Fix an assertion failure related to an exclusive backup.

Previously multiple sessions could execute pg_start_backup() and
pg_stop_backup() to start and stop an exclusive backup at the same time.
This could trigger the assertion failure of
"FailedAssertion("!(XLogCtl->Insert.exclusiveBackup)".
This happend because, even while pg_start_backup() was starting
an exclusive backup, other session could run pg_stop_backup()
concurrently and mark the backup as not-in-progress unconditionally.

This patch introduces ExclusiveBackupState indicating the state of
an exclusive backup. This state is used to ensure that there is only
one session running pg_start_backup() or pg_stop_backup() at
the same time, to avoid the assertion failure.

Back-patch to all supported versions.

Author: Michael Paquier
Reviewed-By: Kyotaro Horiguchi and me
Reported-By: Andreas Seltenreich
Discussion: <87mvktojme.fsf@credativ.de>

src/backend/access/transam/xlog.c

@@ -472,6 +472,29 @@ typedef union WALInsertLockPadded
 	char		pad[PG_CACHE_LINE_SIZE];
 } WALInsertLockPadded;
 
+/*
+ * State of an exclusive backup, necessary to control concurrent activities
+ * across sessions when working on exclusive backups.
+ *
+ * EXCLUSIVE_BACKUP_NONE means that there is no exclusive backup actually
+ * running, to be more precise pg_start_backup() is not being executed for
+ * an exclusive backup and there is no exclusive backup in progress.
+ * EXCLUSIVE_BACKUP_STARTING means that pg_start_backup() is starting an
+ * exclusive backup.
+ * EXCLUSIVE_BACKUP_IN_PROGRESS means that pg_start_backup() has finished
+ * running and an exclusive backup is in progress. pg_stop_backup() is
+ * needed to finish it.
+ * EXCLUSIVE_BACKUP_STOPPING means that pg_stop_backup() is stopping an
+ * exclusive backup.
+ */
+typedef enum ExclusiveBackupState
+{
+	EXCLUSIVE_BACKUP_NONE = 0,
+	EXCLUSIVE_BACKUP_STARTING,
+	EXCLUSIVE_BACKUP_IN_PROGRESS,
+	EXCLUSIVE_BACKUP_STOPPING
+} ExclusiveBackupState;
+
 /*
  * Shared state data for WAL insertion.
  */
@@ -513,13 +536,15 @@ typedef struct XLogCtlInsert
 	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.
+	 * exclusiveBackupState indicates the state of an exclusive backup
+	 * (see comments of ExclusiveBackupState for more details).
+	 * 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;
+	ExclusiveBackupState exclusiveBackupState;
 	int			nonExclusiveBackups;
 	XLogRecPtr	lastBackupStart;
 
@@ -858,6 +883,7 @@ static void xlog_outrec(StringInfo buf, XLogReaderState *record);
 #endif
 static void xlog_outdesc(StringInfo buf, XLogReaderState *record);
 static void pg_start_backup_callback(int code, Datum arg);
+static void pg_stop_backup_callback(int code, Datum arg);
 static bool read_backup_label(XLogRecPtr *checkPointLoc,
 				  bool *backupEndRequired, bool *backupFromStandby);
 static bool read_tablespace_map(List **tablespaces);
@@ -10016,7 +10042,12 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 	WALInsertLockAcquireExclusive();
 	if (exclusive)
 	{
-		if (XLogCtl->Insert.exclusiveBackup)
+		/*
+		 * At first, mark that we're now starting an exclusive backup,
+		 * to ensure that there are no other sessions currently running
+		 * pg_start_backup() or pg_stop_backup().
+		 */
+		if (XLogCtl->Insert.exclusiveBackupState != EXCLUSIVE_BACKUP_NONE)
 		{
 			WALInsertLockRelease();
 			ereport(ERROR,
@@ -10024,7 +10055,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 					 errmsg("a backup is already in progress"),
 					 errhint("Run pg_stop_backup() and try again.")));
 		}
-		XLogCtl->Insert.exclusiveBackup = true;
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STARTING;
 	}
 	else
 		XLogCtl->Insert.nonExclusiveBackups++;
@@ -10279,7 +10310,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 		{
 			/*
 			 * Check for existing backup label --- implies a backup is already
-			 * running.  (XXX given that we checked exclusiveBackup above,
+			 * running.  (XXX given that we checked exclusiveBackupState above,
 			 * maybe it would be OK to just unlink any such label file?)
 			 */
 			if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
@@ -10360,6 +10391,16 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 	}
 	PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
 
+	/*
+	 * Mark that start phase has correctly finished for an exclusive backup.
+	 */
+	if (exclusive)
+	{
+		WALInsertLockAcquireExclusive();
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
+		WALInsertLockRelease();
+	}
+
 	/*
 	 * We're done.  As a convenience, return the starting WAL location.
 	 */
@@ -10378,8 +10419,8 @@ pg_start_backup_callback(int code, Datum arg)
 	WALInsertLockAcquireExclusive();
 	if (exclusive)
 	{
-		Assert(XLogCtl->Insert.exclusiveBackup);
-		XLogCtl->Insert.exclusiveBackup = false;
+		Assert(XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_STARTING);
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_NONE;
 	}
 	else
 	{
@@ -10387,7 +10428,7 @@ pg_start_backup_callback(int code, Datum arg)
 		XLogCtl->Insert.nonExclusiveBackups--;
 	}
 
-	if (!XLogCtl->Insert.exclusiveBackup &&
+	if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
 		XLogCtl->Insert.nonExclusiveBackups == 0)
 	{
 		XLogCtl->Insert.forcePageWrites = false;
@@ -10395,6 +10436,24 @@ pg_start_backup_callback(int code, Datum arg)
 	WALInsertLockRelease();
 }
 
+/*
+ * Error cleanup callback for pg_stop_backup
+ */
+static void
+pg_stop_backup_callback(int code, Datum arg)
+{
+	bool		exclusive = DatumGetBool(arg);
+
+	/* Update backup status on failure */
+	WALInsertLockAcquireExclusive();
+	if (exclusive)
+	{
+		Assert(XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_STOPPING);
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
+	}
+	WALInsertLockRelease();
+}
+
 /*
  * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
  * function.
@@ -10457,41 +10516,29 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 			  errmsg("WAL level not sufficient for making an online backup"),
 				 errhint("wal_level must be set to \"replica\" or \"logical\" at server start.")));
 
+	if (exclusive)
+	{
 		/*
-	 * OK to update backup counters and forcePageWrites
+		 * At first, mark that we're now stopping an exclusive backup,
+		 * to ensure that there are no other sessions currently running
+		 * pg_start_backup() or pg_stop_backup().
 		 */
 		WALInsertLockAcquireExclusive();
-	if (exclusive)
-	{
-		if (!XLogCtl->Insert.exclusiveBackup)
+		if (XLogCtl->Insert.exclusiveBackupState != EXCLUSIVE_BACKUP_IN_PROGRESS)
 		{
 			WALInsertLockRelease();
 			ereport(ERROR,
 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					 errmsg("exclusive backup not in progress")));
 		}
-		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;
-	}
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STOPPING;
 		WALInsertLockRelease();
 
-	if (exclusive)
+		/*
+		 * Remove backup_label. In case of failure, the state for an exclusive
+		 * backup is switched back to in-progress.
+		 */
+		PG_ENSURE_ERROR_CLEANUP(pg_stop_backup_callback, (Datum) BoolGetDatum(exclusive));
 		{
 			/*
 			 * Read the existing label file into memory.
@@ -10501,6 +10548,7 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 
 			if (stat(BACKUP_LABEL_FILE, &statbuf))
 			{
+				/* should not happen per the upper checks */
 				if (errno != ENOENT)
 					ereport(ERROR,
 							(errcode_for_file_access(),
@@ -10543,6 +10591,35 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 			 */
 			unlink(TABLESPACE_MAP);
 		}
+		PG_END_ENSURE_ERROR_CLEANUP(pg_stop_backup_callback, (Datum) BoolGetDatum(exclusive));
+	}
+
+	/*
+	 * OK to update backup counters and forcePageWrites
+	 */
+	WALInsertLockAcquireExclusive();
+	if (exclusive)
+	{
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_NONE;
+	}
+	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.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
+		XLogCtl->Insert.nonExclusiveBackups == 0)
+	{
+		XLogCtl->Insert.forcePageWrites = false;
+	}
+	WALInsertLockRelease();
 
 	/*
 	 * Read and parse the START WAL LOCATION line (this code is pretty crude,
@@ -10780,7 +10857,7 @@ do_pg_abort_backup(void)
 	Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
 	XLogCtl->Insert.nonExclusiveBackups--;
 
-	if (!XLogCtl->Insert.exclusiveBackup &&
+	if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
 		XLogCtl->Insert.nonExclusiveBackups == 0)
 	{
 		XLogCtl->Insert.forcePageWrites = false;