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Introduce Streaming Replication.

Browse Source 
This includes two new kinds of postmaster processes, walsenders and
walreceiver. Walreceiver is responsible for connecting to the primary server
and streaming WAL to disk, while walsender runs in the primary server and
streams WAL from disk to the client.

Documentation still needs work, but the basics are there. We will probably
pull the replication section to a new chapter later on, as well as the
sections describing file-based replication. But let's do that as a separate
patch, so that it's easier to see what has been added/changed. This patch
also adds a new section to the chapter about FE/BE protocol, documenting the
protocol used by walsender/walreceivxer.

Bump catalog version because of two new functions,
pg_last_xlog_receive_location() and pg_last_xlog_replay_location(), for
monitoring the progress of replication.

Fujii Masao, with additional hacking by me
This commit is contained in:
Heikki Linnakangas
2010-01-15 09:19:10 +00:00
parent 4cbe473938
commit 40f908bdcd
53 changed files with 3567 additions and 220 deletions
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851
src/backend/replication/walsender.c Normal file
View File

@@ -0,0 +1,851 @@
/*-------------------------------------------------------------------------
*
* walsender.c
*
* The WAL sender process (walsender) is new as of Postgres 8.5. It takes
* charge of XLOG streaming sender in the primary server. At first, it is
* started by the postmaster when the walreceiver in the standby server
* connects to the primary server and requests XLOG streaming replication,
* i.e., unlike any auxiliary process, it is not an always-running process.
* It attempts to keep reading XLOG records from the disk and sending them
* to the standby server, as long as the connection is alive (i.e., like
* any backend, there is an one to one relationship between a connection
* and a walsender process).
*
* Normal termination is by SIGTERM, which instructs the walsender to
* close the connection and exit(0) at next convenient moment. Emergency
* termination is by SIGQUIT; like any backend, the walsender will simply
* abort and exit on SIGQUIT. A close of the connection and a FATAL error
* are treated as not a crash but approximately normal termination;
* the walsender will exit quickly without sending any more XLOG records.
*
* If the server is shut down, postmaster sends us SIGUSR2 after all
* regular backends have exited and the shutdown checkpoint has been written.
* This instruct walsender to send any outstanding WAL, including the
* shutdown checkpoint record, and then exit.
*
* Note that there can be more than one walsender process concurrently.
*
* Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/replication/walsender.c,v 1.1 2010/01/15 09:19:03 heikki Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include <unistd.h>
#include "access/xlog_internal.h"
#include "catalog/pg_type.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "replication/walsender.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lock.h"
#include "storage/pmsignal.h"
#include "tcop/tcopprot.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
/* Array of WalSnds in shared memory */
WalSndCtlData *WalSndCtl = NULL;
/* My slot in the shared memory array */
static WalSnd *MyWalSnd = NULL;
/* Global state */
bool am_walsender = false; /* Am I a walsender process ? */
/* User-settable parameters for walsender */
int MaxWalSenders = 0; /* the maximum number of concurrent walsenders */
int WalSndDelay = 200; /* max sleep time between some actions */
#define NAPTIME_PER_CYCLE 100 /* max sleep time between cycles (100ms) */
/*
* These variables are used similarly to openLogFile/Id/Seg/Off,
* but for walsender to read the XLOG.
*/
static int sendFile = -1;
static uint32 sendId = 0;
static uint32 sendSeg = 0;
static uint32 sendOff = 0;
/*
* How far have we sent WAL already? This is also advertised in
* MyWalSnd->sentPtr.
*/
static XLogRecPtr sentPtr = {0, 0};
/* Flags set by signal handlers for later service in main loop */
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t shutdown_requested = false;
static volatile sig_atomic_t ready_to_stop = false;
/* Signal handlers */
static void WalSndSigHupHandler(SIGNAL_ARGS);
static void WalSndShutdownHandler(SIGNAL_ARGS);
static void WalSndQuickDieHandler(SIGNAL_ARGS);
/* Prototypes for private functions */
static int WalSndLoop(void);
static void InitWalSnd(void);
static void WalSndHandshake(void);
static void WalSndKill(int code, Datum arg);
static void XLogRead(char *buf, XLogRecPtr recptr, Size nbytes);
static bool XLogSend(StringInfo outMsg);
static void CheckClosedConnection(void);
/*
* How much WAL to send in one message? Must be >= XLOG_BLCKSZ.
*/
#define MAX_SEND_SIZE (XLOG_SEG_SIZE / 2)
/* Main entry point for walsender process */
int
WalSenderMain(void)
{
MemoryContext walsnd_context;
if (!superuser())
ereport(FATAL,
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
errmsg("must be superuser to start walsender")));
/* Create a per-walsender data structure in shared memory */
InitWalSnd();
/*
* Create a memory context that we will do all our work in. We do this so
* that we can reset the context during error recovery and thereby avoid
* possible memory leaks. Formerly this code just ran in
* TopMemoryContext, but resetting that would be a really bad idea.
*
* XXX: we don't actually attempt error recovery in walsender, we just
* close the connection and exit.
*/
walsnd_context = AllocSetContextCreate(TopMemoryContext,
"Wal Sender",
ALLOCSET_DEFAULT_MINSIZE,
ALLOCSET_DEFAULT_INITSIZE,
ALLOCSET_DEFAULT_MAXSIZE);
MemoryContextSwitchTo(walsnd_context);
/* Unblock signals (they were blocked when the postmaster forked us) */
PG_SETMASK(&UnBlockSig);
/* Tell the standby that walsender is ready for receiving commands */
ReadyForQuery(DestRemote);
/* Handle handshake messages before streaming */
WalSndHandshake();
/* Main loop of walsender */
return WalSndLoop();
}
static void
WalSndHandshake(void)
{
StringInfoData input_message;
bool replication_started = false;
initStringInfo(&input_message);
while (!replication_started)
{
int firstchar;
/* Wait for a command to arrive */
firstchar = pq_getbyte();
/*
* Check for any other interesting events that happened while we
* slept.
*/
if (got_SIGHUP)
{
got_SIGHUP = false;
ProcessConfigFile(PGC_SIGHUP);
}
if (firstchar == EOF)
{
/* standby disconnected */
ereport(COMMERROR,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("unexpected EOF on standby connection")));
}
else
{
/*
* Read the message contents. This is expected to be done without
* blocking because we've been able to get message type code.
*/
if (pq_getmessage(&input_message, 0))
firstchar = EOF; /* suitable message already logged */
}
/* Handle the very limited subset of commands expected in this phase */
switch (firstchar)
{
case 'Q': /* Query message */
{
const char *query_string;
XLogRecPtr recptr;
query_string = pq_getmsgstring(&input_message);
pq_getmsgend(&input_message);
if (strcmp(query_string, "IDENTIFY_SYSTEM") == 0)
{
StringInfoData buf;
char sysid[32];
char tli[11];
/*
* Reply with a result set with one row, two columns.
* First col is system ID, and second if timeline ID
*/
snprintf(sysid, sizeof(sysid), UINT64_FORMAT,
GetSystemIdentifier());
snprintf(tli, sizeof(tli), "%u", ThisTimeLineID);
/* Send a RowDescription message */
pq_beginmessage(&buf, 'T');
pq_sendint(&buf, 2, 2); /* 2 fields */
/* first field */
pq_sendstring(&buf, "systemid"); /* col name */
pq_sendint(&buf, 0, 4); /* table oid */
pq_sendint(&buf, 0, 2); /* attnum */
pq_sendint(&buf, TEXTOID, 4); /* type oid */
pq_sendint(&buf, -1, 2); /* typlen */
pq_sendint(&buf, 0, 4); /* typmod */
pq_sendint(&buf, 0, 2); /* format code */
/* second field */
pq_sendstring(&buf, "timeline"); /* col name */
pq_sendint(&buf, 0, 4); /* table oid */
pq_sendint(&buf, 0, 2); /* attnum */
pq_sendint(&buf, INT4OID, 4); /* type oid */
pq_sendint(&buf, 4, 2); /* typlen */
pq_sendint(&buf, 0, 4); /* typmod */
pq_sendint(&buf, 0, 2); /* format code */
pq_endmessage(&buf);
/* Send a DataRow message */
pq_beginmessage(&buf, 'D');
pq_sendint(&buf, 2, 2); /* # of columns */
pq_sendint(&buf, strlen(sysid), 4); /* col1 len */
pq_sendbytes(&buf, (char *) &sysid, strlen(sysid));
pq_sendint(&buf, strlen(tli), 4); /* col2 len */
pq_sendbytes(&buf, (char *) tli, strlen(tli));
pq_endmessage(&buf);
/* Send CommandComplete and ReadyForQuery messages */
EndCommand("SELECT", DestRemote);
ReadyForQuery(DestRemote);
}
else if (sscanf(query_string, "START_REPLICATION %X/%X",
&recptr.xlogid, &recptr.xrecoff) == 2)
{
StringInfoData buf;
/* Send a CopyOutResponse message, and start streaming */
pq_beginmessage(&buf, 'H');
pq_sendbyte(&buf, 0);
pq_sendint(&buf, 0, 2);
pq_endmessage(&buf);
/*
* Initialize position to the received one, then
* the xlog records begin to be shipped from that position
*/
sentPtr = recptr;
/* break out of the loop */
replication_started = true;
}
else
{
ereport(FATAL,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("invalid standby query string: %s", query_string)));
}
break;
}
/* 'X' means that the standby is closing the connection */
case 'X':
proc_exit(0);
case EOF:
ereport(ERROR,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("unexpected EOF on standby connection")));
default:
ereport(FATAL,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("invalid standby handshake message type %d", firstchar)));
}
}
}
/*
* Check if the remote end has closed the connection.
*/
static void
CheckClosedConnection(void)
{
unsigned char firstchar;
int r;
r = pq_getbyte_if_available(&firstchar);
if (r < 0)
{
/* no data available */
if (errno == EAGAIN || errno == EWOULDBLOCK)
return;
/*
* Ok if interrupted, though it shouldn't really happen with
* a non-blocking operation.
*/
if (errno == EINTR)
return;
ereport(COMMERROR,
(errcode_for_socket_access(),
errmsg("could not receive data from client: %m")));
}
if (r == 0)
{
/* standby disconnected unexpectedly */
ereport(ERROR,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("unexpected EOF on standby connection")));
}
/* Handle the very limited subset of commands expected in this phase */
switch (firstchar)
{
/*
* 'X' means that the standby is closing down the socket. EOF means
* unexpected loss of standby connection. Either way, perform normal
* shutdown.
*/
case 'X':
proc_exit(0);
default:
ereport(FATAL,
(errcode(ERRCODE_PROTOCOL_VIOLATION),
errmsg("invalid standby closing message type %d",
firstchar)));
}
}
/* Main loop of walsender process */
static int
WalSndLoop(void)
{
StringInfoData output_message;
initStringInfo(&output_message);
/* Loop forever */
for (;;)
{
int remain; /* remaining time (ms) */
/*
* Emergency bailout if postmaster has died. This is to avoid the
* necessity for manual cleanup of all postmaster children.
*/
if (!PostmasterIsAlive(true))
exit(1);
/* Process any requests or signals received recently */
if (got_SIGHUP)
{
got_SIGHUP = false;
ProcessConfigFile(PGC_SIGHUP);
}
/*
* When SIGUSR2 arrives, we send all outstanding logs up to the
* shutdown checkpoint record (i.e., the latest record) and exit.
*/
if (ready_to_stop)
{
XLogSend(&output_message);
shutdown_requested = true;
}
/* Normal exit from the walsender is here */
if (shutdown_requested)
{
/* Inform the standby that XLOG streaming was done */
pq_puttextmessage('C', "COPY 0");
pq_flush();
proc_exit(0);
}
/*
* Nap for the configured time or until a message arrives.
*
* On some platforms, signals won't interrupt the sleep. To ensure we
* respond reasonably promptly when someone signals us, break down the
* sleep into NAPTIME_PER_CYCLE (ms) increments, and check for
* interrupts after each nap.
*/
remain = WalSndDelay;
while (remain > 0)
{
if (got_SIGHUP || shutdown_requested || ready_to_stop)
break;
/*
* Check to see whether a message from the standby or an interrupt
* from other processes has arrived.
*/
pg_usleep(remain > NAPTIME_PER_CYCLE ? NAPTIME_PER_CYCLE : remain);
CheckClosedConnection();
remain -= NAPTIME_PER_CYCLE;
}
/* Attempt to send the log once every loop */
if (!XLogSend(&output_message))
goto eof;
}
/* can't get here because the above loop never exits */
return 1;
eof:
/*
* Reset whereToSendOutput to prevent ereport from attempting
* to send any more messages to the standby.
*/
if (whereToSendOutput == DestRemote)
whereToSendOutput = DestNone;
proc_exit(0);
return 1; /* keep the compiler quiet */
}
/* Initialize a per-walsender data structure for this walsender process */
static void
InitWalSnd(void)
{
/* use volatile pointer to prevent code rearrangement */
volatile WalSndCtlData *walsndctl = WalSndCtl;
int i;
/*
* WalSndCtl should be set up already (we inherit this by fork() or
* EXEC_BACKEND mechanism from the postmaster).
*/
Assert(walsndctl != NULL);
Assert(MyWalSnd == NULL);
/*
* Find a free walsender slot and reserve it. If this fails, we must be
* out of WalSnd structures.
*/
for (i = 0; i < MaxWalSenders; i++)
{
volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
SpinLockAcquire(&walsnd->mutex);
if (walsnd->pid != 0)
{
SpinLockRelease(&walsnd->mutex);
continue;
}
else
{
/* found */
MyWalSnd = (WalSnd *) walsnd;
walsnd->pid = MyProcPid;
MemSet(&MyWalSnd->sentPtr, 0, sizeof(XLogRecPtr));
SpinLockRelease(&walsnd->mutex);
break;
}
}
if (MyWalSnd == NULL)
ereport(FATAL,
(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
errmsg("sorry, too many standbys already")));
/* Arrange to clean up at walsender exit */
on_shmem_exit(WalSndKill, 0);
}
/* Destroy the per-walsender data structure for this walsender process */
static void
WalSndKill(int code, Datum arg)
{
Assert(MyWalSnd != NULL);
/*
* Mark WalSnd struct no longer in use. Assume that no lock is required
* for this.
*/
MyWalSnd->pid = 0;
/* WalSnd struct isn't mine anymore */
MyWalSnd = NULL;
}
/*
* Read 'nbytes' bytes from WAL into 'buf', starting at location 'recptr'
*/
void
XLogRead(char *buf, XLogRecPtr recptr, Size nbytes)
{
char path[MAXPGPATH];
uint32 startoff;
while (nbytes > 0)
{
int segbytes;
int readbytes;
startoff = recptr.xrecoff % XLogSegSize;
if (sendFile < 0 || !XLByteInSeg(recptr, sendId, sendSeg))
{
/* Switch to another logfile segment */
if (sendFile >= 0)
close(sendFile);
XLByteToSeg(recptr, sendId, sendSeg);
XLogFilePath(path, ThisTimeLineID, sendId, sendSeg);
sendFile = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
if (sendFile < 0)
ereport(FATAL, /* XXX: Why FATAL? */
(errcode_for_file_access(),
errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
path, sendId, sendSeg)));
sendOff = 0;
}
/* Need to seek in the file? */
if (sendOff != startoff)
{
if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not seek in log file %u, segment %u to offset %u: %m",
sendId, sendSeg, startoff)));
sendOff = startoff;
}
/* How many bytes are within this segment? */
if (nbytes > (XLogSegSize - startoff))
segbytes = XLogSegSize - startoff;
else
segbytes = nbytes;
readbytes = read(sendFile, buf, segbytes);
if (readbytes <= 0)
ereport(FATAL,
(errcode_for_file_access(),
errmsg("could not read from log file %u, segment %u, offset %u, "
"length %lu: %m",
sendId, sendSeg, sendOff, (unsigned long) segbytes)));
/* Update state for read */
XLByteAdvance(recptr, readbytes);
sendOff += readbytes;
nbytes -= readbytes;
buf += readbytes;
}
}
/*
* Read all WAL that's been written (and flushed) since last cycle, and send
* it to client.
*
* Returns true if OK, false if trouble.
*/
static bool
XLogSend(StringInfo outMsg)
{
XLogRecPtr SendRqstPtr;
char activitymsg[50];
/* use volatile pointer to prevent code rearrangement */
volatile WalSnd *walsnd = MyWalSnd;
/*
* Invalid position means that we have not yet received the initial
* CopyData message from the slave that indicates where to start the
* streaming.
*/
if (sentPtr.xlogid == 0 &&
sentPtr.xrecoff == 0)
return true;
/* Attempt to send all the records which were written to the disk */
SendRqstPtr = GetWriteRecPtr();
/* Quick exit if nothing to do */
if (!XLByteLT(sentPtr, SendRqstPtr))
return true;
/*
* Since successive pages in a segment are consecutively written,
* we can gather multiple records together by issuing just one
* read() call, and send them as one CopyData message at one time;
* nmsgs is the number of CopyData messages sent in this XLogSend;
* npages is the number of pages we have determined can be read and
* sent together; startpos is the starting position of reading and
* sending in the first page, startoff is the file offset at which
* it should go and endpos is the end position of reading and
* sending in the last page. We must initialize all of them to
* keep the compiler quiet.
*/
while (XLByteLT(sentPtr, SendRqstPtr))
{
XLogRecPtr startptr;
XLogRecPtr endptr;
Size nbytes;
/*
* Figure out how much to send in one message. If there's less than
* MAX_SEND_SIZE bytes to send, send everything. Otherwise send
* MAX_SEND_SIZE bytes, but round to page boundary for efficiency.
*/
startptr = sentPtr;
endptr = startptr;
XLByteAdvance(endptr, MAX_SEND_SIZE);
/*
* Round down to page boundary. This is not only for performance
* reasons, walreceiver relies on the fact that we never split a WAL
* record across two messages. Since a long WAL record is split at
* page boundary into continuation records, page boundary is always
* safe cut-off point. We also assume that SendRqstPtr never points
* in the middle of a WAL record.
*/
endptr.xrecoff -= (endptr.xrecoff % XLOG_BLCKSZ);
if (XLByteLT(SendRqstPtr, endptr))
endptr = SendRqstPtr;
/*
* OK to read and send the log.
*
* We don't need to convert the xlogid/xrecoff from host byte order
* to network byte order because the both server can be expected to
* have the same byte order. If they have the different order, we
* don't reach here.
*/
pq_sendbytes(outMsg, (char *) &startptr, sizeof(startptr));
if (endptr.xlogid != startptr.xlogid)
{
Assert(endptr.xlogid == startptr.xlogid + 1);
nbytes = endptr.xrecoff + XLogFileSize - startptr.xrecoff;
}
else
nbytes = endptr.xrecoff - startptr.xrecoff;
sentPtr = endptr;
/*
* Read the log into the output buffer directly to prevent
* extra memcpy calls.
*/
enlargeStringInfo(outMsg, nbytes);
XLogRead(&outMsg->data[outMsg->len], startptr, nbytes);
outMsg->len += nbytes;
outMsg->data[outMsg->len] = '\0';
pq_putmessage('d', outMsg->data, outMsg->len);
resetStringInfo(outMsg);
}
/* Update shared memory status */
SpinLockAcquire(&walsnd->mutex);
walsnd->sentPtr = sentPtr;
SpinLockRelease(&walsnd->mutex);
/* Flush pending output */
if (pq_flush())
return false;
/* Report progress of XLOG streaming in PS display */
snprintf(activitymsg, sizeof(activitymsg), "streaming %X/%X",
sentPtr.xlogid, sentPtr.xrecoff);
set_ps_display(activitymsg, false);
return true;
}
/* SIGHUP: set flag to re-read config file at next convenient time */
static void
WalSndSigHupHandler(SIGNAL_ARGS)
{
got_SIGHUP = true;
}
/* SIGTERM: set flag to shut down */
static void
WalSndShutdownHandler(SIGNAL_ARGS)
{
shutdown_requested = true;
}
/*
* WalSndQuickDieHandler() occurs when signalled SIGQUIT by the postmaster.
*
* Some backend has bought the farm,
* so we need to stop what we're doing and exit.
*/
static void
WalSndQuickDieHandler(SIGNAL_ARGS)
{
PG_SETMASK(&BlockSig);
/*
* We DO NOT want to run proc_exit() callbacks -- we're here because
* shared memory may be corrupted, so we don't want to try to clean up our
* transaction. Just nail the windows shut and get out of town. Now that
* there's an atexit callback to prevent third-party code from breaking
* things by calling exit() directly, we have to reset the callbacks
* explicitly to make this work as intended.
*/
on_exit_reset();
/*
* Note we do exit(2) not exit(0). This is to force the postmaster into a
* system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
* backend. This is necessary precisely because we don't clean up our
* shared memory state. (The "dead man switch" mechanism in pmsignal.c
* should ensure the postmaster sees this as a crash, too, but no harm
* in being doubly sure.)
*/
exit(2);
}
/* SIGUSR2: set flag to do a last cycle and shut down afterwards */
static void
WalSndLastCycleHandler(SIGNAL_ARGS)
{
ready_to_stop = true;
}
/* Set up signal handlers */
void
WalSndSignals(void)
{
/* Set up signal handlers */
pqsignal(SIGHUP, WalSndSigHupHandler); /* set flag to read config file */
pqsignal(SIGINT, SIG_IGN); /* not used */
pqsignal(SIGTERM, WalSndShutdownHandler); /* request shutdown */
pqsignal(SIGQUIT, WalSndQuickDieHandler); /* hard crash time */
pqsignal(SIGALRM, SIG_IGN);
pqsignal(SIGPIPE, SIG_IGN);
pqsignal(SIGUSR1, SIG_IGN); /* not used */
pqsignal(SIGUSR2, WalSndLastCycleHandler); /* request a last cycle and shutdown */
/* Reset some signals that are accepted by postmaster but not here */
pqsignal(SIGCHLD, SIG_DFL);
pqsignal(SIGTTIN, SIG_DFL);
pqsignal(SIGTTOU, SIG_DFL);
pqsignal(SIGCONT, SIG_DFL);
pqsignal(SIGWINCH, SIG_DFL);
}
/* Report shared-memory space needed by WalSndShmemInit */
Size
WalSndShmemSize(void)
{
Size size = 0;
size = offsetof(WalSndCtlData, walsnds);
size = add_size(size, mul_size(MaxWalSenders, sizeof(WalSnd)));
return size;
}
/* Allocate and initialize walsender-related shared memory */
void
WalSndShmemInit(void)
{
bool found;
int i;
WalSndCtl = (WalSndCtlData *)
ShmemInitStruct("Wal Sender Ctl", WalSndShmemSize(), &found);
if (WalSndCtl == NULL)
ereport(FATAL,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("not enough shared memory for walsender")));
if (found)
return; /* already initialized */
/* Initialize the data structures */
MemSet(WalSndCtl, 0, WalSndShmemSize());
for (i = 0; i < MaxWalSenders; i++)
{
WalSnd *walsnd = &WalSndCtl->walsnds[i];
SpinLockInit(&walsnd->mutex);
}
}
/*
* Returns the oldest Send position among walsenders. Or InvalidXLogRecPtr
* if none.
*/
XLogRecPtr
GetOldestWALSendPointer(void)
{
XLogRecPtr oldest = {0, 0};
int i;
bool found = false;
for (i = 0; i < MaxWalSenders; i++)
{
/* use volatile pointer to prevent code rearrangement */
volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
XLogRecPtr recptr;
if (walsnd->pid == 0)
continue;
SpinLockAcquire(&walsnd->mutex);
recptr = walsnd->sentPtr;
SpinLockRelease(&walsnd->mutex);
if (recptr.xlogid == 0 && recptr.xrecoff == 0)
continue;
if (!found || XLByteLT(recptr, oldest))
oldest = recptr;
found = true;
}
return oldest;
}