Remember asking for feedback during walsender shutdown.

anarazel · anarazel · commit 596f936055c5 · 2016-04-28T22:11:18.000-07:00
Since 5a991ef we're explicitly asking for feedback from the receiving side when shutting down walsender, if there's not yet replicated data. Unfortunately we didn't remember (i.e. set waiting_for_ping_response to true) having asked for feedback, leading to scenarios in which replies were requested at a high frequency. I can't reproduce this problem on my laptop, I think that's because the problem requires a significant TCP window to manifest due to the !pq_is_send_pending() condition. But since this clearly is a bug, let's fix it. There's quite possibly more wrong than just this though. While fiddling with WalSndDone(), I rewrote a hard to understand comment about looking at the flush vs. the write position. Reported-By: Nick Cleaton, Magnus Hagander Author: Nick Cleaton Discussion: CAFgz3kus=rC_avEgBV=+hRK5HYJ8vXskJRh8yEAbahJGTzF2VQ@mail.gmail.com CABUevExsjROqDcD0A2rnJ6HK6FuKGyewJr3PL12pw85BHFGS2Q@mail.gmail.com Backpatch: 9.4, were 5a991ef introduced the use of feedback messages during shutdown.
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
@@ -2488,13 +2488,14 @@ WalSndDone(WalSndSendDataCallback send_data)
 	send_data();
 
 	/*
-	 * 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.
+	 * To figure out whether all WAL has successfully been replicated, check
+	 * flush location if valid, write otherwise. Tools like pg_receivexlog
+	 * will usually (unless in synchronous mode) return an invalid flush
+	 * location.
 	 */
 	replicatedPtr = XLogRecPtrIsInvalid(MyWalSnd->flush) ?
 		MyWalSnd->write : MyWalSnd->flush;
+
 	if (WalSndCaughtUp && sentPtr == replicatedPtr &&
 		!pq_is_send_pending())
 	{
@@ -2505,7 +2506,10 @@ WalSndDone(WalSndSendDataCallback send_data)
 		proc_exit(0);
 	}
 	if (!waiting_for_ping_response)
+	{
 		WalSndKeepalive(true);
+		waiting_for_ping_response = true;
+	}
 }
 
 /*
