Re: signal handling in plpython

On 2016-10-14 13:50:35 -0400, Tom Lane wrote:
> Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
> > On 14 October 2016 16:22:12 EEST, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> I don't know whether to laugh or cry, but that code is a joke. Just
> >> silently fail if you can't get the lock?
>
> > Heh, ok, let me rephrase: I believe it's *intended* to be callable from a signal handler :). Whether it actually works is another question. Perhaps there's some mitigating conditions there, I don't know.
>
> > For our use case, it's actually not too bad if Py_AddPendingCall gives up and does nothing. Then the python function will simply not be interrupted until next SPI call, which is the current situation anyway.
>
> I dunno. If the failure were very low-probability, you could maybe live
> with that behavior, but I'm not sure it is. Presumably the Python
> interpreter loop is taking that lock once per statement (at least), so
> that it can tell if there's something to do. That'd suggest that the
> fraction of time in which the lock is held is not negligible.

Since that's how python's signal handling works, I'm not sure that's
really our problem.

Greetings,

Andres Freund