walsnd: Don't set waiting_for_ping_response spuriously

Ashutosh Bapat noticed that when logical walsender needs to wait for
WAL, and it realizes that it must send a keepalive message to
walreceiver to update the sent-LSN, which *does not* request a reply
from walreceiver, it wrongly sets the flag that it's going to wait for
that reply.  That means that any future would-be sender of feedback
messages ends up not sending a feedback message, because they all
believe that a reply is expected.

With built-in logical replication there's not much harm in this, because
WalReceiverMain will send a ping-back every wal_receiver_timeout/2
anyway; but with other logical replication systems (e.g. pglogical) it
can cause significant pain.

This problem was introduced in commit 41d5f8a, where the
request-reply flag was changed from true to false to WalSndKeepalive,
without at the same time removing the line that sets
waiting_for_ping_response.

Just removing that line would be a sufficient fix, but it seems better
to shift the responsibility of setting the flag to WalSndKeepalive
itself instead of requiring caller to do it; this is clearly less
error-prone.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reported-by: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>
Backpatch: 9.5 and up
Discussion: https://postgr.es/m/20200806225558.GA22401@alvherre.pgsql

Author: alvherre

src/backend/replication/walsender.c

@@ -151,7 +151,7 @@ static XLogRecPtr sendTimeLineValidUpto = InvalidXLogRecPtr;
  * How far have we sent WAL already? This is also advertised in
  * MyWalSnd->sentPtr.  (Actually, this is the next WAL location to send.)
  */
-static XLogRecPtr sentPtr = 0;
+static XLogRecPtr sentPtr = InvalidXLogRecPtr;
 
 /* Buffers for constructing outgoing messages and processing reply messages. */
 static StringInfoData output_message;
@@ -1451,10 +1451,7 @@ WalSndWaitForWal(XLogRecPtr loc)
 		if (MyWalSnd->flush < sentPtr &&
 			MyWalSnd->write < sentPtr &&
 			!waiting_for_ping_response)
-		{
 			WalSndKeepalive(false);
-			waiting_for_ping_response = true;
-		}
 
 		/* check whether we're done */
 		if (loc <= RecentFlushPtr)
@@ -2932,10 +2929,7 @@ WalSndDone(WalSndSendDataCallback send_data)
 		proc_exit(0);
 	}
 	if (!waiting_for_ping_response)
-	{
 		WalSndKeepalive(true);
-		waiting_for_ping_response = true;
-	}
 }
 
 /*
@@ -3432,10 +3426,13 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
 }
 
 /*
-  * This function is used to send a keepalive message to standby.
-  * If requestReply is set, sets a flag in the message requesting the standby
-  * to send a message back to us, for heartbeat purposes.
-  */
+ * Send a keepalive message to standby.
+ *
+ * If requestReply is set, the message requests the other party to send
+ * a message back to us, for heartbeat purposes.  We also set a flag to
+ * let nearby code that we're waiting for that response, to avoid
+ * repeated requests.
+ */
 static void
 WalSndKeepalive(bool requestReply)
 {
@@ -3450,6 +3447,10 @@ WalSndKeepalive(bool requestReply)
 
 	/* ... and send it wrapped in CopyData */
 	pq_putmessage_noblock('d', output_message.data, output_message.len);
+
+	/* Set local flag */
+	if (requestReply)
+		waiting_for_ping_response = true;
 }
 
 /*
@@ -3480,7 +3481,6 @@ WalSndKeepaliveIfNecessary(void)
 	if (last_processing >= ping_time)
 	{
 		WalSndKeepalive(true);
-		waiting_for_ping_response = true;
 
 		/* Try to flush pending output to the client */
 		if (pq_flush_if_writable() != 0)