Fix pg_receivexlog --synchronous

Make pg_receivexlog work correctly with —-synchronous without slots

Backpatch to 9.5

Gabriele Bartolini, reviewed by Michael Paquier and Simon Riggs

src/bin/pg_basebackup/receivelog.c

@@ -503,25 +503,27 @@ ReceiveXlogStream(PGconn *conn, StreamCtl *stream)
 	if (!CheckServerVersionForStreaming(conn))
 		return false;
 
-	if (replication_slot != NULL)
-	{
 	/*
-		 * Report the flush position, so the primary can know what WAL we'll
-		 * possibly re-request, and remove older WAL safely.
+	 * Decide whether we want to report the flush position. If we report
+	 * the flush position, the primary will know what WAL we'll
+	 * possibly re-request, and it can then remove older WAL safely.
+	 * We must always do that when we are using slots.
 	 *
-		 * We only report it when a slot has explicitly been used, because
-		 * reporting the flush position makes one eligible as a synchronous
+	 * Reporting the flush position makes one eligible as a synchronous
 	 * replica. People shouldn't include generic names in
 	 * synchronous_standby_names, but we've protected them against it so
-		 * far, so let's continue to do so in the situations when possible. If
-		 * they've got a slot, though, we need to report the flush position,
-		 * so that the master can remove WAL.
+	 * far, so let's continue to do so unless specifically requested.
 	 */
+	if (replication_slot != NULL)
+	{
 		reportFlushPosition = true;
 		sprintf(slotcmd, "SLOT \"%s\" ", replication_slot);
 	}
 	else
 	{
+		if (stream->synchronous)
+			reportFlushPosition = true;
+		else
 			reportFlushPosition = false;
 		slotcmd[0] = 0;
 	}