bug#51110: Native-comp leaks fds open on /dev/ptmx

[Eli Zaretskii]

> From: Stephen Gildea <stepheng+emacs@gildea.com>
> cc: Andrea Corallo <akrl@sdf.org>, 51110@debbugs.gnu.org
> Date: Sat, 09 Oct 2021 10:01:43 -0700
> 
> Apparently what's happening is that Emacs forks off a child Emacs to do
> the compile and allocates a pty to the child.  These ptys are not
> closed until the compile completes.  It seems Emacs needs to better
> manage the number of simultaneous open ptys.  Or at least be able to
> wait and retry if an open fails.

Emacs limits the number of these background processes, see
native-comp-async-jobs-number.  (The value zero means use half of the
number of execution units available in the system's processor.)  So we
generally shouldn't have more than that number of open pipes at any
given time.  For the obvious reasons they cannot be closed until the
sub-process exits.