Re: Bug in walsender when calling out to do_pg_stop_backup (and others?)

On Oct5, 2011, at 15:30 , Magnus Hagander wrote:
> When walsender calls out to do_pg_stop_backup() (during base backups),
> it is not possible to terminate the process with a SIGTERM - it
> requires a SIGKILL. This can leave unkillable backends for example if
> archive_mode is on and archive_command is failing (or not set). A
> similar thing would happen in other cases if walsender calls out to
> something that would block (do_pg_start_backup() for example), but the
> stop one is easy to provoke.

Hm, this seems to be related to another buglet I noticed a while ago,
but then forgot about again. If one terminates pg_basebackup while it's
waiting for all required WAL to be archived, the backend process only
exits once that waiting phase is over. If, like in your failure case,
archive_command fails indefinity (or isn't set), the backend process
stays around forever.

Your patch would improve that only insofar as it'd at least allow an
immediate shutdown request to succeed - as it stands, that doesn't work
because, as you mentioned, the blocked walsender doesn't handle SIGTERM.

The question is, should we do more? To me, it'd make sense to terminate
a backend once it's connection is gone. We could, for example, make
pq_flush() set a global flag, and make CHECK_FOR_INTERRUPTS handle a
broken connection that same way as a SIGINT or SIGTERM.

Thoughts?

> ISTM one way to fix it is the attached, which is to have walsender set
> the "global" flags saying that we have received sigterm, which in turn
> causes the CHECK_FOR_INTERRUPTS() calls in the routines to properly
> exit the process. AFAICT it works fine. Any holes in this approach?

Seems sensible, but my knowledge about these code paths is quite limited..

> Second, I wonder if we should add a SIGINT handler as well, that would
> make it possible to send a cancel signal. Given that the end result
> would be the same (at least if we want to keep with the "walsender is
> simple" path), I'm not sure it's necessary - but it would at least
> help those doing pg_cancel_backend()... thoughts?

If all that's needed is a simple SIGINT handler that sets one flag, it'd
say let's add it.

best regards,
Florian Pflug