Ashutosh Bapat noticed that WalSndWaitForWal() is setting
waiting_for_ping_response after sending a keepalive that does *not*
request a reply. The bad consequence is that other callers that do
require a reply end up in not sending a keepalive, because they think it
was already sent previously. So the whole thing gets stuck.
He found that commit 41d5f8ad734 failed to remove the setting of
waiting_for_ping_response after changing the "request" parameter
WalSndKeepalive from true to false; that seems to have been an omission
and it breaks the algorithm. Thread at [1].
The simplest fix is just to remove the line that sets
waiting_for_ping_response, but I think it is less error-prone to have
WalSndKeepalive set the flag itself, instead of expecting its callers to
do it (and know when to). Patch attached. Also rewords some related
commentary.
[1] https://postgr.es/m/flat/blu436-smtp25712b7ef9fc2adeb87c522dc...@phx.gbl
--
Álvaro Herrera Valdivia, Chile
>From bf5ca106817744c1ba300ecb8c86d230789988e0 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvhe...@alvh.no-ip.org>
Date: Thu, 6 Aug 2020 14:57:09 -0400
Subject: [PATCH] Fix waiting_for_ping in walsender
---
src/backend/replication/walsender.c | 29 ++++++++++++++---------------
1 file changed, 14 insertions(+), 15 deletions(-)
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 5e2210dd7b..c239902e0e 100644
--- a/src/backend/replication/walsender.c
+++ b/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;
@@ -1444,17 +1444,13 @@ 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, 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.
+ * send a ping containing the flush location. A reply from standby is
+ * not needed and would be wasteful.
*/
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 +2928,7 @@ WalSndDone(WalSndSendDataCallback send_data)
proc_exit(0);
}
if (!waiting_for_ping_response)
- {
WalSndKeepalive(true);
- waiting_for_ping_response = true;
- }
}
/*
@@ -3432,10 +3425,13 @@ 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 +3446,10 @@ 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 +3480,6 @@ 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)
--
2.20.1
