Re: POC: make mxidoff 64 bits

On 06/12/2025 01:36, Heikki Linnakangas wrote:
> On 05/12/2025 15:42, Ashutosh Bapat wrote:
>> + $newnode->start;
>> + my $new_dump = get_dump_for_comparison($newnode, "newnode_${tag}
>> _dump");
>> + $newnode->stop;
>>
>>
>> There is no code which actually looks at the multixact offsets here to
>> make sure that the conversion happened correctly. I guess the test
>> relies on visibility checks for that. Anyway, we need a comment
>> explaining why just comparing the contents of the table is enough to
>> ensure correct conversion. Better if we can add an explicit test that
>> the offsets were converted correctly. I don't have any idea of how to
>> do that right now, though. Maybe use pg_get_multixact_members()
>> somehow in the query to extract data out of the table?
>
> Agreed, the verification here is quite weak. I didn't realize that
> pg_get_multixact_members() exists! That might indeed be handy here, but
> I'm not sure how exactly to construct the test. A direct C function like
> test_create_multixact() in test_multixact.c would be handy here, but
> we'd need to compile and do run that in the old cluster, which seems
> difficult.

I added verification of all the multixids between oldest and next
multixid, using pg_get_multixact_members(). The test now calls
pg_get_multixact_members() for all updating multixids in the range,
before and after the upgrade, and compares the results.

The verification ignores locking-only multixids. Verifying their
correctness would need a little more code because they're not fully
preserved by the upgrade.

I also expanded the test to cover multixid wraparound. It only covered
mxoffset wraparound previously.

New patch set attached. Only test changes compared to patch set v28.

- Heikki