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Re: Fix bug in multixact Oldest*MXactId initialization and access

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Yura Sokolov <y.sokolov@postgrespro.ru>, Sami Imseih <samimseih@gmail.com>
Cc: Chao Li <li.evan.chao@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-03T15:11:05Z
Lists: pgsql-hackers

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On 03/03/2026 14:02, Yura Sokolov wrote:
> 02.03.2026 22:09, Sami Imseih пишет:
>>>> It says “Each is indexed by ProcNumber”, but it’s no longer accurate for OldestMemberMXactId prepared-xact entries, which now use index (procno - FIRST_PREPARED_XACT_PROC_NUMBER).
>>>
>>> Fixed those and some other comment work, and pushed. Thanks!
>>>
>>
>> Thanks! what are your thoughts about adding a test like the one
>> here [1] ? This allows us to test correct handling of prepared
>> transaction dummy procs. The asserts added will not cover
>> this case.
>>
>> What do you think?
>>
>> [1] [https://www.postgresql.org/message-id/CAA5RZ0twq5bNMq0r0QNoopQnAEv%2BJ3qJNCrLs7HVqTEntBhJ%3Dg%40mail.gmail.com]
> I support: test for bug fixed is a good thing.

Ok, here's another version of Sami's repro. I realized that it doesn't 
even need concurrent sessions, so I moved it to the main regression test 
suite, into the 'prepared_xacts' test. Looks good?

- Heikki

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add test for row-locking and multixids with prepared transactions

  2. Skip prepared_xacts test if max_prepared_transactions < 2

  3. Fix OldestMemberMXactId and OldestVisibleMXactId array usage

  4. Redefine backend ID to be an index into the proc array

  5. Fix an old bug in multixact and two-phase commit. Prepared transactions can