bug#14474: 24.3.50; Zombie subprocesses (again)

[Paul Eggert]

[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.