Fix bug in clean shutdown of walsender that pg_receiving is connecting to.

On clean shutdown, walsender waits for all WAL to be replicated to a standby,
and exits. It determined whether that replication had been completed by
checking whether its sent location had been equal to a standby's flush
location. Unfortunately this condition never becomes true when the standby
such as pg_receivexlog which always returns an invalid flush location is
connecting to walsender, and then walsender waits forever.

This commit changes walsender so that it just checks a standby's write
location if a flush location is invalid.

Back-patch to 9.1 where enough infrastructure for this exists.

src/backend/replication/walsender.c

@@ -2446,10 +2446,20 @@ XLogSendLogical(void)
 static void
 WalSndDone(WalSndSendDataCallback send_data)
 {
+	XLogRecPtr	replicatedPtr;
+
 	/* ... let's just be real sure we're caught up ... */
 	send_data();
 
-	if (WalSndCaughtUp && sentPtr == MyWalSnd->flush &&
+	/*
+	 * Check a write location to see whether all the WAL have
+	 * successfully been replicated if this walsender is connecting
+	 * to a standby such as pg_receivexlog which always returns
+	 * an invalid flush location. Otherwise, check a flush location.
+	 */
+	replicatedPtr = XLogRecPtrIsInvalid(MyWalSnd->flush) ?
+		MyWalSnd->write : MyWalSnd->flush;
+	if (WalSndCaughtUp && sentPtr == replicatedPtr &&
 		!pq_is_send_pending())
 	{
 		/* Inform the standby that XLOG streaming is done */