Re: XLogReadRecord() error in XlogReadTwoPhaseData()

Noah Misch <noah(at)leadboat(dot)com> writes:
> Tom Lane reported another instance today:
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tadarida&dt=2021-11-11%2013%3A29%3A58

> Each of the three failures happened on a sparc64 Debian+gcc machine. I had
> tried ~8000 iterations on thorntail, another sparc64 Debian+gcc animal,
> without reproducing this.

>>> As a first step, let's report the actual XLogReadRecord() error message.
>>> Attached.

>> Good catch! This looks good.

> Pushed.

Well, we didn't have to wait too long [1]:

# at t/003_cic_2pc.pl line 143.
# 'pgbench: error: client 0 script 1 aborted in command 4 query 0: ERROR: could not read two-phase state from WAL at 0/159EF88: unexpected pageaddr 0/0 in log segment 000000010000000000000001, offset 5890048
# pgbench: error: client 2 script 3 aborted in command 2 query 0: ERROR: canceling statement due to lock timeout
# pgbench: fatal: Run was aborted; the above results are incomplete.

I suppose "unexpected pageaddr 0/0" is most easily explained by supposing
that XlogReadTwoPhaseData tried to read a WAL page that hadn't been
written out yet. Have we got any synchronization around that?

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tadarida&dt=2021-11-17%2013%3A01%3A24