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Commit900429d

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walsnd: Don't set waiting_for_ping_response spuriously
Ashutosh Bapat noticed that when logical walsender needs to wait forWAL, and it realizes that it must send a keepalive message towalreceiver to update the sent-LSN, which *does not* request a replyfrom walreceiver, it wrongly sets the flag that it's going to wait forthat reply. That means that any future would-be sender of feedbackmessages ends up not sending a feedback message, because they allbelieve that a reply is expected.With built-in logical replication there's not much harm in this, becauseWalReceiverMain will send a ping-back every wal_receiver_timeout/2anyway; but with other logical replication systems (e.g. pglogical) itcan cause significant pain.This problem was introduced in commit41d5f8a, where therequest-reply flag was changed from true to false to WalSndKeepalive,without at the same time removing the line that setswaiting_for_ping_response.Just removing that line would be a sufficient fix, but it seems betterto shift the responsibility of setting the flag to WalSndKeepaliveitself instead of requiring caller to do it; this is clearly lesserror-prone.Author: Álvaro Herrera <alvherre@alvh.no-ip.org>Reported-by: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>Backpatch: 9.5 and upDiscussion:https://postgr.es/m/20200806225558.GA22401@alvherre.pgsql
1 parentf1c3a41 commit900429d

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‎src/backend/replication/walsender.c

Lines changed: 12 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -151,7 +151,7 @@ static XLogRecPtr sendTimeLineValidUpto = InvalidXLogRecPtr;
151151
* How far have we sent WAL already? This is also advertised in
152152
* MyWalSnd->sentPtr. (Actually, this is the next WAL location to send.)
153153
*/
154-
staticXLogRecPtrsentPtr=0;
154+
staticXLogRecPtrsentPtr=InvalidXLogRecPtr;
155155

156156
/* Buffers for constructing outgoing messages and processing reply messages. */
157157
staticStringInfoDataoutput_message;
@@ -1451,10 +1451,7 @@ WalSndWaitForWal(XLogRecPtr loc)
14511451
if (MyWalSnd->flush<sentPtr&&
14521452
MyWalSnd->write<sentPtr&&
14531453
!waiting_for_ping_response)
1454-
{
14551454
WalSndKeepalive(false);
1456-
waiting_for_ping_response= true;
1457-
}
14581455

14591456
/* check whether we're done */
14601457
if (loc <=RecentFlushPtr)
@@ -2932,10 +2929,7 @@ WalSndDone(WalSndSendDataCallback send_data)
29322929
proc_exit(0);
29332930
}
29342931
if (!waiting_for_ping_response)
2935-
{
29362932
WalSndKeepalive(true);
2937-
waiting_for_ping_response= true;
2938-
}
29392933
}
29402934

29412935
/*
@@ -3432,10 +3426,13 @@ pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
34323426
}
34333427

34343428
/*
3435-
* This function is used to send a keepalive message to standby.
3436-
* If requestReply is set, sets a flag in the message requesting the standby
3437-
* to send a message back to us, for heartbeat purposes.
3438-
*/
3429+
* Send a keepalive message to standby.
3430+
*
3431+
* If requestReply is set, the message requests the other party to send
3432+
* a message back to us, for heartbeat purposes. We also set a flag to
3433+
* let nearby code that we're waiting for that response, to avoid
3434+
* repeated requests.
3435+
*/
34393436
staticvoid
34403437
WalSndKeepalive(boolrequestReply)
34413438
{
@@ -3450,6 +3447,10 @@ WalSndKeepalive(bool requestReply)
34503447

34513448
/* ... and send it wrapped in CopyData */
34523449
pq_putmessage_noblock('d',output_message.data,output_message.len);
3450+
3451+
/* Set local flag */
3452+
if (requestReply)
3453+
waiting_for_ping_response= true;
34533454
}
34543455

34553456
/*
@@ -3480,7 +3481,6 @@ WalSndKeepaliveIfNecessary(void)
34803481
if (last_processing >=ping_time)
34813482
{
34823483
WalSndKeepalive(true);
3483-
waiting_for_ping_response= true;
34843484

34853485
/* Try to flush pending output to the client */
34863486
if (pq_flush_if_writable()!=0)

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