Re: postmaster deadlock while logging after syslogger exited

On 2017-11-06 15:35:03 -0500, Tom Lane wrote:
> David Pacheco <dap(at)joyent(dot)com> writes:
> > I ran into what appears to be a deadlock in the logging subsystem. It
> > looks like what happened was that the syslogger process exited because it
> > ran out of memory. But before the postmaster got a chance to handle the
> > SIGCLD to restart it, it handled a SIGUSR1 to start an autovacuum worker.
> > That also failed, and the postmaster went to log a message about it, but
> > it's blocked on the pipe that's normally connected to the syslogger,
> > presumably because the pipe is full because the syslogger is gone and
> > hasn't read from it.
>
> Ugh.

I'm somewhat inclined to say that one has to live with this if the
system is so resource constrainted that processes barely using memory
get killed.

We could work around a situation like that if we made postmaster use a
*different* pipe as stderr than the one we're handing to normal
backends. If postmaster created a new pipe and closed the read end
whenever forking a syslogger, we should get EPIPEs when writing after
syslogger died and could fall back to proper stderr or such.

Greetings,

Andres Freund