From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Add check for invalid offset at multixid truncation
Date: Thu, 15 Jan 2026 15:03:28 +0000
Message-ID: <E1vgOsh-000d0i-39@gemulon.postgresql.org> (raw)
Add check for invalid offset at multixid truncation
If a multixid with zero offset is left behind after a crash, and that
multixid later becomes the oldest multixid, truncation might try to
look up its offset and read the zero value. In the worst case, we
might incorrectly use the zero offset to truncate valid SLRU segments
that are still needed. I'm not sure if that can happen in practice, or
if there are some other lower-level safeguards or incidental reasons
that prevent the caller from passing an unwritten multixid as the
oldest multi. But better safe than sorry, so let's add an explicit
check for it.
In stable branches, we should perhaps do the same check for
'oldestOffset', i.e. the offset of the old oldest multixid (in master,
'oldestOffset' is gone). But if the old oldest multixid has an invalid
offset, the damage has been done already, and we would never advance
past that point. It's not clear what we should do in that case. The
check that this commit adds will prevent such an multixid with invalid
offset from becoming the oldest multixid in the first place, which
seems enough for now.
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: Discussion: https://www.postgresql.org/message-id/000301b2-5b81-4938-bdac-90f6eb660843@iki.fi
Backpatch-through: 14
Branch
------
REL_14_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/46be454ca4131e9731577d392897803d951e71c8
Modified Files
--------------
src/backend/access/transam/multixact.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
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To: pgsql-committers@postgresql.org
Cc: heikki.linnakangas@iki.fi, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Add check for invalid offset at multixid truncation
In-Reply-To: <E1vgOsh-000d0i-39@gemulon.postgresql.org>
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