Re: max_files_per_processes vs others uses of file descriptors

On 2017-08-07 17:05:06 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2017-08-07 16:52:42 -0400, Tom Lane wrote:
> >> No, I don't think so. If you're depending on the NUM_RESERVED_FDS
> >> headroom for anything meaningful, *you're doing it wrong*. You should be
> >> getting an FD via fd.c, so that there is an opportunity to free up an FD
> >> (by closing a VFD) if you're up against system limits. Relying on
> >> NUM_RESERVED_FDS headroom can only protect against EMFILE not ENFILE.
>
> > How would this work for libpq based stuff like postgres fdw? Or some
> > random PL doing something with files? There's very little headroom here.
>
> Probably the best we can hope for there is to have fd.c provide a function
> "close an FD please", which postgres_fdw could call if libpq fails because
> of ENFILE/EMFILE, and then retry.

Unless that takes up a slot in fd.c while in use, that'll still leave us
open to failures to open files in some critical parts, unless I miss
something.

And then we'd have to teach similar things to PLs etc. I agree that
having some more slop isn't a proper solution, but only having ~30 fds
as slop on the most common systems seems mightily small.

> (Though I'm unsure how reliably postgres_fdw can detect that failure
> reason right now --- I don't know that we preserve errno on the way
> out of PQconnect.)

Yea, probably not really...

Regards,

Andres