Horton

Re: POC: make mxidoff 64 bits

Maxim Orlov <orlovmg@gmail.com>

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From: Maxim Orlov <orlovmg@gmail.com>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: wenhui qiu <qiuwenhuifx@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-10-30T06:13:54Z
Lists: pgsql-hackers

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 partial read handling in pg_upgrade's multixact conversion

  2. Increase timeout in multixid_conversion upgrade test

  3. Improve sanity checks on multixid members length

  4. Clarify comment on multixid offset wraparound check

  5. Never store 0 as the nextMXact

  6. Add runtime checks for bogus multixact offsets

  7. Widen MultiXactOffset to 64 bits

  8. Move pg_multixact SLRU page format definitions to a separate header

  9. Convert confusing macros in multixact.c to static inline functions

  10. Index SLRUs by 64-bit integers rather than by 32-bit integers

  11. Cope with possible failure of the oldest MultiXact to exist.

Attachments

Unfortunately, I need to admit that I have messed a bit with v18.
I forgot to sync the pg_upgrade portion with the rest of the patch,
among other things. Here's a proper version with additional testing.

pg_upgrade/t/007_mxoff.pl is not meant to be committed, it's just
for test purposes. In order to run it, env var oldinstall must be set.

On Tue, 28 Oct 2025 at 17:17, Heikki Linnakangas <hlinnaka@iki.fi> wrote:

> On 27/10/2025 17:54, Maxim Orlov wrote:
>
>
> If backend C looks up multixid 101 in between steps 3 and 4, it would
> read the offset incorrectly, because 'base' isn't set yet.
>
> Hmm, maybe I miss something? We set page base on first write of any
offset on the page, not only the first one. In other words, there
should never be a case when we read an offset without a previously
defined page base. Correct me if I'm wrong:
1. Backend A assigned mxact=100, offset=1000.
2. Backend B assigned mxact=101, offset=1010.
3. Backend B calls RecordNewMultiXact()/MXOffsetWrite() and
    set page base=1010, offset plus 0^0x80000000 bit while
    holding lock on the page.
4. Backend C looks up for the mxact=101 by calling MXOffsetRead()
    and should get exactly what he's looking for:
    base (1010) + offset (0) minus 0x80000000 bit.
5. Backend A calls RecordNewMultiXact() and sets his offset using
    existing base from step 3.


-- 
Best regards,
Maxim Orlov.