Remove pg_backup_start_callback and reuse similar code

We had two copies of almost identical logic to revert shared memory
state when a running backup aborts; we can remove
pg_backup_start_callback if we adapt do_pg_abort_backup so that it can
be used for this purpose too.

However, in order for this to work, we have to repurpose the flag passed
to do_pg_abort_backup.  It used to indicate whether to throw a warning
(and the only caller always passed true).  It now indicates whether the
callback is being called at start time (in which case the session backup
state is known not to have been set to RUNNING yet, so action is always
taken) or shmem time (in which case action is only taken if the session
backup state is RUNNING).  Thus the meaning of the flag is no longer
superfluous, but it's actually quite critical to get right.  I (Álvaro)
chose to change the polarity and the code flow re. the flag from what
Bharath submitted, for coding clarity.

Co-authored-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://www.postgresql.org/message-id/20221013111330.564fk5tkwe3ha77l%40alvherre.pgsql

src/backend/access/transam/xlog.c

@@ -679,8 +679,6 @@ static void ReadControlFile(void);
 static void UpdateControlFile(void);
 static char *str_time(pg_time_t tnow);
 
-static void pg_backup_start_callback(int code, Datum arg);
-
 static int	get_sync_bit(int method);
 
 static void CopyXLogRecordToWAL(int write_len, bool isLogSwitch,
@@ -8271,7 +8269,7 @@ void
 do_pg_backup_start(const char *backupidstr, bool fast, List **tablespaces,
 				   BackupState *state, StringInfo tblspcmapfile)
 {
-	bool		backup_started_in_recovery = false;
+	bool		backup_started_in_recovery;
 
 	Assert(state != NULL);
 	backup_started_in_recovery = RecoveryInProgress();
@@ -8320,8 +8318,12 @@ do_pg_backup_start(const char *backupidstr, bool fast, List **tablespaces,
 	XLogCtl->Insert.forcePageWrites = true;
 	WALInsertLockRelease();
 
-	/* Ensure we release forcePageWrites if fail below */
-	PG_ENSURE_ERROR_CLEANUP(pg_backup_start_callback, (Datum) 0);
+	/*
+	 * Ensure we release forcePageWrites if fail below. NB -- for this to work
+	 * correctly, it is critical that sessionBackupState is only updated after
+	 * this block is over.
+	 */
+	PG_ENSURE_ERROR_CLEANUP(do_pg_abort_backup, DatumGetBool(true));
 	{
 		bool		gotUniqueStartpoint = false;
 		DIR		   *tblspcdir;
@@ -8531,7 +8533,7 @@ do_pg_backup_start(const char *backupidstr, bool fast, List **tablespaces,
 
 		state->starttime = (pg_time_t) time(NULL);
 	}
-	PG_END_ENSURE_ERROR_CLEANUP(pg_backup_start_callback, (Datum) 0);
+	PG_END_ENSURE_ERROR_CLEANUP(do_pg_abort_backup, DatumGetBool(true));
 
 	state->started_in_recovery = backup_started_in_recovery;
 
@@ -8541,23 +8543,6 @@ do_pg_backup_start(const char *backupidstr, bool fast, List **tablespaces,
 	sessionBackupState = SESSION_BACKUP_RUNNING;
 }
 
-/* Error cleanup callback for pg_backup_start */
-static void
-pg_backup_start_callback(int code, Datum arg)
-{
-	/* Update backup counters and forcePageWrites on failure */
-	WALInsertLockAcquireExclusive();
-
-	Assert(XLogCtl->Insert.runningBackups > 0);
-	XLogCtl->Insert.runningBackups--;
-
-	if (XLogCtl->Insert.runningBackups == 0)
-	{
-		XLogCtl->Insert.forcePageWrites = false;
-	}
-	WALInsertLockRelease();
-}
-
 /*
  * Utility routine to fetch the session-level status of a backup running.
  */
@@ -8850,38 +8835,42 @@ do_pg_backup_stop(BackupState *state, bool waitforarchive)
  * system out of backup mode, thus making it a lot more safe to call from
  * an error handler.
  *
- * The caller can pass 'arg' as 'true' or 'false' to control whether a warning
- * is emitted.
+ * 'arg' indicates that it's being called during backup setup; so
+ * sessionBackupState has not been modified yet, but runningBackups has
+ * already been incremented.  When it's false, then it's invoked as a
+ * before_shmem_exit handler, and therefore we must not change state
+ * unless sessionBackupState indicates that a backup is actually running.
  *
- * NB: This gets used as a before_shmem_exit handler, hence the odd-looking
- * signature.
+ * NB: This gets used as a PG_ENSURE_ERROR_CLEANUP callback and
+ * before_shmem_exit handler, hence the odd-looking signature.
  */
 void
 do_pg_abort_backup(int code, Datum arg)
 {
-	bool		emit_warning = DatumGetBool(arg);
+	bool		during_backup_start = DatumGetBool(arg);
 
-	/*
-	 * Quick exit if session does not have a running backup.
-	 */
-	if (sessionBackupState != SESSION_BACKUP_RUNNING)
-		return;
+	/* Only one of these conditions can be true */
+	Assert(during_backup_start ^
+		   (sessionBackupState == SESSION_BACKUP_RUNNING));
 
-	WALInsertLockAcquireExclusive();
-	Assert(XLogCtl->Insert.runningBackups > 0);
-	XLogCtl->Insert.runningBackups--;
-
-	if (XLogCtl->Insert.runningBackups == 0)
+	if (during_backup_start || sessionBackupState != SESSION_BACKUP_NONE)
 	{
-		XLogCtl->Insert.forcePageWrites = false;
-	}
+		WALInsertLockAcquireExclusive();
+		Assert(XLogCtl->Insert.runningBackups > 0);
+		XLogCtl->Insert.runningBackups--;
 
-	sessionBackupState = SESSION_BACKUP_NONE;
-	WALInsertLockRelease();
+		if (XLogCtl->Insert.runningBackups == 0)
+		{
+			XLogCtl->Insert.forcePageWrites = false;
+		}
+
+		sessionBackupState = SESSION_BACKUP_NONE;
+		WALInsertLockRelease();
 
-	if (emit_warning)
-		ereport(WARNING,
-				(errmsg("aborting backup due to backend exiting before pg_backup_stop was called")));
+		if (!during_backup_start)
+			ereport(WARNING,
+					errmsg("aborting backup due to backend exiting before pg_backup_stop was called"));
+	}
 }
 
 /*
@@ -8895,7 +8884,7 @@ register_persistent_abort_backup_handler(void)
 
 	if (already_done)
 		return;
-	before_shmem_exit(do_pg_abort_backup, DatumGetBool(true));
+	before_shmem_exit(do_pg_abort_backup, DatumGetBool(false));
 	already_done = true;
 }