Allow a streaming replication standby to follow a timeline switch.

Before this patch, streaming replication would refuse to start replicating
if the timeline in the primary doesn't exactly match the standby. The
situation where it doesn't match is when you have a master, and two
standbys, and you promote one of the standbys to become new master.
Promoting bumps up the timeline ID, and after that bump, the other standby
would refuse to continue.

There's significantly more timeline related logic in streaming replication
now. First of all, when a standby connects to primary, it will ask the
primary for any timeline history files that are missing from the standby.
The missing files are sent using a new replication command TIMELINE_HISTORY,
and stored in standby's pg_xlog directory. Using the timeline history files,
the standby can follow the latest timeline present in the primary
(recovery_target_timeline='latest'), just as it can follow new timelines
appearing in an archive directory.

START_REPLICATION now takes a TIMELINE parameter, to specify exactly which
timeline to stream WAL from. This allows the standby to request the primary
to send over WAL that precedes the promotion. The replication protocol is
changed slightly (in a backwards-compatible way although there's little hope
of streaming replication working across major versions anyway), to allow
replication to stop when the end of timeline reached, putting the walsender
back into accepting a replication command.

Many thanks to Amit Kapila for testing and reviewing various versions of
this patch.

src/include/replication/walreceiver.h

@@ -14,6 +14,7 @@
 
 #include "access/xlog.h"
 #include "access/xlogdefs.h"
+#include "storage/latch.h"
 #include "storage/spin.h"
 #include "pgtime.h"
 
@@ -40,7 +41,9 @@ typedef enum
 	WALRCV_STOPPED,				/* stopped and mustn't start up again */
 	WALRCV_STARTING,			/* launched, but the process hasn't
 								 * initialized yet */
-	WALRCV_RUNNING,				/* walreceiver is running */
+	WALRCV_STREAMING,			/* walreceiver is streaming */
+	WALRCV_WAITING,				/* stopped streaming, waiting for orders */
+	WALRCV_RESTARTING,			/* asked to restart streaming */
 	WALRCV_STOPPING				/* requested to stop, but still running */
 } WalRcvState;
 
@@ -57,19 +60,23 @@ typedef struct
 	pg_time_t	startTime;
 
 	/*
-	 * receiveStart is the first byte position that will be received. When
-	 * startup process starts the walreceiver, it sets receiveStart to the
-	 * point where it wants the streaming to begin.
+	 * receiveStart and receiveStartTLI indicate the first byte position
+	 * and timeline that will be received. When startup process starts the
+	 * walreceiver, it sets these to the point where it wants the streaming
+	 * to begin.
 	 */
 	XLogRecPtr	receiveStart;
+	TimeLineID	receiveStartTLI;
 
 	/*
 	 * receivedUpto-1 is the last byte position that has already been
-	 * received.  At the first startup of walreceiver, receivedUpto is set to
-	 * receiveStart. After that, walreceiver updates this whenever it flushes
-	 * the received WAL to disk.
+	 * received, and receivedTLI is the timeline it came from.  At the first
+	 * startup of walreceiver, these are set to receiveStart and
+	 * receiveStartTLI. After that, walreceiver updates these whenever it
+	 * flushes the received WAL to disk.
 	 */
 	XLogRecPtr	receivedUpto;
+	TimeLineID	receivedTLI;
 
 	/*
 	 * latestChunkStart is the starting byte position of the current "batch"
@@ -97,16 +104,34 @@ typedef struct
 	char		conninfo[MAXCONNINFO];
 
 	slock_t		mutex;			/* locks shared variables shown above */
+
+	/*
+	 * Latch used by startup process to wake up walreceiver after telling it
+	 * where to start streaming (after setting receiveStart and
+	 * receiveStartTLI).
+	 */
+	Latch		latch;
 } WalRcvData;
 
 extern WalRcvData *WalRcv;
 
 /* libpqwalreceiver hooks */
-typedef bool (*walrcv_connect_type) (char *conninfo, XLogRecPtr startpoint);
+typedef void (*walrcv_connect_type) (char *conninfo);
 extern PGDLLIMPORT walrcv_connect_type walrcv_connect;
 
-typedef bool (*walrcv_receive_type) (int timeout, unsigned char *type,
-												 char **buffer, int *len);
+typedef void (*walrcv_identify_system_type) (TimeLineID *primary_tli);
+extern PGDLLIMPORT walrcv_identify_system_type walrcv_identify_system;
+
+typedef void (*walrcv_readtimelinehistoryfile_type) (TimeLineID tli, char **filename, char **content, int *size);
+extern PGDLLIMPORT walrcv_readtimelinehistoryfile_type walrcv_readtimelinehistoryfile;
+
+typedef bool (*walrcv_startstreaming_type) (TimeLineID tli, XLogRecPtr startpoint);
+extern PGDLLIMPORT walrcv_startstreaming_type walrcv_startstreaming;
+
+typedef void (*walrcv_endstreaming_type) (void);
+extern PGDLLIMPORT walrcv_endstreaming_type walrcv_endstreaming;
+
+typedef int (*walrcv_receive_type) (int timeout, char **buffer);
 extern PGDLLIMPORT walrcv_receive_type walrcv_receive;
 
 typedef void (*walrcv_send_type) (const char *buffer, int nbytes);
@@ -122,9 +147,10 @@ extern void WalReceiverMain(void) __attribute__((noreturn));
 extern Size WalRcvShmemSize(void);
 extern void WalRcvShmemInit(void);
 extern void ShutdownWalRcv(void);
-extern bool WalRcvInProgress(void);
-extern void RequestXLogStreaming(XLogRecPtr recptr, const char *conninfo);
-extern XLogRecPtr GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart);
+extern bool WalRcvStreaming(void);
+extern bool WalRcvRunning(void);
+extern void RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo);
+extern XLogRecPtr GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart, TimeLineID *receiveTLI);
 extern int	GetReplicationApplyDelay(void);
 extern int	GetReplicationTransferLatency(void);
 

src/include/replication/walsender.h

@@ -19,7 +19,6 @@
 /* global state */
 extern bool am_walsender;
 extern bool am_cascading_walsender;
-extern volatile sig_atomic_t walsender_ready_to_stop;
 extern bool wake_wal_senders;
 
 /* user-settable parameters */

src/include/replication/walsender_private.h

@@ -95,7 +95,7 @@ extern WalSndCtlData *WalSndCtl;
 
 
 extern void WalSndSetState(WalSndState state);
-extern void XLogRead(char *buf, XLogRecPtr startptr, Size count);
+extern void XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count);
 
 /*
  * Internal functions for parsing the replication grammar, in repl_gram.y and