Re: [HACKERS] max_files_per_processes vs others uses of file descriptors

Andres Freund <andres@anarazel.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.  (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.)
        regards, tom lane