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Re: IPC/MultixactCreation on the Standby server

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Álvaro Herrera <alvherre@kurilemu.de>
Cc: Dmitry Yurichev <dsy.075@yandex.ru>, Andrey Borodin <x4mmm@yandex-team.ru>, pgsql-hackers@lists.postgresql.org, Ivan Bykov <i.bykov@modernsys.ru>, Kirill Reshke <reshkekirill@gmail.com>
Date: 2025-12-05T09:49:25Z
Lists: pgsql-hackers 
On 04/12/2025 16:36, Álvaro Herrera wrote:
> On 2025-Dec-04, Heikki Linnakangas wrote:
> 
>> While working on the 64-bit multixid offsets patch, I noticed one more bug
>> with this. At offset wraparound, when we set the next multixid's offset in
>> RecordNewMultiXact, we incorrectly set it to 0 instead of 1. We're supposed
>> to skip over offset 1, because 0 is reserved to mean invalid. We do that
>> correctly when setting the "current" multixid's offset, because the caller
>> of RecordNewMultiXact has already skipped over offset 0, but I missed it for
>> the next offset.
> 
> Ouch.

Committed the fix.

>> I tried to modify the new wraparound TAP test to reproduce that, but it
>> turned out to be difficult because you need to have multiple backends
>> assigning multixids concurrently to hit that.
> 
> Hmm, would it make sense to add a pgbench-based test on
> src/test/modules/xid_wraparound?  That module is already known to be
> expensive, and it doesn't run unless explicitly enabled, so I think it's
> not a bad fit.

Doesn't seem worth it. The repro with pgbench is a bit unreliable too. 
It actually only readily reproduces on 'master'. In backbranches, I also 
had to add a random sleep before RecordNewMultiXact() to trigger it. I 
think that's because on 'master', I removed the special case in 
GetMultiXactIdMembers() to use the in-memory nextOffset value instead of 
reading the next offset from the SLRU page, when we're reading the last 
assigned multixid.

Furthermore, we will hopefully get rid of offset wraparounds soon with 
the 64-bit offsets.

- 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. Fix test to work with non-8kB block sizes

  2. Fix setting next multixid's offset at offset wraparound

  3. Add test for multixid wraparound

  4. Set next multixid's offset when creating a new multixid

  5. psql: Improve tab completion for large objects.

  6. Refactor some repetitive SLRU code

  7. Transaction IDs wrap around, per my proposal of 13-Aug-01. More