Feedback-ID: i0fe9450f:Fastmail
Date: Fri, 5 Jun 2026 11:14:31 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Xuneng Zhou <xunengzhou@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18158: Assert in pgstat_report_stat() fails when a backend
 shutting down with stats pending
Message-ID: <aiIxB2oawj8pZeON@paquier.xyz>
References: <18158-88f667028dbc7e7b@postgresql.org>
 <58b5d6bf-7dca-41e5-8000-e12981205ada@gmail.com>
 <CABPTF7XdDoLAoLKo7pOHmSqHniHf46Pw8Z=iqNx2uYi4J5ixBA@mail.gmail.com>
 <agQgZKF6q3rYRX7O@paquier.xyz>
 <CABPTF7VgzdBP_ZPKf2eZ79ry0fVKzcgCkB5gy60HGXCy0EQb0w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
	protocol="application/pgp-signature"; boundary="f1dkMevmLII8QzUu"
Content-Disposition: inline
In-Reply-To: 
 <CABPTF7VgzdBP_ZPKf2eZ79ry0fVKzcgCkB5gy60HGXCy0EQb0w@mail.gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/aiIxB2oawj8pZeON%40paquier.xyz>
Precedence: bulk


--f1dkMevmLII8QzUu
Content-Type: multipart/mixed; boundary="TwV5nfSZvOa/8eah"
Content-Disposition: inline


--TwV5nfSZvOa/8eah
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

On Thu, May 14, 2026 at 03:19:34PM +0800, Xuneng Zhou wrote:
> I agree that applying the change wholesale could be too invasive. If
> we decide not to address the ordering issue, removing this assertion
> seems reasonable, since the non-assert path does not actually provide
> the required guarantee.

Attached is a patch for 15~17 that is able to achieve this goal:
- Lift the assertion on shutdown for WAL senders, but keep it
everywhere else.
- Secondary trick in pgstat_shutdown_hook to cope with the fact that
some stats make still be around at shutdown for a WAL sender.

I cannot reproduce the failure re-using the trick sent by Alexander at
the top of the thread.

Thoughts or objections?
--
Michael

--TwV5nfSZvOa/8eah
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment;
	filename=0001-Lift-shutdown-assertion-in-pgstats-for-WAL-senders.patch
Content-Transfer-Encoding: quoted-printable

=46rom 62bae7f32ea095e0130af762bdf6ef39a7c5d388 Mon Sep 17 00:00:00 2001
=46rom: Michael Paquier <michael@paquier.xyz>
Date: Fri, 5 Jun 2026 10:42:47 +0900
Subject: [PATCH] Lift shutdown assertion in pgstats for WAL senders

Backpatch-through: 15-17
---
 src/backend/utils/activity/pgstat.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activi=
ty/pgstat.c
index 33025cf4aad3..b7cf6fa88ff4 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -97,6 +97,7 @@
 #include "lib/dshash.h"
 #include "pgstat.h"
 #include "port/atomics.h"
+#include "replication/walsender.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
 #include "storage/lwlock.h"
@@ -516,8 +517,12 @@ pgstat_shutdown_hook(int code, Datum arg)
=20
 	pgstat_report_stat(true);
=20
-	/* there shouldn't be any pending changes left */
-	Assert(dlist_is_empty(&pgStatPending));
+	/*
+	 * There shouldn't be any pending changes left, unless this is a WAL
+	 * sender that would shut down after the checkpointer has flushed the
+	 * stats.
+	 */
+	Assert(dlist_is_empty(&pgStatPending) || am_walsender);
 	dlist_init(&pgStatPending);
=20
 	pgstat_detach_shmem();
@@ -608,7 +613,13 @@ pgstat_report_stat(bool force)
 	 * assert that before the checks above, as there is an unconditional
 	 * pgstat_report_stat() call in pgstat_shutdown_hook() - which at least
 	 * the process that ran pgstat_before_server_shutdown() will still call.
+	 *
+	 * WAL senders would be shut down after the checkpointer and may still
+	 * have stats.  Skip them.
 	 */
+	if (pgStatLocal.shmem->is_shutdown && am_walsender)
+		return 0;
+
 	Assert(!pgStatLocal.shmem->is_shutdown);
=20
 	if (force)
--=20
2.54.0


--TwV5nfSZvOa/8eah--

--f1dkMevmLII8QzUu
Content-Type: application/pgp-signature; name=signature.asc

-----BEGIN PGP SIGNATURE-----
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=ZR5C
-----END PGP SIGNATURE-----

--f1dkMevmLII8QzUu--