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 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.
