bug#17036: Continuation for Emacs: invoking a process on exit?

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: rgm@gnu.org,  17036@debbugs.gnu.org,  schwab@linux-m68k.org,  rrt@sc3d.org
> Date: Sun, 17 Apr 2022 14:08:49 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > As implemented, it won't work reliably on MS-Windows, because execvp
> > there doesn't do what you think it should.  I think we should use
> > sys_spawnve instead.
> 
> Ah, I grepped for execvp to see whether we already used it, but didn't
> notice that the hits were from Gnulib.
> 
> I'm not familiar with sys_spawnve -- can you do the adjustments?

I don't think I will have the time.  It isn't a simple job, because
just calling sys_spawnve will not do -- that function currently
supports only Emacs sub-processes.

FTR, I will document below the potential issues with the current
implementation of kill-emacs/restart-emacs:

  . when kill-emacs is called with RESTART non-nil, the value of ARG
    is ignored; this should at least be documented;
  . the exit status of the restarted Emacs is discarded, so it will
    not be available to the parent program, at least on MS-Windows,
    and also if execvp fails for some reason;
  . the semantics of the file descriptors open in the original Emacs
    process is not clear to me: will they remain open in the restarted
    Emacs, if the original Emacs opened them without CLOEXEC?
  . does the restarted Emacs belong to the same process group? should
    it?
  . on MS-Windows, if any of the argv[] command-line arguments have
    embedded whitespace, the restarted Emacs will not get the same
    elements in its argv[] array, because the Windows API for starting
    processes accepts the command-line arguments as a single string