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.
@@ -10457,20 +10516,91 @@ 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.")));
 
-	/*
-	 * OK to update backup counters and forcePageWrites
-	 */
-	WALInsertLockAcquireExclusive();
 	if (exclusive)
 	{
-		if (!XLogCtl->Insert.exclusiveBackup)
+		/*
+		 * 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 (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;
+		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STOPPING;
+		WALInsertLockRelease();
+
+		/*
+		 * 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.
+			 */
+			struct stat statbuf;
+			int			r;
+
+			if (stat(BACKUP_LABEL_FILE, &statbuf))
+			{
+				/* should not happen per the upper checks */
+				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)));
+
+			/*
+			 * Remove tablespace_map file if present, it is created only if there
+			 * are tablespaces.
+			 */
+			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
 	{
@@ -10484,66 +10614,13 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 		XLogCtl->Insert.nonExclusiveBackups--;
 	}
 
-	if (!XLogCtl->Insert.exclusiveBackup &&
+	if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
 		XLogCtl->Insert.nonExclusiveBackups == 0)
 	{
 		XLogCtl->Insert.forcePageWrites = false;
 	}
 	WALInsertLockRelease();
 
-	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)));
-
-		/*
-		 * Remove tablespace_map file if present, it is created only if there
-		 * are tablespaces.
-		 */
-		unlink(TABLESPACE_MAP);
-	}
-
 	/*
 	 * Read and parse the START WAL LOCATION line (this code is pretty crude,
 	 * but we are not expecting any variability in the file format).
@@ -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;