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bug#14474: 24.3.50; Zombie subprocesses (again)
From: |
Paul Eggert |
Subject: |
bug#14474: 24.3.50; Zombie subprocesses (again) |
Date: |
Wed, 05 Jun 2013 10:21:46 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6 |
I found another problem with trying to have Emacs use glib's child watcher.
glib's signal handling code uses SA_RESTART and SA_NOCLDSTOP.
Both flags are non-starters for Emacs. SA_NOCLDSTOP, I suppose,
could be conditionalized based on the discussion in Gnome bug
reports 701538 and 562501. But SA_RESTART is more of a worry.
An interactive Emacs doesn't want SA_RESTART, because Emacs wants
long-running syscalls to be interrupted after a signal, not
restarted.
I thought of a way to work around this problem: have Emacs catch
SIGCHLD using its own flags, and call glib's SIGCHLD handler as part
of Emacs's SIGCHLD handler. So I installed the patch quoted at the
end of this message into the Emacs trunk as bzr 112859. If you've
had D-bus problems please try this new approach.
This raises three more questions for glib, though. First,
why does glib use SA_RESTART? If it's to avoid having application
syscalls fail with errno==EINTR, then we're OK. But if it's to
avoid having glib's internal syscalls fail with errno==EINTR, then
we have a problem, as that can happen with the following patch
(and it can also happen with vanilla Emacs 24.3).
Second, should there be a more robust way for Emacs to invoke
glib's SIGCHLD handler. The code below is a bit of a hack:
it uses g_source_unref (g_child_watch_source_new (0)) to
create and free a dummy SIGCHLD source, the only reason being
to trick glib into installing its SIGCHLD handler. It also assumes
that glib does not use SA_SIGINFO. This all seems fairly fragile.
Third, if a glib memory allocation fails, what does Emacs do?
Emacs tries hard not to exit when there's a memory allocation failure,
but I worry that glib will simply call 'exit' if malloc fails, which
is not good.
=== modified file 'src/ChangeLog'
--- src/ChangeLog 2013-06-05 12:17:02 +0000
+++ src/ChangeLog 2013-06-05 17:04:13 +0000
@@ -1,3 +1,17 @@
+2013-06-05 Paul Eggert <eggert@cs.ucla.edu>
+
+ Chain glib's SIGCHLD handler from Emacs's (Bug#14474).
+ * process.c (dummy_handler): New function.
+ (lib_child_handler): New static var.
+ (handle_child_signal): Invoke it.
+ (catch_child_signal): If a library has set up a signal handler,
+ save it into lib_child_handler.
+ (init_process_emacs): If using glib and not on Windows, tickle glib's
+ child-handling code so that it initializes its private SIGCHLD handler.
+ * syssignal.h (SA_SIGINFO): Default to 0.
+ * xterm.c (x_term_init): Remove D-bus hack that I installed on May
+ 31; it should no longer be needed now.
+
2013-06-05 Michael Albinus <michael.albinus@gmx.de>
* emacs.c (main) [HAVE_GFILENOTIFY]: Call globals_of_gfilenotify.
=== modified file 'src/process.c'
--- src/process.c 2013-06-03 18:47:35 +0000
+++ src/process.c 2013-06-05 17:04:13 +0000
@@ -6100,6 +6100,12 @@
might inadvertently reap a GTK-created process that happened to
have the same process ID. */
+/* LIB_CHILD_HANDLER is a SIGCHLD handler that Emacs calls while doing
+ its own SIGCHLD handling. On POSIXish systems, glib needs this to
+ keep track of its own children. The default handler does nothing. */
+static void dummy_handler (int sig) {}
+static signal_handler_t volatile lib_child_handler = dummy_handler;
+
/* Handle a SIGCHLD signal by looking for known child processes of
Emacs whose status have changed. For each one found, record its
new status.
@@ -6184,6 +6190,8 @@
}
}
}
+
+ lib_child_handler (sig);
}
static void
@@ -7035,9 +7043,13 @@
void
catch_child_signal (void)
{
- struct sigaction action;
+ struct sigaction action, old_action;
emacs_sigaction_init (&action, deliver_child_signal);
- sigaction (SIGCHLD, &action, 0);
+ sigaction (SIGCHLD, &action, &old_action);
+ eassert (! (old_action.sa_flags & SA_SIGINFO));
+ if (old_action.sa_handler != SIG_DFL && old_action.sa_handler != SIG_IGN
+ && old_action.sa_handler != deliver_child_signal)
+ lib_child_handler = old_action.sa_handler;
}
@@ -7055,6 +7067,11 @@
if (! noninteractive || initialized)
#endif
{
+#if defined HAVE_GLIB && !defined WINDOWSNT
+ /* Tickle glib's child-handling code so that it initializes its
+ private SIGCHLD handler. */
+ g_source_unref (g_child_watch_source_new (0));
+#endif
catch_child_signal ();
}
=== modified file 'src/syssignal.h'
--- src/syssignal.h 2013-01-02 16:13:04 +0000
+++ src/syssignal.h 2013-06-05 17:04:13 +0000
@@ -50,6 +50,10 @@
# define NSIG NSIG_MINIMUM
#endif
+#ifndef SA_SIGINFO
+# define SA_SIGINFO 0
+#endif
+
#ifndef emacs_raise
# define emacs_raise(sig) raise (sig)
#endif
=== modified file 'src/xterm.c'
--- src/xterm.c 2013-05-31 01:41:52 +0000
+++ src/xterm.c 2013-06-05 17:04:13 +0000
@@ -9897,13 +9897,6 @@
XSetLocaleModifiers ("");
- /* If D-Bus is not already configured, inhibit D-Bus autolaunch,
- as autolaunch can mess up Emacs's SIGCHLD handler.
- FIXME: Rewrite subprocess handlers to use glib's child watchers.
- See Bug#14474. */
- if (! egetenv ("DBUS_SESSION_BUS_ADDRESS"))
- xputenv ("DBUS_SESSION_BUS_ADDRESS=unix:path=/dev/null");
-
/* Emacs can only handle core input events, so make sure
Gtk doesn't use Xinput or Xinput2 extensions. */
xputenv ("GDK_CORE_DEVICE_EVENTS=1");