Fix a race condition that causes Emacs to mess up glib.

The symptom is a diagnostic "GLib-WARNING **: In call to
g_spawn_sync(), exit status of a child process was requested but
SIGCHLD action was set to SIG_IGN and ECHILD was received by
waitpid(), so exit status can't be returned."  The diagnostic
is partly wrong, as the SIGCHLD action is not set to SIG_IGN.
The real bug is a race condition between Emacs and glib: Emacs
does a waitpid (-1, ...) and reaps glib's subprocess by mistake,
so that glib can't find it.  Work around the bug by invoking
waitpid only on subprocesses that Emacs itself creates.
* process.c (create_process, record_child_status_change):
Don't use special value -1 in pid field, as the caller now must
know the pid rather than having the callee infer it.  The
inference was sometimes incorrect anyway, due to another race.
(create_process): Set new 'alive' member if child is created.
(process_status_retrieved): New function.
(record_child_status_change): Use it.
Accept negative 1st argument, which means to wait for the
processes that Emacs already knows about.  Move special-case code
for DOS_NT (which lacks WNOHANG) here, from caller.  Keep track of
processes that have already been waited for, by testing and
clearing new 'alive' member.
(CAN_HANDLE_MULTIPLE_CHILDREN): Remove, as record_child_status_change
now does this internally.
(handle_child_signal): Let record_child_status_change do all
the work, since we do not want to reap all exited child processes,
only the child processes that Emacs itself created.
* process.h (Lisp_Process): New boolean member 'alive'.

Fixes: debbugs:8855

1 files changed, 29 insertions, 0 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 3a88b9c7ce2..2f040accd9a 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,34 @@
 2012-11-03  Paul Eggert  <eggert@cs.ucla.edu>
 
+        Fix a race condition that causes Emacs to mess up glib (Bug#8855).
+        The symptom is a diagnostic "GLib-WARNING **: In call to
+        g_spawn_sync(), exit status of a child process was requested but
+        SIGCHLD action was set to SIG_IGN and ECHILD was received by
+        waitpid(), so exit status can't be returned."  The diagnostic
+        is partly wrong, as the SIGCHLD action is not set to SIG_IGN.
+        The real bug is a race condition between Emacs and glib: Emacs
+        does a waitpid (-1, ...) and reaps glib's subprocess by mistake,
+        so that glib can't find it.  Work around the bug by invoking
+        waitpid only on subprocesses that Emacs itself creates.
+        * process.c (create_process, record_child_status_change):
+        Don't use special value -1 in pid field, as the caller now must
+        know the pid rather than having the callee infer it.  The
+        inference was sometimes incorrect anyway, due to another race.
+        (create_process): Set new 'alive' member if child is created.
+        (process_status_retrieved): New function.
+        (record_child_status_change): Use it.
+        Accept negative 1st argument, which means to wait for the
+        processes that Emacs already knows about.  Move special-case code
+        for DOS_NT (which lacks WNOHANG) here, from caller.  Keep track of
+        processes that have already been waited for, by testing and
+        clearing new 'alive' member.
+        (CAN_HANDLE_MULTIPLE_CHILDREN): Remove, as record_child_status_change
+        now does this internally.
+        (handle_child_signal): Let record_child_status_change do all
+        the work, since we do not want to reap all exited child processes,
+        only the child processes that Emacs itself created.
+        * process.h (Lisp_Process): New boolean member 'alive'.
+
         Omit duplicate definitions no longer needed with gcc -g3.
         * lisp.h (GCTYPEBITS, GCALIGNMENT, ARRAY_MARK_FLAG, PSEUDOVECTOR_FLAG)
         (VALMASK, MOST_POSITIVE_FIXNUM, MOST_NEGATIVE_FIXNUM):