Horton

Re: Weird failure with latches in curculio on v15

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Michael Paquier <michael@paquier.xyz>, Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Fujii Masao <fujii@postgresql.org>, Nathan Bossart <nathandbossart@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-02-03T07:24:03Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:
> Ugh, I think I might understand what's happening:

> The signal arrives just after the fork() (within system()). Because we
> have all our processes configure themselves as process group leaders,
> and we signal the entire process group (c.f. signal_child()), both the
> child process and the parent will process the signal. So we'll end up
> doing a proc_exit() in both. As both are trying to remove themselves
> from the same PGPROC etc entry, that doesn't end well.

Ugh ...

> I don't see how we can solve that properly as long as we use system().

... but I don't see how that's system()'s fault?  Doing the fork()
ourselves wouldn't change anything about that.

> A workaround for the back branches could be to have a test in
> StartupProcShutdownHandler() that tests if MyProcPid == getpid(), and
> not do the proc_exit() if they don't match. We probably should just do
> an _exit() in that case.

Might work.

> OTOH, the current approach only works on systems with setsid(2) support,
> so we probably shouldn't rely so hard on it anyway.

setsid(2) is required since SUSv2, so I'm not sure which systems
are of concern here ... other than Redmond's of course.

			regards, tom lane

Commits

  1. Avoid calling proc_exit() in processes forked by system().

  2. Move extra code out of the Pre/PostRestoreCommand() section.

  3. Revert refactoring of restore command code to shell_restore.c

  4. Refactor code in charge of running shell-based recovery commands

  5. Clean up inconsistent use of fflush().

  6. Report wait events for local shell commands like archive_command.