Always treat a standby returning an an invalid flush location as async

This ensures that a standby such as pg_receivexlog will not be selected
as sync standby - which would cause the master to block waiting for
a location that could never happen.

Fujii Masao

src/backend/replication/syncrep.c

@@ -379,10 +379,12 @@ SyncRepReleaseWaiters(void)
 	/*
 	 * If this WALSender is serving a standby that is not on the list of
 	 * potential standbys then we have nothing to do. If we are still starting
-	 * up or still running base backup, then leave quickly also.
+	 * up, still running base backup or the current flush position is still
+	 * invalid, then leave quickly also.
 	 */
 	if (MyWalSnd->sync_standby_priority == 0 ||
-		MyWalSnd->state < WALSNDSTATE_STREAMING)
+		MyWalSnd->state < WALSNDSTATE_STREAMING ||
+		XLogRecPtrIsInvalid(MyWalSnd->flush))
 		return;
 
 	/*
@@ -402,7 +404,8 @@ SyncRepReleaseWaiters(void)
 			walsnd->state == WALSNDSTATE_STREAMING &&
 			walsnd->sync_standby_priority > 0 &&
 			(priority == 0 ||
-			 priority > walsnd->sync_standby_priority))
+			 priority > walsnd->sync_standby_priority) &&
+			!XLogRecPtrIsInvalid(walsnd->flush))
 		{
 			priority = walsnd->sync_standby_priority;
 			syncWalSnd = walsnd;

src/backend/replication/walreceiver.c

@@ -276,6 +276,11 @@ WalReceiverMain(void)
 	walrcv_connect(conninfo, startpoint);
 	DisableWalRcvImmediateExit();
 
+	/* Initialize LogstreamResult, reply_message and feedback_message */
+	LogstreamResult.Write = LogstreamResult.Flush = GetXLogReplayRecPtr();
+	MemSet(&reply_message, 0, sizeof(reply_message));
+	MemSet(&feedback_message, 0, sizeof(feedback_message));
+
 	/* Loop until end-of-streaming or error */
 	for (;;)
 	{

src/backend/replication/walsender.c

@@ -1443,12 +1443,19 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
 
 		if (walsnd->pid != 0)
 		{
-			sync_priority[i] = walsnd->sync_standby_priority;
+			/*
+			 * Treat a standby such as a pg_basebackup background process
+			 * which always returns an invalid flush location, as an
+			 * asynchronous standby.
+			 */
+			sync_priority[i] = XLogRecPtrIsInvalid(walsnd->flush) ?
+				0 : walsnd->sync_standby_priority;
 
 			if (walsnd->state == WALSNDSTATE_STREAMING &&
 				walsnd->sync_standby_priority > 0 &&
 				(priority == 0 ||
-				 priority > walsnd->sync_standby_priority))
+				 priority > walsnd->sync_standby_priority) &&
+				!XLogRecPtrIsInvalid(walsnd->flush))
 			{
 				priority = walsnd->sync_standby_priority;
 				sync_standby = i;