bug#49682: 27.2.50; accept-process-output within accept-process-output hangs emacs

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Rajeev N <rajeev.jnk@sivalik.com>,  49682@debbugs.gnu.org
> Date: Wed, 21 Jul 2021 23:32:16 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> (let () (run-with-timer 0 nil #'url-retrieve "https://www.gnu.org/";
> >> #'ignore) (diary-mail-entries))
> >
> > Don't do that, because it will hurt.  Mixing timers with subprocesses
> > is asking for trouble.
> 
> That's not my experience, really -- url-queue.el is based around firing
> off `url-retrieve' from timers, and I've never seen any problems.

Sheer luck, IMO.

I don't really understand the need to run subprocesses from timers,
since subprocess support already includes async operation and built-in
time-outs.