Subject: Race condition in server-crash testing

From: Tom Lane <[email-masked]>
Date: Mon, 04 Apr 2022 00:50:27 -0400

My pet dinosaur gaur just failed [1] in
src/test/recovery/t/022_crash_temp_files.pl, which does this:

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my $ret = PostgreSQL::Test::Utils::system_log('pg_ctl', 'kill', 'KILL', $pid);
is($ret, 0, 'killed process with KILL');

# Close psql session
$killme->finish;
$killme2->finish;

# Wait till server restarts
$node->poll_query_until('postgres', undef, '');
-----

It's hard to be totally sure, but I think what happened is that
gaur hit the in-hindsight-obvious race condition in this code:
we managed to execute a successful iteration of poll_query_until
before the postmaster had noticed its dead child and commenced
the restart.  The test lines after these are not prepared to see
failure-to-connect.

It's not obvious to me how to remove this race condition.
Thoughts?

			regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=gaur&dt=2022-04-03%2021%3A14%3A41

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From: Andres Freund <[email-masked]>
Date: Sun, 3 Apr 2022 22:07:21 -0700

Hi,

> It's not obvious to me how to remove this race condition.

Maybe we can use pump_until() with the psql that's not getting killed? With a
non-matching regex? That'd only return once the backend was killed by
postmaster, afaics?

Greetings,

Andres Freund

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From: Noah Misch <[email-masked]>
Date: Sun, 3 Apr 2022 22:52:18 -0700

> non-matching regex? That'd only return once the backend was killed by

Sounds good; I suspect that will be better than any of the ideas I scratched
down when
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hoverfly&dt=2021-08-31%2015%3A00%3A52
failed the same way.  For what it's worth, those were:

- Check that pg_postmaster_start_time() has changed.  But that runs into EPIPE
  trouble, requiring a write to a file or an eval{} to trap the EPIPE.
- Likewise, but check "select checkpoint_lsn from pg_control_checkpoint();".
- Poll pg_controldata until a new checkpoint happens.  Compare checkpoint LSN.
  Use checkpoint_timeout=1h to avoid non-end-of-recovery checkpoints.
- Poll logfile until "all server processes terminated; reinitializing".  Can
  be fooled with certain log_min_messages settings, but so can our other
  log-scraping tests.
- Grab the pid of e.g. the checkpointer and poll for that process to be gone.
  Can be fooled by PID reuse.