Re: elog.c query_id support vs shutdown

On Sat, Aug 07, 2021 at 04:44:07PM -0700, Andres Freund wrote:
>
> As currently implemented those pgstat_get_my_query_id() calls are not
> safe. It's fine during backend startup because MyBEEntry is not set, but
> during shutdown that's not ok, because we never unset MyBEEntry.
>
> andres(at)awork3:~/src/postgresql$ /home/andres/build/postgres/dev-assert/vpath/src/backend/postgres --single postgres -D /srv/dev/pgdev-dev/ -c 'log_line_prefix=%Q' -c log_min_messages=debug1
> [...]
> PostgreSQL stand-alone backend 15devel
> backend> 0NOTICE: shutting down
> 0DEBUG: performing replication slot checkpoint
> Segmentation fault

Ouch

> I suspect that to make the elog.c usage safe, we'll have to clear MyBEEntry in
> pgstat_beshutdown_hook().

I agree, and a quick test indeed fix your scenario. It also seems like a good
thing to do overall.

I didn't find any other problematic corner cases, but I'm not that familiar with
pgstat, especially after the recent activity.