GNU bug report logs - #14474
24.3.50; Zombie subprocesses (again)

Previous Next

Full log

View this message in rfc822 format

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Colin Walters <walters <at> verbum.org>
Cc: Michael Heerdegen <michael_heerdegen <at> web.de>, Jan Djärv <jan.h.d <at> swipnet.se>, 14474 <at> debbugs.gnu.org
Subject: bug#14474: 24.3.50; Zombie subprocesses (again)
Date: Mon, 27 May 2013 10:36:48 -0700

[The context is
http://bugs.gnu.org/14474
]

On 05/27/2013 05:46 AM, Colin Walters wrote:

> Basically it's going to be very hard over time to avoid codepaths
> in the GTK+ stack that don't call g_spawn_*() indirectly, thus
> installing a SIGCHLD handler

Thanks.  In that case, shouldn't the glib documentation be
changed to warn application developers not to install a SIGCHLD
handler as well?  Currently it warns them only to not call
waitpid(-1, ...).

Are application developers allowed to temporarily mask SIGCHLD?
Emacs does that a lot.

>> One possibility is to see if we can get Emacs to use
>> > glib's child watcher.
> That'd be best obviously.

I suspect so too, but it requires more expertise in
glib than I have (which is, basically, nothing).
If I understand things correctly, if Emacs is using
Gtk it should

 * never call sigaction (SIGCHLD, ...) or signal (SIGCHLD, ...)
   or waitpid (-1, ...).
   E.g., remove the current call to sigaction (SIGCHLD, ...),
   in src/process.c's init_process_emacs.
   
 * Whenever Emacs creates a child process, use the
   following pattern:

       block SIGCHLD;
       pid = vfork ();
       if (pid > 0)
         {
           record pid in Emacs's process table, as location 'loc';
           record in *loc that glib is watching this pid;
           g_child_watch_add (pid, watcher, loc);
         }
       unblock SIGCHLD;

  * never call waitpid (pid, ...) if PID is recorded
    in Emacs's process table as something that glib is
    watching.

  * Add a glue function ("watcher", above) that does
    something like this:

      void watcher (GPid pid, gint status, gpointer loc) {
	block SIGCHLD
        record that PID exited with status STATUS, by modifying *LOC,
	  sort of like's what currently done in handle_child_signal;
        if (input_available_clear_time)
	  *input_available_clear_time = make_emacs_time (0, 0);
        unblock SIGCHLD
     }

But this sounds incomplete.  No doubt there's something
about the main loop, or setting up the watchers, that I don't
know about.  E.g., how does one remove the watcher once it
has fired and told us that the process has exited?

I'll CC: this to Jan Djärv, who knows about gtk, to
see if he can help.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.