epoch */
		{
			/* use volatile pointer to prevent code rearrangement */
			volatile XLogCtlData *xlogctl = XLogCtl;

			SpinLockAcquire(&xlogctl->info_lck);
			xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
			xlogctl->ckptXid = checkPoint.nextXid;
			SpinLockRelease(&xlogctl->info_lck);
		}

		/* TLI should not change in an on-line checkpoint */
		if (checkPoint.ThisTimeLineID != ThisTimeLineID)
			ereport(PANIC,
					(errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
							checkPoint.ThisTimeLineID, ThisTimeLineID)));

		RecoveryRestartPoint(&checkPoint);
	}
	else if (info == XLOG_END_OF_RECOVERY)
	{
		xl_end_of_recovery xlrec;

		memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));

		/*
		 * For Hot Standby, we could treat this like a Shutdown Checkpoint,
		 * but this case is rarer and harder to test, so the benefit doesn't
		 * outweigh the potential extra cost of maintenance.
		 */

		/*
		 * We should've already switched to the new TLI before replaying this
		 * record.
		 */
		if (xlrec.ThisTimeLineID != ThisTimeLineID)
			ereport(PANIC,
					(errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
							xlrec.ThisTimeLineID, ThisTimeLineID)));
	}
	else if (info == XLOG_NOOP)
	{
		/* nothing to do here */
	}
	else if (info == XLOG_SWITCH)
	{
		/* nothing to do here */
	}
	else if (info == XLOG_RESTORE_POINT)
	{
		/* nothing to do here */
	}
	else if (info == XLOG_FPI)
	{
		char	   *data;
		BkpBlock	bkpb;

		/*
		 * Full-page image (FPI) records contain a backup block stored "inline"
		 * in the normal data since the locking when writing hint records isn't
		 * sufficient to use the normal backup block mechanism, which assumes
		 * exclusive lock on the buffer supplied.
		 *
		 * Since the only change in these backup block are hint bits, there
		 * are no recovery conflicts generated.
		 *
		 * This also means there is no corresponding API call for this, so an
		 * smgr implementation has no need to implement anything. Which means
		 * nothing is needed in md.c etc
		 */
		data = XLogRecGetData(record);
		memcpy(&bkpb, data, sizeof(BkpBlock));
		data += sizeof(BkpBlock);

		RestoreBackupBlockContents(lsn, bkpb, data, false, false);
	}
	else if (info == XLOG_BACKUP_END)
	{
		XLogRecPtr	startpoint;

		memcpy(&startpoint, XLogRecGetData(record), sizeof(startpoint));

		if (ControlFile->backupStartPoint == startpoint)
		{
			/*
			 * We have reached the end of base backup, the point where
			 * pg_stop_backup() was done. The data on disk is now consistent.
			 * Reset backupStartPoint, and update minRecoveryPoint to make
			 * sure we don't allow starting up at an earlier point even if
			 * recovery is stopped and restarted soon after this.
			 */
			elog(DEBUG1, "end of backup reached");

			LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

			if (ControlFile->minRecoveryPoint < lsn)
			{
				ControlFile->minRecoveryPoint = lsn;
				ControlFile->minRecoveryPointTLI = ThisTimeLineID;
			}
			ControlFile->backupStartPoint = InvalidXLogRecPtr;
			ControlFile->backupEndRequired = false;
			UpdateControlFile();

			LWLockRelease(ControlFileLock);
		}
	}
	else if (info == XLOG_PARAMETER_CHANGE)
	{
		xl_parameter_change xlrec;

		/* Update our copy of the parameters in pg_control */
		memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_parameter_change));

		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
		ControlFile->MaxConnections = xlrec.MaxConnections;
		ControlFile->max_worker_processes = xlrec.max_worker_processes;
		ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts;
		ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
		ControlFile->wal_level = xlrec.wal_level;

		/*
		 * Update minRecoveryPoint to ensure that if recovery is aborted, we
		 * recover back up to this point before allowing hot standby again.
		 * This is particularly important if wal_level was set to 'archive'
		 * before, and is now 'hot_standby', to ensure you don't run queries
		 * against the WAL preceding the wal_level change. Same applies to
		 * decreasing max_* settings.
		 */
		minRecoveryPoint = ControlFile->minRecoveryPoint;
		minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
		if (minRecoveryPoint != 0 && minRecoveryPoint < lsn)
		{
			ControlFile->minRecoveryPoint = lsn;
			ControlFile->minRecoveryPointTLI = ThisTimeLineID;
		}

		UpdateControlFile();
		LWLockRelease(ControlFileLock);

		/* Check to see if any changes to max_connections give problems */
		CheckRequiredParameterValues();
	}
	else if (info == XLOG_FPW_CHANGE)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;
		bool		fpw;

		memcpy(&fpw, XLogRecGetData(record), sizeof(bool));

		/*
		 * Update the LSN of the last replayed XLOG_FPW_CHANGE record so that
		 * do_pg_start_backup() and do_pg_stop_backup() can check whether
		 * full_page_writes has been disabled during online backup.
		 */
		if (!fpw)
		{
			SpinLockAcquire(&xlogctl->info_lck);
			if (xlogctl->lastFpwDisableRecPtr < ReadRecPtr)
				xlogctl->lastFpwDisableRecPtr = ReadRecPtr;
			SpinLockRelease(&xlogctl->info_lck);
		}

		/* Keep track of full_page_writes */
		lastFullPageWrites = fpw;
	}
}

#ifdef WAL_DEBUG

static void
xlog_outrec(StringInfo buf, XLogRecord *record)
{
	int			i;

	appendStringInfo(buf, "prev %X/%X; xid %u",
					 (uint32) (record->xl_prev >> 32),
					 (uint32) record->xl_prev,
					 record->xl_xid);

	appendStringInfo(buf, "; len %u",
					 record->xl_len);

	for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
	{
		if (record->xl_info & XLR_BKP_BLOCK(i))
			appendStringInfo(buf, "; bkpb%d", i);
	}

	appendStringInfo(buf, ": %s", RmgrTable[record->xl_rmid].rm_name);
}
#endif   /* WAL_DEBUG */


/*
 * Return the (possible) sync flag used for opening a file, depending on the
 * value of the GUC wal_sync_method.
 */
static int
get_sync_bit(int method)
{
	int			o_direct_flag = 0;

	/* If fsync is disabled, never open in sync mode */
	if (!enableFsync)
		return 0;

	/*
	 * Optimize writes by bypassing kernel cache with O_DIRECT when using
	 * O_SYNC/O_FSYNC and O_DSYNC.	But only if archiving and streaming are
	 * disabled, otherwise the archive command or walsender process will read
	 * the WAL soon after writing it, which is guaranteed to cause a physical
	 * read if we bypassed the kernel cache. We also skip the
	 * posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
	 * reason.
	 *
	 * Never use O_DIRECT in walreceiver process for similar reasons; the WAL
	 * written by walreceiver is normally read by the startup process soon
	 * after its written. Also, walreceiver performs unaligned writes, which
	 * don't work with O_DIRECT, so it is required for correctness too.
	 */
	if (!XLogIsNeeded() && !AmWalReceiverProcess())
		o_direct_flag = PG_O_DIRECT;

	switch (method)
	{
			/*
			 * enum values for all sync options are defined even if they are
			 * not supported on the current platform.  But if not, they are
			 * not included in the enum option array, and therefore will never
			 * be seen here.
			 */
		case SYNC_METHOD_FSYNC:
		case SYNC_METHOD_FSYNC_WRITETHROUGH:
		case SYNC_METHOD_FDATASYNC:
			return 0;
#ifdef OPEN_SYNC_FLAG
		case SYNC_METHOD_OPEN:
			return OPEN_SYNC_FLAG | o_direct_flag;
#endif
#ifdef OPEN_DATASYNC_FLAG
		case SYNC_METHOD_OPEN_DSYNC:
			return OPEN_DATASYNC_FLAG | o_direct_flag;
#endif
		default:
			/* can't happen (unless we are out of sync with option array) */
			elog(ERROR, "unrecognized wal_sync_method: %d", method);
			return 0;			/* silence warning */
	}
}

/*
 * GUC support
 */
void
assign_xlog_sync_method(int new_sync_method, void *extra)
{
	if (sync_method != new_sync_method)
	{
		/*
		 * To ensure that no blocks escape unsynced, force an fsync on the
		 * currently open log segment (if any).  Also, if the open flag is
		 * changing, close the log file so it will be reopened (with new flag
		 * bit) at next use.
		 */
		if (openLogFile >= 0)
		{
			if (pg_fsync(openLogFile) != 0)
				ereport(PANIC,
						(errcode_for_file_access(),
						 errmsg("could not fsync log segment %s: %m",
							  XLogFileNameP(ThisTimeLineID, openLogSegNo))));
			if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
				XLogFileClose();
		}
	}
}


/*
 * Issue appropriate kind of fsync (if any) for an XLOG output file.
 *
 * 'fd' is a file descriptor for the XLOG file to be fsync'd.
 * 'log' and 'seg' are for error reporting purposes.
 */
void
issue_xlog_fsync(int fd, XLogSegNo segno)
{
	switch (sync_method)
	{
		case SYNC_METHOD_FSYNC:
			if (pg_fsync_no_writethrough(fd) != 0)
				ereport(PANIC,
						(errcode_for_file_access(),
						 errmsg("could not fsync log file %s: %m",
								XLogFileNameP(ThisTimeLineID, segno))));
			break;
#ifdef HAVE_FSYNC_WRITETHROUGH
		case SYNC_METHOD_FSYNC_WRITETHROUGH:
			if (pg_fsync_writethrough(fd) != 0)
				ereport(PANIC,
						(errcode_for_file_access(),
					  errmsg("could not fsync write-through log file %s: %m",
							 XLogFileNameP(ThisTimeLineID, segno))));
			break;
#endif
#ifdef HAVE_FDATASYNC
		case SYNC_METHOD_FDATASYNC:
			if (pg_fdatasync(fd) != 0)
				ereport(PANIC,
						(errcode_for_file_access(),
						 errmsg("could not fdatasync log file %s: %m",
								XLogFileNameP(ThisTimeLineID, segno))));
			break;
#endif
		case SYNC_METHOD_OPEN:
		case SYNC_METHOD_OPEN_DSYNC:
			/* write synced it already */
			break;
		default:
			elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
			break;
	}
}

/*
 * Return the filename of given log segment, as a palloc'd string.
 */
char *
XLogFileNameP(TimeLineID tli, XLogSegNo segno)
{
	char	   *result = palloc(MAXFNAMELEN);

	XLogFileName(result, tli, segno);
	return result;
}

/*
 * do_pg_start_backup is the workhorse of the user-visible pg_start_backup()
 * function. It creates the necessary starting checkpoint and constructs the
 * backup label file.
 *
 * There are two kind of backups: exclusive and non-exclusive. An exclusive
 * backup is started with pg_start_backup(), and there can be only one active
 * at a time. The backup label file of an exclusive backup is written to
 * $PGDATA/backup_label, and it is removed by pg_stop_backup().
 *
 * A non-exclusive backup is used for the streaming base backups (see
 * src/backend/replication/basebackup.c). The difference to exclusive backups
 * is that the backup label file is not written to disk. Instead, its would-be
 * contents are returned in *labelfile, and the caller is responsible for
 * including it in the backup archive as 'backup_label'. There can be many
 * non-exclusive backups active at the same time, and they don't conflict
 * with an exclusive backup either.
 *
 * Returns the minimum WAL position that must be present to restore from this
 * backup, and the corresponding timeline ID in *starttli_p.
 *
 * Every successfully started non-exclusive backup must be stopped by calling
 * do_pg_stop_backup() or do_pg_abort_backup().
 */
XLogRecPtr
do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
				   char **labelfile)
{
	bool		exclusive = (labelfile == NULL);
	bool		backup_started_in_recovery = false;
	XLogRecPtr	checkpointloc;
	XLogRecPtr	startpoint;
	TimeLineID	starttli;
	pg_time_t	stamp_time;
	char		strfbuf[128];
	char		xlogfilename[MAXFNAMELEN];
	XLogSegNo	_logSegNo;
	struct stat stat_buf;
	FILE	   *fp;
	StringInfoData labelfbuf;

	backup_started_in_recovery = RecoveryInProgress();

	if (!superuser() && !has_rolreplication(GetUserId()))
		ereport(ERROR,
				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
		   errmsg("must be superuser or replication role to run a backup")));

	/*
	 * Currently only non-exclusive backup can be taken during recovery.
	 */
	if (backup_started_in_recovery && exclusive)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("recovery is in progress"),
				 errhint("WAL control functions cannot be executed during recovery.")));

	/*
	 * During recovery, we don't need to check WAL level. Because, if WAL
	 * level is not sufficient, it's impossible to get here during recovery.
	 */
	if (!backup_started_in_recovery && !XLogIsNeeded())
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
			  errmsg("WAL level not sufficient for making an online backup"),
				 errhint("wal_level must be set to \"archive\" or \"hot_standby\" at server start.")));

	if (strlen(backupidstr) > MAXPGPATH)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
				 errmsg("backup label too long (max %d bytes)",
						MAXPGPATH)));

	/*
	 * Mark backup active in shared memory.  We must do full-page WAL writes
	 * during an on-line backup even if not doing so at other times, because
	 * it's quite possible for the backup dump to obtain a "torn" (partially
	 * written) copy of a database page if it reads the page concurrently with
	 * our write to the same page.	This can be fixed as long as the first
	 * write to the page in the WAL sequence is a full-page write. Hence, we
	 * turn on forcePageWrites and then force a CHECKPOINT, to ensure there
	 * are no dirty pages in shared memory that might get dumped while the
	 * backup is in progress without having a corresponding WAL record.  (Once
	 * the backup is complete, we need not force full-page writes anymore,
	 * since we expect that any pages not modified during the backup interval
	 * must have been correctly captured by the backup.)
	 *
	 * Note that forcePageWrites has no effect during an online backup from
	 * the standby.
	 *
	 * We must hold all the insertion slots to change the value of
	 * forcePageWrites, to ensure adequate interlocking against XLogInsert().
	 */
	WALInsertSlotAcquire(true);
	if (exclusive)
	{
		if (XLogCtl->Insert.exclusiveBackup)
		{
			WALInsertSlotRelease();
			ereport(ERROR,
					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
					 errmsg("a backup is already in progress"),
					 errhint("Run pg_stop_backup() and try again.")));
		}
		XLogCtl->Insert.exclusiveBackup = true;
	}
	else
		XLogCtl->Insert.nonExclusiveBackups++;
	XLogCtl->Insert.forcePageWrites = true;
	WALInsertSlotRelease();

	/* Ensure we release forcePageWrites if fail below */
	PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
	{
		bool		gotUniqueStartpoint = false;

		/*
		 * Force an XLOG file switch before the checkpoint, to ensure that the
		 * WAL segment the checkpoint is written to doesn't contain pages with
		 * old timeline IDs.  That would otherwise happen if you called
		 * pg_start_backup() right after restoring from a PITR archive: the
		 * first WAL segment containing the startup checkpoint has pages in
		 * the beginning with the old timeline ID.	That can cause trouble at
		 * recovery: we won't have a history file covering the old timeline if
		 * pg_xlog directory was not included in the base backup and the WAL
		 * archive was cleared too before starting the backup.
		 *
		 * This also ensures that we have emitted a WAL page header that has
		 * XLP_BKP_REMOVABLE off before we emit the checkpoint record.
		 * Therefore, if a WAL archiver (such as pglesslog) is trying to
		 * compress out removable backup blocks, it won't remove any that
		 * occur after this point.
		 *
		 * During recovery, we skip forcing XLOG file switch, which means that
		 * the backup taken during recovery is not available for the special
		 * recovery case described above.
		 */
		if (!backup_started_in_recovery)
			RequestXLogSwitch();

		do
		{
			bool		checkpointfpw;

			/*
			 * Force a CHECKPOINT.	Aside from being necessary to prevent torn
			 * page problems, this guarantees that two successive backup runs
			 * will have different checkpoint positions and hence different
			 * history file names, even if nothing happened in between.
			 *
			 * During recovery, establish a restartpoint if possible. We use
			 * the last restartpoint as the backup starting checkpoint. This
			 * means that two successive backup runs can have same checkpoint
			 * positions.
			 *
			 * Since the fact that we are executing do_pg_start_backup()
			 * during recovery means that checkpointer is running, we can use
			 * RequestCheckpoint() to establish a restartpoint.
			 *
			 * We use CHECKPOINT_IMMEDIATE only if requested by user (via
			 * passing fast = true).  Otherwise this can take awhile.
			 */
			RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT |
							  (fast ? CHECKPOINT_IMMEDIATE : 0));

			/*
			 * Now we need to fetch the checkpoint record location, and also
			 * its REDO pointer.  The oldest point in WAL that would be needed
			 * to restore starting from the checkpoint is precisely the REDO
			 * pointer.
			 */
			LWLockAcquire(ControlFileLock, LW_SHARED);
			checkpointloc = ControlFile->checkPoint;
			startpoint = ControlFile->checkPointCopy.redo;
			starttli = ControlFile->checkPointCopy.ThisTimeLineID;
			checkpointfpw = ControlFile->checkPointCopy.fullPageWrites;
			LWLockRelease(ControlFileLock);

			if (backup_started_in_recovery)
			{
				/* use volatile pointer to prevent code rearrangement */
				volatile XLogCtlData *xlogctl = XLogCtl;
				XLogRecPtr	recptr;

				/*
				 * Check to see if all WAL replayed during online backup
				 * (i.e., since last restartpoint used as backup starting
				 * checkpoint) contain full-page writes.
				 */
				SpinLockAcquire(&xlogctl->info_lck);
				recptr = xlogctl->lastFpwDisableRecPtr;
				SpinLockRelease(&xlogctl->info_lck);

				if (!checkpointfpw || startpoint <= recptr)
					ereport(ERROR,
						  (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
						   errmsg("WAL generated with full_page_writes=off was replayed "
								  "since last restartpoint"),
						   errhint("This means that the backup being taken on the standby "
								   "is corrupt and should not be used. "
								   "Enable full_page_writes and run CHECKPOINT on the master, "
								   "and then try an online backup again.")));

				/*
				 * During recovery, since we don't use the end-of-backup WAL
				 * record and don't write the backup history file, the
				 * starting WAL location doesn't need to be unique. This means
				 * that two base backups started at the same time might use
				 * the same checkpoint as starting locations.
				 */
				gotUniqueStartpoint = true;
			}

			/*
			 * If two base backups are started at the same time (in WAL sender
			 * processes), we need to make sure that they use different
			 * checkpoints as starting locations, because we use the starting
			 * WAL location as a unique identifier for the base backup in the
			 * end-of-backup WAL record and when we write the backup history
			 * file. Perhaps it would be better generate a separate unique ID
			 * for each backup instead of forcing another checkpoint, but
			 * taking a checkpoint right after another is not that expensive
			 * either because only few buffers have been dirtied yet.
			 */
			WALInsertSlotAcquire(true);
			if (XLogCtl->Insert.lastBackupStart < startpoint)
			{
				XLogCtl->Insert.lastBackupStart = startpoint;
				gotUniqueStartpoint = true;
			}
			WALInsertSlotRelease();
		} while (!gotUniqueStartpoint);

		XLByteToSeg(startpoint, _logSegNo);
		XLogFileName(xlogfilename, ThisTimeLineID, _logSegNo);

		/*
		 * Construct backup label file
		 */
		initStringInfo(&labelfbuf);

		/* Use the log timezone here, not the session timezone */
		stamp_time = (pg_time_t) time(NULL);
		pg_strftime(strfbuf, sizeof(strfbuf),
					"%Y-%m-%d %H:%M:%S %Z",
					pg_localtime(&stamp_time, log_timezone));
		appendStringInfo(&labelfbuf, "START WAL LOCATION: %X/%X (file %s)\n",
			 (uint32) (startpoint >> 32), (uint32) startpoint, xlogfilename);
		appendStringInfo(&labelfbuf, "CHECKPOINT LOCATION: %X/%X\n",
					 (uint32) (checkpointloc >> 32), (uint32) checkpointloc);
		appendStringInfo(&labelfbuf, "BACKUP METHOD: %s\n",
						 exclusive ? "pg_start_backup" : "streamed");
		appendStringInfo(&labelfbuf, "BACKUP FROM: %s\n",
						 backup_started_in_recovery ? "standby" : "master");
		appendStringInfo(&labelfbuf, "START TIME: %s\n", strfbuf);
		appendStringInfo(&labelfbuf, "LABEL: %s\n", backupidstr);

		/*
		 * Okay, write the file, or return its contents to caller.
		 */
		if (exclusive)
		{
			/*
			 * Check for existing backup label --- implies a backup is already
			 * running.  (XXX given that we checked exclusiveBackup above,
			 * maybe it would be OK to just unlink any such label file?)
			 */
			if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
			{
				if (errno != ENOENT)
					ereport(ERROR,
							(errcode_for_file_access(),
							 errmsg("could not stat file \"%s\": %m",
									BACKUP_LABEL_FILE)));
			}
			else
				ereport(ERROR,
						(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
						 errmsg("a backup is already in progress"),
						 errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
								 BACKUP_LABEL_FILE)));

			fp = AllocateFile(BACKUP_LABEL_FILE, "w");

			if (!fp)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not create file \"%s\": %m",
								BACKUP_LABEL_FILE)));
			if (fwrite(labelfbuf.data, labelfbuf.len, 1, fp) != 1 ||
				fflush(fp) != 0 ||
				pg_fsync(fileno(fp)) != 0 ||
				ferror(fp) ||
				FreeFile(fp))
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not write file \"%s\": %m",
								BACKUP_LABEL_FILE)));
			pfree(labelfbuf.data);
		}
		else
			*labelfile = labelfbuf.data;
	}
	PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));

	/*
	 * We're done.  As a convenience, return the starting WAL location.
	 */
	if (starttli_p)
		*starttli_p = starttli;
	return startpoint;
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
	bool		exclusive = DatumGetBool(arg);

	/* Update backup counters and forcePageWrites on failure */
	WALInsertSlotAcquire(true);
	if (exclusive)
	{
		Assert(XLogCtl->Insert.exclusiveBackup);
		XLogCtl->Insert.exclusiveBackup = false;
	}
	else
	{
		Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
		XLogCtl->Insert.nonExclusiveBackups--;
	}

	if (!XLogCtl->Insert.exclusiveBackup &&
		XLogCtl->Insert.nonExclusiveBackups == 0)
	{
		XLogCtl->Insert.forcePageWrites = false;
	}
	WALInsertSlotRelease();
}

/*
 * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
 * function.

 * If labelfile is NULL, this stops an exclusive backup. Otherwise this stops
 * the non-exclusive backup specified by 'labelfile'.
 *
 * Returns the last WAL position that must be present to restore from this
 * backup, and the corresponding timeline ID in *stoptli_p.
 */
XLogRecPtr
do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
{
	bool		exclusive = (labelfile == NULL);
	bool		backup_started_in_recovery = false;
	XLogRecPtr	startpoint;
	XLogRecPtr	stoppoint;
	TimeLineID	stoptli;
	XLogRecData rdata;
	pg_time_t	stamp_time;
	char		strfbuf[128];
	char		histfilepath[MAXPGPATH];
	char		startxlogfilename[MAXFNAMELEN];
	char		stopxlogfilename[MAXFNAMELEN];
	char		lastxlogfilename[MAXFNAMELEN];
	char		histfilename[MAXFNAMELEN];
	char		backupfrom[20];
	XLogSegNo	_logSegNo;
	FILE	   *lfp;
	FILE	   *fp;
	char		ch;
	int			seconds_before_warning;
	int			waits = 0;
	bool		reported_waiting = false;
	char	   *remaining;
	char	   *ptr;
	uint32		hi,
				lo;

	backup_started_in_recovery = RecoveryInProgress();

	if (!superuser() && !has_rolreplication(GetUserId()))
		ereport(ERROR,
				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
		 (errmsg("must be superuser or replication role to run a backup"))));

	/*
	 * Currently only non-exclusive backup can be taken during recovery.
	 */
	if (backup_started_in_recovery && exclusive)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("recovery is in progress"),
				 errhint("WAL control functions cannot be executed during recovery.")));

	/*
	 * During recovery, we don't need to check WAL level. Because, if WAL
	 * level is not sufficient, it's impossible to get here during recovery.
	 */
	if (!backup_started_in_recovery && !XLogIsNeeded())
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
			  errmsg("WAL level not sufficient for making an online backup"),
				 errhint("wal_level must be set to \"archive\" or \"hot_standby\" at server start.")));

	/*
	 * OK to update backup counters and forcePageWrites
	 */
	WALInsertSlotAcquire(true);
	if (exclusive)
		XLogCtl->Insert.exclusiveBackup = false;
	else
	{
		/*
		 * The user-visible pg_start/stop_backup() functions that operate on
		 * exclusive backups can be called at any time, but for non-exclusive
		 * backups, it is expected that each do_pg_start_backup() call is
		 * matched by exactly one do_pg_stop_backup() call.
		 */
		Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
		XLogCtl->Insert.nonExclusiveBackups--;
	}

	if (!XLogCtl->Insert.exclusiveBackup &&
		XLogCtl->Insert.nonExclusiveBackups == 0)
	{
		XLogCtl->Insert.forcePageWrites = false;
	}
	WALInsertSlotRelease();

	if (exclusive)
	{
		/*
		 * Read the existing label file into memory.
		 */
		struct stat statbuf;
		int			r;

		if (stat(BACKUP_LABEL_FILE, &statbuf))
		{
			if (errno != ENOENT)
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not stat file \"%s\": %m",
								BACKUP_LABEL_FILE)));
			ereport(ERROR,
					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
					 errmsg("a backup is not in progress")));
		}

		lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
		if (!lfp)
		{
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not read file \"%s\": %m",
							BACKUP_LABEL_FILE)));
		}
		labelfile = palloc(statbuf.st_size + 1);
		r = fread(labelfile, statbuf.st_size, 1, lfp);
		labelfile[statbuf.st_size] = '\0';

		/*
		 * Close and remove the backup label file
		 */
		if (r != 1 || ferror(lfp) || FreeFile(lfp))
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not read file \"%s\": %m",
							BACKUP_LABEL_FILE)));
		if (unlink(BACKUP_LABEL_FILE) != 0)
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not remove file \"%s\": %m",
							BACKUP_LABEL_FILE)));
	}

	/*
	 * Read and parse the START WAL LOCATION line (this code is pretty crude,
	 * but we are not expecting any variability in the file format).
	 */
	if (sscanf(labelfile, "START WAL LOCATION: %X/%X (file %24s)%c",
			   &hi, &lo, startxlogfilename,
			   &ch) != 4 || ch != '\n')
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
	startpoint = ((uint64) hi) << 32 | lo;
	remaining = strchr(labelfile, '\n') + 1;	/* %n is not portable enough */

	/*
	 * Parse the BACKUP FROM line. If we are taking an online backup from the
	 * standby, we confirm that the standby has not been promoted during the
	 * backup.
	 */
	ptr = strstr(remaining, "BACKUP FROM:");
	if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
	if (strcmp(backupfrom, "standby") == 0 && !backup_started_in_recovery)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("the standby was promoted during online backup"),
				 errhint("This means that the backup being taken is corrupt "
						 "and should not be used. "
						 "Try taking another online backup.")));

	/*
	 * During recovery, we don't write an end-of-backup record. We assume that
	 * pg_control was backed up last and its minimum recovery point can be
	 * available as the backup end location. Since we don't have an
	 * end-of-backup record, we use the pg_control value to check whether
	 * we've reached the end of backup when starting recovery from this
	 * backup. We have no way of checking if pg_control wasn't backed up last
	 * however.
	 *
	 * We don't force a switch to new WAL file and wait for all the required
	 * files to be archived. This is okay if we use the backup to start the
	 * standby. But, if it's for an archive recovery, to ensure all the
	 * required files are available, a user should wait for them to be
	 * archived, or include them into the backup.
	 *
	 * We return the current minimum recovery point as the backup end
	 * location. Note that it can be greater than the exact backup end
	 * location if the minimum recovery point is updated after the backup of
	 * pg_control. This is harmless for current uses.
	 *
	 * XXX currently a backup history file is for informational and debug
	 * purposes only. It's not essential for an online backup. Furthermore,
	 * even if it's created, it will not be archived during recovery because
	 * an archiver is not invoked. So it doesn't seem worthwhile to write a
	 * backup history file during recovery.
	 */
	if (backup_started_in_recovery)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;
		XLogRecPtr	recptr;

		/*
		 * Check to see if all WAL replayed during online backup contain
		 * full-page writes.
		 */
		SpinLockAcquire(&xlogctl->info_lck);
		recptr = xlogctl->lastFpwDisableRecPtr;
		SpinLockRelease(&xlogctl->info_lck);

		if (startpoint <= recptr)
			ereport(ERROR,
					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
			   errmsg("WAL generated with full_page_writes=off was replayed "
					  "during online backup"),
			 errhint("This means that the backup being taken on the standby "
					 "is corrupt and should not be used. "
				 "Enable full_page_writes and run CHECKPOINT on the master, "
					 "and then try an online backup again.")));


		LWLockAcquire(ControlFileLock, LW_SHARED);
		stoppoint = ControlFile->minRecoveryPoint;
		stoptli = ControlFile->minRecoveryPointTLI;
		LWLockRelease(ControlFileLock);

		if (stoptli_p)
			*stoptli_p = stoptli;
		return stoppoint;
	}

	/*
	 * Write the backup-end xlog record
	 */
	rdata.data = (char *) (&startpoint);
	rdata.len = sizeof(startpoint);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;
	stoppoint = XLogInsert(RM_XLOG_ID, XLOG_BACKUP_END, &rdata);
	stoptli = ThisTimeLineID;

	/*
	 * Force a switch to a new xlog segment file, so that the backup is valid
	 * as soon as archiver moves out the current segment file.
	 */
	RequestXLogSwitch();

	XLByteToPrevSeg(stoppoint, _logSegNo);
	XLogFileName(stopxlogfilename, ThisTimeLineID, _logSegNo);

	/* Use the log timezone here, not the session timezone */
	stamp_time = (pg_time_t) time(NULL);
	pg_strftime(strfbuf, sizeof(strfbuf),
				"%Y-%m-%d %H:%M:%S %Z",
				pg_localtime(&stamp_time, log_timezone));

	/*
	 * Write the backup history file
	 */
	XLByteToSeg(startpoint, _logSegNo);
	BackupHistoryFilePath(histfilepath, ThisTimeLineID, _logSegNo,
						  (uint32) (startpoint % XLogSegSize));
	fp = AllocateFile(histfilepath, "w");
	if (!fp)
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not create file \"%s\": %m",
						histfilepath)));
	fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
		(uint32) (startpoint >> 32), (uint32) startpoint, startxlogfilename);
	fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
			(uint32) (stoppoint >> 32), (uint32) stoppoint, stopxlogfilename);
	/* transfer remaining lines from label to history file */
	fprintf(fp, "%s", remaining);
	fprintf(fp, "STOP TIME: %s\n", strfbuf);
	if (fflush(fp) || ferror(fp) || FreeFile(fp))
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not write file \"%s\": %m",
						histfilepath)));

	/*
	 * Clean out any no-longer-needed history files.  As a side effect, this
	 * will post a .ready file for the newly created history file, notifying
	 * the archiver that history file may be archived immediately.
	 */
	CleanupBackupHistory();

	/*
	 * If archiving is enabled, wait for all the required WAL files to be
	 * archived before returning. If archiving isn't enabled, the required WAL
	 * needs to be transported via streaming replication (hopefully with
	 * wal_keep_segments set high enough), or some more exotic mechanism like
	 * polling and copying files from pg_xlog with script. We have no
	 * knowledge of those mechanisms, so it's up to the user to ensure that he
	 * gets all the required WAL.
	 *
	 * We wait until both the last WAL file filled during backup and the
	 * history file have been archived, and assume that the alphabetic sorting
	 * property of the WAL files ensures any earlier WAL files are safely
	 * archived as well.
	 *
	 * We wait forever, since archive_command is supposed to work and we
	 * assume the admin wanted his backup to work completely. If you don't
	 * wish to wait, you can set statement_timeout.  Also, some notices are
	 * issued to clue in anyone who might be doing this interactively.
	 */
	if (waitforarchive && XLogArchivingActive())
	{
		XLByteToPrevSeg(stoppoint, _logSegNo);
		XLogFileName(lastxlogfilename, ThisTimeLineID, _logSegNo);

		XLByteToSeg(startpoint, _logSegNo);
		BackupHistoryFileName(histfilename, ThisTimeLineID, _logSegNo,
							  (uint32) (startpoint % XLogSegSize));

		seconds_before_warning = 60;
		waits = 0;

		while (XLogArchiveIsBusy(lastxlogfilename) ||
			   XLogArchiveIsBusy(histfilename))
		{
			CHECK_FOR_INTERRUPTS();

			if (!reported_waiting && waits > 5)
			{
				ereport(NOTICE,
						(errmsg("pg_stop_backup cleanup done, waiting for required WAL segments to be archived")));
				reported_waiting = true;
			}

			pg_usleep(1000000L);

			if (++waits >= seconds_before_warning)
			{
				seconds_before_warning *= 2;	/* This wraps in >10 years... */
				ereport(WARNING,
						(errmsg("pg_stop_backup still waiting for all required WAL segments to be archived (%d seconds elapsed)",
								waits),
						 errhint("Check that your archive_command is executing properly.  "
								 "pg_stop_backup can be canceled safely, "
								 "but the database backup will not be usable without all the WAL segments.")));
			}
		}

		ereport(NOTICE,
				(errmsg("pg_stop_backup complete, all required WAL segments have been archived")));
	}
	else if (waitforarchive)
		ereport(NOTICE,
				(errmsg("WAL archiving is not enabled; you must ensure that all required WAL segments are copied through other means to complete the backup")));

	/*
	 * We're done.  As a convenience, return the ending WAL location.
	 */
	if (stoptli_p)
		*stoptli_p = stoptli;
	return stoppoint;
}


/*
 * do_pg_abort_backup: abort a running backup
 *
 * This does just the most basic steps of do_pg_stop_backup(), by taking the
 * system out of backup mode, thus making it a lot more safe to call from
 * an error handler.
 *
 * NB: This is only for aborting a non-exclusive backup that doesn't write
 * backup_label. A backup started with pg_stop_backup() needs to be finished
 * with pg_stop_backup().
 */
void
do_pg_abort_backup(void)
{
	WALInsertSlotAcquire(true);
	Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
	XLogCtl->Insert.nonExclusiveBackups--;

	if (!XLogCtl->Insert.exclusiveBackup &&
		XLogCtl->Insert.nonExclusiveBackups == 0)
	{
		XLogCtl->Insert.forcePageWrites = false;
	}
	WALInsertSlotRelease();
}

/*
 * Get latest redo apply position.
 *
 * Exported to allow WALReceiver to read the pointer directly.
 */
XLogRecPtr
GetXLogReplayRecPtr(TimeLineID *replayTLI)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	XLogRecPtr	recptr;
	TimeLineID	tli;

	SpinLockAcquire(&xlogctl->info_lck);
	recptr = xlogctl->lastReplayedEndRecPtr;
	tli = xlogctl->lastReplayedTLI;
	SpinLockRelease(&xlogctl->info_lck);

	if (replayTLI)
		*replayTLI = tli;
	return recptr;
}

/*
 * Get latest WAL insert pointer
 */
XLogRecPtr
GetXLogInsertRecPtr(void)
{
	volatile XLogCtlInsert *Insert = &XLogCtl->Insert;
	uint64		current_bytepos;

	SpinLockAcquire(&Insert->insertpos_lck);
	current_bytepos = Insert->CurrBytePos;
	SpinLockRelease(&Insert->insertpos_lck);

	return XLogBytePosToRecPtr(current_bytepos);
}

/*
 * Get latest WAL write pointer
 */
XLogRecPtr
GetXLogWriteRecPtr(void)
{
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		LogwrtResult = xlogctl->LogwrtResult;
		SpinLockRelease(&xlogctl->info_lck);
	}

	return LogwrtResult.Write;
}

/*
 * Returns the redo pointer of the last checkpoint or restartpoint. This is
 * the oldest point in WAL that we still need, if we have to restart recovery.
 */
void
GetOldestRestartPoint(XLogRecPtr *oldrecptr, TimeLineID *oldtli)
{
	LWLockAcquire(ControlFileLock, LW_SHARED);
	*oldrecptr = ControlFile->checkPointCopy.redo;
	*oldtli = ControlFile->checkPointCopy.ThisTimeLineID;
	LWLockRelease(ControlFileLock);
}

/*
 * read_backup_label: check to see if a backup_label file is present
 *
 * If we see a backup_label during recovery, we assume that we are recovering
 * from a backup dump file, and we therefore roll forward from the checkpoint
 * identified by the label file, NOT what pg_control says.	This avoids the
 * problem that pg_control might have been archived one or more checkpoints
 * later than the start of the dump, and so if we rely on it as the start
 * point, we will fail to restore a consistent database state.
 *
 * Returns TRUE if a backup_label was found (and fills the checkpoint
 * location and its REDO location into *checkPointLoc and RedoStartLSN,
 * respectively); returns FALSE if not. If this backup_label came from a
 * streamed backup, *backupEndRequired is set to TRUE. If this backup_label
 * was created during recovery, *backupFromStandby is set to TRUE.
 */
static bool
read_backup_label(XLogRecPtr *checkPointLoc, bool *backupEndRequired,
				  bool *backupFromStandby)
{
	char		startxlogfilename[MAXFNAMELEN];
	TimeLineID	tli;
	FILE	   *lfp;
	char		ch;
	char		backuptype[20];
	char		backupfrom[20];
	uint32		hi,
				lo;

	*backupEndRequired = false;
	*backupFromStandby = false;

	/*
	 * See if label file is present
	 */
	lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
	if (!lfp)
	{
		if (errno != ENOENT)
			ereport(FATAL,
					(errcode_for_file_access(),
					 errmsg("could not read file \"%s\": %m",
							BACKUP_LABEL_FILE)));
		return false;			/* it's not there, all is fine */
	}

	/*
	 * Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
	 * is pretty crude, but we are not expecting any variability in the file
	 * format).
	 */
	if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
			   &hi, &lo, &tli, startxlogfilename, &ch) != 5 || ch != '\n')
		ereport(FATAL,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
	RedoStartLSN = ((uint64) hi) << 32 | lo;
	if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
			   &hi, &lo, &ch) != 3 || ch != '\n')
		ereport(FATAL,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
	*checkPointLoc = ((uint64) hi) << 32 | lo;

	/*
	 * BACKUP METHOD and BACKUP FROM lines are new in 9.2. We can't restore
	 * from an older backup anyway, but since the information on it is not
	 * strictly required, don't error out if it's missing for some reason.
	 */
	if (fscanf(lfp, "BACKUP METHOD: %19s\n", backuptype) == 1)
	{
		if (strcmp(backuptype, "streamed") == 0)
			*backupEndRequired = true;
	}

	if (fscanf(lfp, "BACKUP FROM: %19s\n", backupfrom) == 1)
	{
		if (strcmp(backupfrom, "standby") == 0)
			*backupFromStandby = true;
	}

	if (ferror(lfp) || FreeFile(lfp))
		ereport(FATAL,
				(errcode_for_file_access(),
				 errmsg("could not read file \"%s\": %m",
						BACKUP_LABEL_FILE)));

	return true;
}

/*
 * Error context callback for errors occurring during rm_redo().
 */
static void
rm_redo_error_callback(void *arg)
{
	XLogRecord *record = (XLogRecord *) arg;
	StringInfoData buf;

	initStringInfo(&buf);
	RmgrTable[record->xl_rmid].rm_desc(&buf,
									   record->xl_info,
									   XLogRecGetData(record));

	/* don't bother emitting empty description */
	if (buf.len > 0)
		errcontext("xlog redo %s", buf.data);

	pfree(buf.data);
}

/*
 * BackupInProgress: check if online backup mode is active
 *
 * This is done by checking for existence of the "backup_label" file.
 */
bool
BackupInProgress(void)
{
	struct stat stat_buf;

	return (stat(BACKUP_LABEL_FILE, &stat_buf) == 0);
}

/*
 * CancelBackup: rename the "backup_label" file to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.old".
 * Note that this will render an online backup in progress useless.
 * To correctly finish an online backup, pg_stop_backup must be called.
 */
void
CancelBackup(void)
{
	struct stat stat_buf;

	/* if the file is not there, return */
	if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
		return;

	/* remove leftover file from previously canceled backup if it exists */
	unlink(BACKUP_LABEL_OLD);

	if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) == 0)
	{
		ereport(LOG,
				(errmsg("online backup mode canceled"),
				 errdetail("\"%s\" was renamed to \"%s\".",
						   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
	}
	else
	{
		ereport(WARNING,
				(errcode_for_file_access(),
				 errmsg("online backup mode was not canceled"),
				 errdetail("Could not rename \"%s\" to \"%s\": %m.",
						   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
	}
}

/*
 * Read the XLOG page containing RecPtr into readBuf (if not read already).
 * Returns number of bytes read, if the page is read successfully, or -1
 * in case of errors.  When errors occur, they are ereport'ed, but only
 * if they have not been previously reported.
 *
 * This is responsible for restoring files from archive as needed, as well
 * as for waiting for the requested WAL record to arrive in standby mode.
 *
 * 'emode' specifies the log level used for reporting "file not found" or
 * "end of WAL" situations in archive recovery, or in standby mode when a
 * trigger file is found. If set to WARNING or below, XLogPageRead() returns
 * false in those situations, on higher log levels the ereport() won't
 * return.
 *
 * In standby mode, if after a successful return of XLogPageRead() the
 * caller finds the record it's interested in to be broken, it should
 * ereport the error with the level determined by
 * emode_for_corrupt_record(), and then set lastSourceFailed
 * and call XLogPageRead() again with the same arguments. This lets
 * XLogPageRead() to try fetching the record from another source, or to
 * sleep and retry.
 */
static int
XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
			 XLogRecPtr targetRecPtr, char *readBuf, TimeLineID *readTLI)
{
	XLogPageReadPrivate *private =
	(XLogPageReadPrivate *) xlogreader->private_data;
	int			emode = private->emode;
	uint32		targetPageOff;
	XLogSegNo targetSegNo PG_USED_FOR_ASSERTS_ONLY;

	XLByteToSeg(targetPagePtr, targetSegNo);
	targetPageOff = targetPagePtr % XLogSegSize;

	/*
	 * See if we need to switch to a new segment because the requested record
	 * is not in the currently open one.
	 */
	if (readFile >= 0 && !XLByteInSeg(targetPagePtr, readSegNo))
	{
		/*
		 * Request a restartpoint if we've replayed too much xlog since the
		 * last one.
		 */
		if (StandbyModeRequested && bgwriterLaunched)
		{
			if (XLogCheckpointNeeded(readSegNo))
			{
				(void) GetRedoRecPtr();
				if (XLogCheckpointNeeded(readSegNo))
					RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
			}
		}

		close(readFile);
		readFile = -1;
		readSource = 0;
	}

	XLByteToSeg(targetPagePtr, readSegNo);

retry:
	/* See if we need to retrieve more data */
	if (readFile < 0 ||
		(readSource == XLOG_FROM_STREAM &&
		 receivedUpto < targetPagePtr + reqLen))
	{
		if (!WaitForWALToBecomeAvailable(targetPagePtr + reqLen,
										 private->randAccess,
										 private->fetching_ckpt,
										 targetRecPtr))
		{
			if (readFile >= 0)
				close(readFile);
			readFile = -1;
			readLen = 0;
			readSource = 0;

			return -1;
		}
	}

	/*
	 * At this point, we have the right segment open and if we're streaming we
	 * know the requested record is in it.
	 */
	Assert(readFile != -1);

	/*
	 * If the current segment is being streamed from master, calculate how
	 * much of the current page we have received already. We know the
	 * requested record has been received, but this is for the benefit of
	 * future calls, to allow quick exit at the top of this function.
	 */
	if (readSource == XLOG_FROM_STREAM)
	{
		if (((targetPagePtr) / XLOG_BLCKSZ) != (receivedUpto / XLOG_BLCKSZ))
			readLen = XLOG_BLCKSZ;
		else
			readLen = receivedUpto % XLogSegSize - targetPageOff;
	}
	else
		readLen = XLOG_BLCKSZ;

	/* Read the requested page */
	readOff = targetPageOff;
	if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
	{
		char		fname[MAXFNAMELEN];

		XLogFileName(fname, curFileTLI, readSegNo);
		ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
				(errcode_for_file_access(),
				 errmsg("could not seek in log segment %s to offset %u: %m",
						fname, readOff)));
		goto next_record_is_invalid;
	}

	if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
	{
		char		fname[MAXFNAMELEN];

		XLogFileName(fname, curFileTLI, readSegNo);
		ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
				(errcode_for_file_access(),
				 errmsg("could not read from log segment %s, offset %u: %m",
						fname, readOff)));
		goto next_record_is_invalid;
	}

	Assert(targetSegNo == readSegNo);
	Assert(targetPageOff == readOff);
	Assert(reqLen <= readLen);

	*readTLI = curFileTLI;
	return readLen;

next_record_is_invalid:
	lastSourceFailed = true;

	if (readFile >= 0)
		close(readFile);
	readFile = -1;
	readLen = 0;
	readSource = 0;

	/* In standby-mode, keep trying */
	if (StandbyMode)
		goto retry;
	else
		return -1;
}

/*
 * Open the WAL segment containing WAL position 'RecPtr'.
 *
 * The segment can be fetched via restore_command, or via walreceiver having
 * streamed the record, or it can already be present in pg_xlog. Checking
 * pg_xlog is mainly for crash recovery, but it will be polled in standby mode
 * too, in case someone copies a new segment directly to pg_xlog. That is not
 * documented or recommended, though.
 *
 * If 'fetching_ckpt' is true, we're fetching a checkpoint record, and should
 * prepare to read WAL starting from RedoStartLSN after this.
 *
 * 'RecPtr' might not point to the beginning of the record we're interested
 * in, it might also point to the page or segment header. In that case,
 * 'tliRecPtr' is the position of the WAL record we're interested in. It is
 * used to decide which timeline to stream the requested WAL from.
 *
 * If the the record is not immediately available, the function returns false
 * if we're not in standby mode. In standby mode, waits for it to become
 * available.
 *
 * When the requested record becomes available, the function opens the file
 * containing it (if not open already), and returns true. When end of standby
 * mode is triggered by the user, and there is no more WAL available, returns
 * false.
 */
static bool
WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
							bool fetching_ckpt, XLogRecPtr tliRecPtr)
{
	static pg_time_t last_fail_time = 0;
	pg_time_t	now;

	/*-------
	 * Standby mode is implemented by a state machine:
	 *
	 * 1. Read from archive (XLOG_FROM_ARCHIVE)
	 * 2. Read from pg_xlog (XLOG_FROM_PG_XLOG)
	 * 3. Check trigger file
	 * 4. Read from primary server via walreceiver (XLOG_FROM_STREAM)
	 * 5. Rescan timelines
	 * 6. Sleep 5 seconds, and loop back to 1.
	 *
	 * Failure to read from the current source advances the state machine to
	 * the next state. In addition, successfully reading a file from pg_xlog
	 * moves the state machine from state 2 back to state 1 (we always prefer
	 * files in the archive over files in pg_xlog).
	 *
	 * 'currentSource' indicates the current state. There are no currentSource
	 * values for "check trigger", "rescan timelines", and "sleep" states,
	 * those actions are taken when reading from the previous source fails, as
	 * part of advancing to the next state.
	 *-------
	 */
	if (!InArchiveRecovery)
		currentSource = XLOG_FROM_PG_XLOG;
	else if (currentSource == 0)
		currentSource = XLOG_FROM_ARCHIVE;

	for (;;)
	{
		int			oldSource = currentSource;

		/*
		 * First check if we failed to read from the current source, and
		 * advance the state machine if so. The failure to read might've
		 * happened outside this function, e.g when a CRC check fails on a
		 * record, or within this loop.
		 */
		if (lastSourceFailed)
		{
			switch (currentSource)
			{
				case XLOG_FROM_ARCHIVE:
					currentSource = XLOG_FROM_PG_XLOG;
					break;

				case XLOG_FROM_PG_XLOG:

					/*
					 * Check to see if the trigger file exists. Note that we
					 * do this only after failure, so when you create the
					 * trigger file, we still finish replaying as much as we
					 * can from archive and pg_xlog before failover.
					 */
					if (StandbyMode && CheckForStandbyTrigger())
					{
						ShutdownWalRcv();
						return false;
					}

					/*
					 * Not in standby mode, and we've now tried the archive
					 * and pg_xlog.
					 */
					if (!StandbyMode)
						return false;

					/*
					 * If primary_conninfo is set, launch walreceiver to try
					 * to stream the missing WAL.
					 *
					 * If fetching_ckpt is TRUE, RecPtr points to the initial
					 * checkpoint location. In that case, we use RedoStartLSN
					 * as the streaming start position instead of RecPtr, so
					 * that when we later jump backwards to start redo at
					 * RedoStartLSN, we will have the logs streamed already.
					 */
					if (PrimaryConnInfo)
					{
						XLogRecPtr	ptr;
						TimeLineID	tli;

						if (fetching_ckpt)
						{
							ptr = RedoStartLSN;
							tli = ControlFile->checkPointCopy.ThisTimeLineID;
						}
						else
						{
							ptr = tliRecPtr;
							tli = tliOfPointInHistory(tliRecPtr, expectedTLEs);

							if (curFileTLI > 0 && tli < curFileTLI)
								elog(ERROR, "according to history file, WAL location %X/%X belongs to timeline %u, but previous recovered WAL file came from timeline %u",
									 (uint32) (ptr >> 32), (uint32) ptr,
									 tli, curFileTLI);
						}
						curFileTLI = tli;
						RequestXLogStreaming(tli, ptr, PrimaryConnInfo);
						receivedUpto = 0;
					}

					/*
					 * Move to XLOG_FROM_STREAM state in either case. We'll
					 * get immediate failure if we didn't launch walreceiver,
					 * and move on to the next state.
					 */
					currentSource = XLOG_FROM_STREAM;
					break;

				case XLOG_FROM_STREAM:

					/*
					 * Failure while streaming. Most likely, we got here
					 * because streaming replication was terminated, or
					 * promotion was triggered. But we also get here if we
					 * find an invalid record in the WAL streamed from master,
					 * in which case something is seriously wrong. There's
					 * little chance that the problem will just go away, but
					 * PANIC is not good for availability either, especially
					 * in hot standby mode. So, we treat that the same as
					 * disconnection, and retry from archive/pg_xlog again.
					 * The WAL in the archive should be identical to what was
					 * streamed, so it's unlikely that it helps, but one can
					 * hope...
					 */

					/*
					 * Before we leave XLOG_FROM_STREAM state, make sure that
					 * walreceiver is not active, so that it won't overwrite
					 * WAL that we restore from archive.
					 */
					if (WalRcvStreaming())
						ShutdownWalRcv();

					/*
					 * Before we sleep, re-scan for possible new timelines if
					 * we were requested to recover to the latest timeline.
					 */
					if (recoveryTargetIsLatest)
					{
						if (rescanLatestTimeLine())
						{
							currentSource = XLOG_FROM_ARCHIVE;
							break;
						}
					}

					/*
					 * XLOG_FROM_STREAM is the last state in our state
					 * machine, so we've exhausted all the options for
					 * obtaining the requested WAL. We're going to loop back
					 * and retry from the archive, but if it hasn't been long
					 * since last attempt, sleep 5 seconds to avoid
					 * busy-waiting.
					 */
					now = (pg_time_t) time(NULL);
					if ((now - last_fail_time) < 5)
					{
						pg_usleep(1000000L * (5 - (now - last_fail_time)));
						now = (pg_time_t) time(NULL);
					}
					last_fail_time = now;
					currentSource = XLOG_FROM_ARCHIVE;
					break;

				default:
					elog(ERROR, "unexpected WAL source %d", currentSource);
			}
		}
		else if (currentSource == XLOG_FROM_PG_XLOG)
		{
			/*
			 * We just successfully read a file in pg_xlog. We prefer files in
			 * the archive over ones in pg_xlog, so try the next file again
			 * from the archive first.
			 */
			if (InArchiveRecovery)
				currentSource = XLOG_FROM_ARCHIVE;
		}

		if (currentSource != oldSource)
			elog(DEBUG2, "switched WAL source from %s to %s after %s",
				 xlogSourceNames[oldSource], xlogSourceNames[currentSource],
				 lastSourceFailed ? "failure" : "success");

		/*
		 * We've now handled possible failure. Try to read from the chosen
		 * source.
		 */
		lastSourceFailed = false;

		switch (currentSource)
		{
			case XLOG_FROM_ARCHIVE:
			case XLOG_FROM_PG_XLOG:
				/* Close any old file we might have open. */
				if (readFile >= 0)
				{
					close(readFile);
					readFile = -1;
				}
				/* Reset curFileTLI if random fetch. */
				if (randAccess)
					curFileTLI = 0;

				/*
				 * Try to restore the file from archive, or read an existing
				 * file from pg_xlog.
				 */
				readFile = XLogFileReadAnyTLI(readSegNo, DEBUG2, currentSource);
				if (readFile >= 0)
					return true;	/* success! */

				/*
				 * Nope, not found in archive or pg_xlog.
				 */
				lastSourceFailed = true;
				break;

			case XLOG_FROM_STREAM:
				{
					bool		havedata;

					/*
					 * Check if WAL receiver is still active.
					 */
					if (!WalRcvStreaming())
					{
						lastSourceFailed = true;
						break;
					}

					/*
					 * Walreceiver is active, so see if new data has arrived.
					 *
					 * We only advance XLogReceiptTime when we obtain fresh
					 * WAL from walreceiver and observe that we had already
					 * processed everything before the most recent "chunk"
					 * that it flushed to disk.  In steady state where we are
					 * keeping up with the incoming data, XLogReceiptTime will
					 * be updated on each cycle. When we are behind,
					 * XLogReceiptTime will not advance, so the grace time
					 * allotted to conflicting queries will decrease.
					 */
					if (RecPtr < receivedUpto)
						havedata = true;
					else
					{
						XLogRecPtr	latestChunkStart;

						receivedUpto = GetWalRcvWriteRecPtr(&latestChunkStart, &receiveTLI);
						if (RecPtr < receivedUpto && receiveTLI == curFileTLI)
						{
							havedata = true;
							if (latestChunkStart <= RecPtr)
							{
								XLogReceiptTime = GetCurrentTimestamp();
								SetCurrentChunkStartTime(XLogReceiptTime);
							}
						}
						else
							havedata = false;
					}
					if (havedata)
					{
						/*
						 * Great, streamed far enough.	Open the file if it's
						 * not open already.  Also read the timeline history
						 * file if we haven't initialized timeline history
						 * yet; it should be streamed over and present in
						 * pg_xlog by now.	Use XLOG_FROM_STREAM so that
						 * source info is set correctly and XLogReceiptTime
						 * isn't changed.
						 */
						if (readFile < 0)
						{
							if (!expectedTLEs)
								expectedTLEs = readTimeLineHistory(receiveTLI);
							readFile = XLogFileRead(readSegNo, PANIC,
													receiveTLI,
													XLOG_FROM_STREAM, false);
							Assert(readFile >= 0);
						}
						else
						{
							/* just make sure source info is correct... */
							readSource = XLOG_FROM_STREAM;
							XLogReceiptSource = XLOG_FROM_STREAM;
							return true;
						}
						break;
					}

					/*
					 * Data not here yet. Check for trigger, then wait for
					 * walreceiver to wake us up when new WAL arrives.
					 */
					if (CheckForStandbyTrigger())
					{
						/*
						 * Note that we don't "return false" immediately here.
						 * After being triggered, we still want to replay all
						 * the WAL that was already streamed. It's in pg_xlog
						 * now, so we just treat this as a failure, and the
						 * state machine will move on to replay the streamed
						 * WAL from pg_xlog, and then recheck the trigger and
						 * exit replay.
						 */
						lastSourceFailed = true;
						break;
					}

					/*
					 * Wait for more WAL to arrive. Time out after 5 seconds,
					 * like when polling the archive, to react to a trigger
					 * file promptly.
					 */
					WaitLatch(&XLogCtl->recoveryWakeupLatch,
							  WL_LATCH_SET | WL_TIMEOUT,
							  5000L);
					ResetLatch(&XLogCtl->recoveryWakeupLatch);
					break;
				}

			default:
				elog(ERROR, "unexpected WAL source %d", currentSource);
		}

		/*
		 * This possibly-long loop needs to handle interrupts of startup
		 * process.
		 */
		HandleStartupProcInterrupts();
	} while (StandbyMode);

	return false;
}

/*
 * Determine what log level should be used to report a corrupt WAL record
 * in the current WAL page, previously read by XLogPageRead().
 *
 * 'emode' is the error mode that would be used to report a file-not-found
 * or legitimate end-of-WAL situation.	 Generally, we use it as-is, but if
 * we're retrying the exact same record that we've tried previously, only
 * complain the first time to keep the noise down.	However, we only do when
 * reading from pg_xlog, because we don't expect any invalid records in archive
 * or in records streamed from master. Files in the archive should be complete,
 * and we should never hit the end of WAL because we stop and wait for more WAL
 * to arrive before replaying it.
 *
 * NOTE: This function remembers the RecPtr value it was last called with,
 * to suppress repeated messages about the same record. Only call this when
 * you are about to ereport(), or you might cause a later message to be
 * erroneously suppressed.
 */
static int
emode_for_corrupt_record(int emode, XLogRecPtr RecPtr)
{
	static XLogRecPtr lastComplaint = 0;

	if (readSource == XLOG_FROM_PG_XLOG && emode == LOG)
	{
		if (RecPtr == lastComplaint)
			emode = DEBUG1;
		else
			lastComplaint = RecPtr;
	}
	return emode;
}

/*
 * Check to see whether the user-specified trigger file exists and whether a
 * promote request has arrived.  If either condition holds, return true.
 */
static bool
CheckForStandbyTrigger(void)
{
	struct stat stat_buf;
	static bool triggered = false;

	if (triggered)
		return true;

	if (IsPromoteTriggered())
	{
		/*
		 * In 9.1 and 9.2 the postmaster unlinked the promote file inside the
		 * signal handler. We now leave the file in place and let the Startup
		 * process do the unlink. This allows Startup to know whether we're
		 * doing fast or normal promotion. Fast promotion takes precedence.
		 */
		if (stat(FAST_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
		{
			unlink(FAST_PROMOTE_SIGNAL_FILE);
			unlink(PROMOTE_SIGNAL_FILE);
			fast_promote = true;
		}
		else if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
		{
			unlink(PROMOTE_SIGNAL_FILE);
			fast_promote = false;
		}

		ereport(LOG, (errmsg("received promote request")));

		ResetPromoteTriggered();
		triggered = true;
		return true;
	}

	if (TriggerFile == NULL)
		return false;

	if (stat(TriggerFile, &stat_buf) == 0)
	{
		ereport(LOG,
				(errmsg("trigger file found: %s", TriggerFile)));
		unlink(TriggerFile);
		triggered = true;
		fast_promote = true;
		return true;
	}
	return false;
}

/*
 * Check to see if a promote request has arrived. Should be
 * called by postmaster after receiving SIGUSR1.
 */
bool
CheckPromoteSignal(void)
{
	struct stat stat_buf;

	if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0 ||
		stat(FAST_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
		return true;

	return false;
}

/*
 * Wake up startup process to replay newly arrived WAL, or to notice that
 * failover has been requested.
 */
void
WakeupRecovery(void)
{
	SetLatch(&XLogCtl->recoveryWakeupLatch);
}

/*
 * Update the WalWriterSleeping flag.
 */
void
SetWalWriterSleeping(bool sleeping)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;

	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->WalWriterSleeping = sleeping;
	SpinLockRelease(&xlogctl->info_lck);
}