Re: POC: make mxidoff 64 bits

Tom Lane <tgl@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

To: Heikki Linnakangas <hlinnaka@iki.fi>

Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Maxim Orlov <orlovmg@gmail.com>, Alexander Korotkov <aekorotkov@gmail.com>, wenhui qiu <qiuwenhuifx@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-12-14T22:55:36Z

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 →

Fix partial read handling in pg_upgrade's multixact conversion
- ac94ce8194e5 19 (unreleased) landed
Increase timeout in multixid_conversion upgrade test
- bd43940b02b2 19 (unreleased) landed
Improve sanity checks on multixid members length
- ecb553ae8211 19 (unreleased) landed
Clarify comment on multixid offset wraparound check
- 170361d7b869 14.21 landed
- b0b52b7123ae 15.16 landed
- 7d42e2367c6b 16.12 landed
- cd1a887fe9bf 17.8 landed
- 3fbad030a24d 18.2 landed
- 366dcdaf5779 19 (unreleased) landed
Never store 0 as the nextMXact
- 87a350e1f284 19 (unreleased) landed
Add runtime checks for bogus multixact offsets
- d4b7bde4183b 19 (unreleased) landed
Widen MultiXactOffset to 64 bits
- bd8d9c9bdfa0 19 (unreleased) landed
Move pg_multixact SLRU page format definitions to a separate header
- bb3b1c4f6462 19 (unreleased) landed
Convert confusing macros in multixact.c to static inline functions
- 0099b9408e8c 17.0 landed
Index SLRUs by 64-bit integers rather than by 32-bit integers
- 4ed8f0913bfd 17.0 cited
Cope with possible failure of the oldest MultiXact to exist.
- b6a3444fa635 9.4.4 cited

Heikki Linnakangas <hlinnaka@iki.fi> writes:
> Ok, I have pushed this. Thanks!

Coverity is unhappy about this bit:

/srv/coverity/git/pgsql-git/postgresql/src/bin/pg_upgrade/multixact_read_v18.c: 282             in GetOldMultiXactIdSingleMember()
276             if (!TransactionIdIsValid(*xactptr))
277             {
278                 /*
279                  * Corner case 2: we are looking at unused slot zero
280                  */
281                 if (offset == 0)
>>>     CID 1676077:         Control flow issues  (DEADCODE)
>>>     Execution cannot reach this statement: "continue;".
282                     continue;
283     
284                 /*
285                  * Otherwise this is an invalid entry that should not be

It sees the earlier test for offset == 0, and evidently is assuming
that the loop's "offset++" will not wrap around.  Now I think that
the point of this check is exactly that "offset++" could have wrapped
around, but the commentary is not so clear that I'm certain this is a
false positive.  If that is the intention, what do you think of
rephrasing this comment as "we have wrapped around to unused slot
zero"?

			regards, tom lane