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walsnd: Don't set waiting_for_ping_response spuriously

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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 41d5f8ad73, 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
This commit is contained in:
Alvaro Herrera 2020-08-08 12:31:55 -04:00
parent 1db9c80f83
commit 1fa6eec974
No known key found for this signature in database
GPG Key ID: 1C20ACB9D5C564AE
1 changed files with 16 additions and 14 deletions
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30
src/backend/replication/walsender.c
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@ -155,7 +155,7 @@ 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;
@ -1352,16 +1352,15 @@ WalSndWaitForWal(XLogRecPtr loc)
/*
* We only send regular messages to the client for full decoded
* transactions, but a synchronous replication and walsender shutdown
* possibly are waiting for a later location. So we send pings
* containing the flush location every now and then.
* possibly are waiting for a later location. So, before sleeping, we
* send a ping containing the flush location. If the receiver is
* otherwise idle, this keepalive will trigger a reply. Processing the
* reply will update these MyWalSnd locations.
*/
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)
@ -2862,10 +2861,7 @@ WalSndDone(WalSndSendDataCallback send_data)
proc_exit(0);
}
if (!waiting_for_ping_response)
{
WalSndKeepalive(true);
waiting_for_ping_response = true;
}
}
/*
@ -3361,10 +3357,13 @@ 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)
{
@ -3379,6 +3378,10 @@ 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;
}
/*
@ -3409,7 +3412,6 @@ 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)