Fix bug in cancellation of non-exclusive backup to avoid assertion failure.

Previously an assertion failure occurred when pg_stop_backup() for
non-exclusive backup was aborted while it's waiting for WAL files to
be archived. This assertion failure happened in do_pg_abort_backup()
which was called when a non-exclusive backup was canceled.
do_pg_abort_backup() assumes that there is at least one non-exclusive
backup running when it's called. But pg_stop_backup() can be canceled
even after it marks the end of non-exclusive backup (e.g.,
during waiting for WAL archiving). This broke the assumption that
do_pg_abort_backup() relies on, and which caused an assertion failure.

This commit changes do_pg_abort_backup() so that it does nothing
when non-exclusive backup has been already marked as completed.
That is, the asssumption is also changed, and do_pg_abort_backup()
now can handle even the case where it's called when there is
no running backup.

Backpatch to 9.6 where SQL-callable non-exclusive backup was added.

Author: Masahiko Sawada and Michael Paquier
Reviewed-By: Robert Haas and Fujii Masao
Discussion: https://www.postgresql.org/message-id/CAD21AoD2L1Fu2c==gnVASMyFAAaq3y-AQ2uEVj-zTCGFFjvmDg@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -10628,13 +10628,20 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 	/*
 	 * Mark that start phase has correctly finished for an exclusive backup.
 	 * Session-level locks are updated as well to reflect that state.
+	 *
+	 * Note that CHECK_FOR_INTERRUPTS() must not occur while updating
+	 * backup counters and session-level lock. Otherwise they can be
+	 * updated inconsistently, and which might cause do_pg_abort_backup()
+	 * to fail.
 	 */
 	if (exclusive)
 	{
 		WALInsertLockAcquireExclusive();
 		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
-		WALInsertLockRelease();
+
+		/* Set session-level lock */
 		sessionBackupState = SESSION_BACKUP_EXCLUSIVE;
+		WALInsertLockRelease();
 	}
 	else
 		sessionBackupState = SESSION_BACKUP_NON_EXCLUSIVE;
@@ -10838,7 +10845,11 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 	}
 
 	/*
-	 * OK to update backup counters and forcePageWrites
+	 * OK to update backup counters, forcePageWrites and session-level lock.
+	 *
+	 * Note that CHECK_FOR_INTERRUPTS() must not occur while updating them.
+	 * Otherwise they can be updated inconsistently, and which might cause
+	 * do_pg_abort_backup() to fail.
 	 */
 	WALInsertLockAcquireExclusive();
 	if (exclusive)
@@ -10862,11 +10873,20 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 	{
 		XLogCtl->Insert.forcePageWrites = false;
 	}
-	WALInsertLockRelease();
 
-	/* Clean up session-level lock */
+	/*
+	 * Clean up session-level lock.
+	 *
+	 * You might think that WALInsertLockRelease() can be called
+	 * before cleaning up session-level lock because session-level
+	 * lock doesn't need to be protected with WAL insertion lock.
+	 * But since CHECK_FOR_INTERRUPTS() can occur in it,
+	 * session-level lock must be cleaned up before it.
+	 */
 	sessionBackupState = SESSION_BACKUP_NONE;
 
+	WALInsertLockRelease();
+
 	/*
 	 * Read and parse the START WAL LOCATION line (this code is pretty crude,
 	 * but we are not expecting any variability in the file format).
@@ -11104,8 +11124,16 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 void
 do_pg_abort_backup(void)
 {
+	/*
+	 * Quick exit if session is not keeping around a non-exclusive backup
+	 * already started.
+	 */
+	if (sessionBackupState == SESSION_BACKUP_NONE)
+		return;
+
 	WALInsertLockAcquireExclusive();
 	Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
+	Assert(sessionBackupState == SESSION_BACKUP_NON_EXCLUSIVE);
 	XLogCtl->Insert.nonExclusiveBackups--;
 
 	if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&

src/backend/replication/basebackup.c

@@ -215,7 +215,7 @@ perform_base_backup(basebackup_options *opt)
 	 * Once do_pg_start_backup has been called, ensure that any failure causes
 	 * us to abort the backup so we don't "leak" a backup counter. For this
 	 * reason, *all* functionality between do_pg_start_backup() and
-	 * do_pg_stop_backup() should be inside the error cleanup block!
+	 * the end of do_pg_stop_backup() should be inside the error cleanup block!
 	 */
 
 	PG_ENSURE_ERROR_CLEANUP(base_backup_cleanup, (Datum) 0);
@@ -324,10 +324,11 @@ perform_base_backup(basebackup_options *opt)
 			else
 				pq_putemptymessage('c');	/* CopyDone */
 		}
+
+		endptr = do_pg_stop_backup(labelfile->data, !opt->nowait, &endtli);
 	}
 	PG_END_ENSURE_ERROR_CLEANUP(base_backup_cleanup, (Datum) 0);
 
-	endptr = do_pg_stop_backup(labelfile->data, !opt->nowait, &endtli);
 
 	if (opt->includewal)
 	{