From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Set next multixid's offset when creating a new multixid
Date: Wed, 03 Dec 2025 17:17:15 +0000
Message-ID: <E1vQqTa-002cln-2N@gemulon.postgresql.org> (raw)
Set next multixid's offset when creating a new multixid
With this commit, the next multixid's offset will always be set on the
offsets page, by the time that a backend might try to read it, so we
no longer need the waiting mechanism with the condition variable. In
other words, this eliminates "corner case 2" mentioned in the
comments.
The waiting mechanism was broken in a few scenarios:
- When nextMulti was advanced without WAL-logging the next
multixid. For example, if a later multixid was already assigned and
WAL-logged before the previous one was WAL-logged, and then the
server crashed. In that case the next offset would never be set in
the offsets SLRU, and a query trying to read it would get stuck
waiting for it. Same thing could happen if pg_resetwal was used to
forcibly advance nextMulti.
- In hot standby mode, a deadlock could happen where one backend waits
for the next multixid assignment record, but WAL replay is not
advancing because of a recovery conflict with the waiting backend.
The old TAP test used carefully placed injection points to exercise
the old waiting code, but now that the waiting code is gone, much of
the old test is no longer relevant. Rewrite the test to reproduce the
IPC/MultixactCreation hang after crash recovery instead, and to verify
that previously recorded multixids stay readable.
Backpatch to all supported versions. In back-branches, we still need
to be able to read WAL that was generated before this fix, so in the
back-branches this includes a hack to initialize the next offsets page
when replaying XLOG_MULTIXACT_CREATE_ID for the last multixid on a
page. On 'master', bump XLOG_PAGE_MAGIC instead to indicate that the
WAL is not compatible.
Author: Andrey Borodin <amborodin@acm.org>
Reviewed-by: Dmitry Yurichev <dsy.075@yandex.ru>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Ivan Bykov <i.bykov@modernsys.ru>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/172e5723-d65f-4eec-b512-14beacb326ce@yandex.ru
Backpatch-through: 14
Branch
------
REL_14_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/81416e1016ddd31a4ad0847957e44ac91495b0dc
Modified Files
--------------
src/backend/access/transam/multixact.c | 193 +++++++++++++++++++++++++--------
1 file changed, 148 insertions(+), 45 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: heikki.linnakangas@iki.fi, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Set next multixid's offset when creating a new multixid
In-Reply-To: <E1vQqTa-002cln-2N@gemulon.postgresql.org>
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