Subject: Re: 24.3.50; Zombie process left after call-process

From: Paul Eggert <[email-masked]>
Date: Wed, 28 Nov 2012 18:30:08 -0800

Eli, thanks for your review.  I'm attaching a
heavily-revised patch, which tries to take your comments
into account, along with several other issues I noticed in
my own analysis and testing.  In response to your comments:

On 11/27/2012 08:00 AM, Eli Zaretskii wrote:

> I'm confused by this commentary

The new patch removes that function and its associated
commentary.

> The feature, whereby this function (call-process) could return a
> description of errno, is hereby lost.  That's quite nasty in the MSDOS
> case

That feature is supported only by MS-DOS, right?  On all
other platforms an error is thrown.  Anyway, the new patch
attempts to preserve the MS-DOS behavior.

> This is wrong for MSDOS, because the temporary file is created before
> attempting to launch the process, and so it needs to be cleaned up
> even if we failed to launch the child process.

The new patch attempts to undo that change for MSDOS.

> This seem to assume that the only trouble in call-process could be
> from some signal.  Why is that true?

By the time that part of the code is reached, any other
failures (e.g., fork failure) have already been diagnosed
and an error would have been thrown or call-process would
have returned before getting to that location.

> Why is this eassert a good idea?  We don't need to enforce the Posix
> spec at a price of crashing Emacs on users, do we?  What bad things
> can happen if you see some other errno here?

It's not a question of enforcing Posix semantics.  It's a
question of catching potentially serious internal
programming errors in Emacs.  That part of the code has been
completely rewritten, so the details of the comment is moot.
However, the new code has an "eassert (errno == EINTR)" in
the new get_child_status function.  I preceded that with a
comment to try to help explain why the eassert is there.

> The condition testing that status is non-NULL must stay, because that
> argument can be NULL, even if Emacs currently never uses that option.

OK, thanks, I did that.

> it would be nice to have somewhere a commentary with an
> overview of how termination of child processes is handled.

I added some, before handle_child_signal.

=== modified file 'src/ChangeLog'
--- src/ChangeLog	2012-11-29 00:36:22 +0000
+++ src/ChangeLog	2012-11-29 02:24:45 +0000
@@ -1,5 +1,62 @@
 2012-11-29  Paul Eggert  <[email-masked]>
 
+	Don't let call-process be a zombie factory (Bug#12980).
+	Fixing this bug required some cleanup of the signal-handling code.
+	As a side effect, this change also fixes a longstanding rare race
+	condition whereby Emacs could mistakenly kill unrelated processes,
+	and it fixes a bug where a second C-g does not kill a recalcitrant
+	synchronous process in GNU/Linux and similar platforms.
+	The patch should also fix the last vestiges of Bug#9488,
+	a bug which has mostly been fixed on the trunk by other changes.
+	* callproc.c, process.h (synch_process_alive, synch_process_death)
+	(synch_process_termsig, sync_process_retcode):
+	Remove.  All uses removed, to simplify analysis and so that
+	less consing is done inside critical sections.
+	* callproc.c (call_process_exited): Remove.  All uses replaced
+	with !synch_process_pid.
+	* callproc.c (synch_process_pid, synch_process_fd): New static vars.
+	These take the role of what used to be in unwind-protect arg.
+	All uses changed.
+	(block_child_signal, unblock_child_signal):
+	New functions, to avoid races that could kill innocent-victim processes.
+	(call_process_kill, call_process_cleanup, Fcall_process): Use them.
+	(call_process_kill): Record killed processes as deleted, so that
+	zombies do not clutter up the system.  Do this inside a critical
+	section, to avoid a race that would allow the clutter.
+	(call_process_cleanup): Fix code so that the second C-g works again
+	on common platforms such as GNU/Linux.
+	(Fcall_process): Create the child process in a critical section,
+	to fix a race condition.  If creating an asynchronous process,
+	record it as deleted so that zombies do not clutter up the system.
+	Do unwind-protect for WINDOWSNT too, as that's simpler in the
+	light of these changes.  Omit unnecessary call to emacs_close
+	before failure, as the unwind-protect code does that.
+	* callproc.c (call_process_cleanup):
+	* w32proc.c (waitpid): Simplify now that synch_process_alive is gone.
+	* process.c (record_deleted_pid): New function, containing
+	code refactored out of Fdelete_process.
+	(Fdelete_process): Use it.
+	(process_status_retrieved): Remove.  All callers changed to use
+	child_status_change.
+	(record_child_status_change): Remove, folding its contents into ...
+	(handle_child_signal): ... this signal handler.  Now, this
+	function is purely a handler for SIGCHLD, and is not called after
+	a synchronous waitpid returns; the synchronous code is moved to
+	wait_for_termination.  There is no need to worry about reaping
+	more than one child now.
+	* sysdep.c (get_child_status, child_status_changed): New functions.
+	(wait_for_termination): Now takes int * status and bool
+	interruptible arguments, too.  Do not record child status change;
+	that's now the caller's responsibility.  All callers changed.
+	Reimplement in terms of get_child_status.
+	(wait_for_termination_1, interruptible_wait_for_termination):
+	Remove.  All callers changed to use wait_for_termination.
+	* syswait.h: Include <stdbool.h>, for bool.
+	(record_child_status_change, interruptible_wait_for_termination):
+	Remove decls.
+	(record_deleted_pid, child_status_changed): New decls.
+	(wait_for_termination): Adjust to API changes noted above.
+
 	* callproc.c (Fcall_process): Don't misreport vfork failure.
 
 2012-11-28  Paul Eggert  <[email-masked]>

=== modified file 'src/callproc.c'
--- src/callproc.c	2012-11-29 00:36:22 +0000
+++ src/callproc.c	2012-11-29 02:24:45 +0000
@@ -67,88 +67,110 @@
 /* Pattern used by call-process-region to make temp files.  */
 static Lisp_Object Vtemp_file_name_pattern;
 
-/* True if we are about to fork off a synchronous process or if we
-   are waiting for it.  */
-bool synch_process_alive;
-
-/* Nonzero => this is a string explaining death of synchronous subprocess.  */
-const char *synch_process_death;
-
-/* Nonzero => this is the signal number that terminated the subprocess.  */
-int synch_process_termsig;
-
-/* If synch_process_death is zero,
-   this is exit code of synchronous subprocess.  */
-int synch_process_retcode;
-
+/* The next two variables are valid only while record-unwind-protect
+   is in place during call-process for a synchronous subprocess.  At
+   other times, their contents are irrelevant.  Doing this via static
+   C variables is more convenient than putting them into the arguments
+   of record-unwind-protect, as they need to be updated at randomish
+   times in the code, and Lisp cannot always store these values as
+   Emacs integers.  It's safe to use static variables here, as the
+   code is never invoked reentrantly.  */
+
+/* If nonzero, a process-ID that has not been reaped.  */
+static pid_t synch_process_pid;
+
+/* If nonnegative, a file descriptor that has not been closed.  */
+static int synch_process_fd;
 
+/* Block SIGCHLD.  */
+
+static void
+block_child_signal (void)
+{
+#ifdef SIGCHLD
+  sigset_t blocked;
+  sigemptyset (&blocked);
+  sigaddset (&blocked, SIGCHLD);
+  pthread_sigmask (SIG_BLOCK, &blocked, 0);
+#endif
+}
+
+/* Unblock SIGCHLD.  */
+
+static void
+unblock_child_signal (void)
+{
+#ifdef SIGCHLD
+  pthread_sigmask (SIG_SETMASK, &empty_mask, 0);
+#endif
+}
+
+/* Clean up when exiting call_process_cleanup.  */
+
+static Lisp_Object
+call_process_kill (Lisp_Object ignored)
+{
+  if (0 <= synch_process_fd)
+    emacs_close (synch_process_fd);
+
+  /* If PID is reapable, kill it and record it as a deleted process.
+     Do this in a critical section.  Unless PID is wedged it will be
+     reaped on receipt of the first SIGCHLD after the critical section.  */
+  if (synch_process_pid)
+    {
+      block_child_signal ();
+      record_deleted_pid (synch_process_pid);
+      EMACS_KILLPG (synch_process_pid, SIGKILL);
+      unblock_child_signal ();
+    }
+
+  return Qnil;
+}
+
 /* Clean up when exiting Fcall_process.
    On MSDOS, delete the temporary file on any kind of termination.
    On Unix, kill the process and any children on termination by signal.  */
 
-/* True if this is termination due to exit.  */
-static bool call_process_exited;
-
-static Lisp_Object
-call_process_kill (Lisp_Object fdpid)
-{
-  int fd;
-  pid_t pid;
-  CONS_TO_INTEGER (Fcar (fdpid), int, fd);
-  CONS_TO_INTEGER (Fcdr (fdpid), pid_t, pid);
-  emacs_close (fd);
-  EMACS_KILLPG (pid, SIGKILL);
-  synch_process_alive = 0;
-  return Qnil;
-}
-
 static Lisp_Object
 call_process_cleanup (Lisp_Object arg)
 {
-  Lisp_Object fdpid = Fcdr (arg);
-  int fd;
-#if defined (MSDOS)
-  Lisp_Object file;
+#ifdef MSDOS
+  Lisp_Object buffer = Fcar (arg);
+  Lisp_Object file = Fcdr (arg);
 #else
-  pid_t pid;
+  Lisp_Object buffer = arg;
 #endif
 
-  Fset_buffer (Fcar (arg));
-  CONS_TO_INTEGER (Fcar (fdpid), int, fd);
-
-#if defined (MSDOS)
-  /* for MSDOS fdpid is really (fd . tempfile)  */
-  file = Fcdr (fdpid);
-  /* FD is -1 and FILE is "" when we didn't actually create a
-     temporary file in call-process.  */
-  if (fd >= 0)
-    emacs_close (fd);
-  if (!(strcmp (SDATA (file), NULL_DEVICE) == 0 || SREF (file, 0) == '\0'))
-    unlink (SDATA (file));
-#else /* not MSDOS */
-  CONS_TO_INTEGER (Fcdr (fdpid), pid_t, pid);
-
-  if (call_process_exited)
-    {
-      emacs_close (fd);
-      return Qnil;
-    }
-
-  if (EMACS_KILLPG (pid, SIGINT) == 0)
+  Fset_buffer (buffer);
+
+#ifndef MSDOS
+  /* If the process still exists, kill its process group.  */
+  if (synch_process_pid)
     {
       ptrdiff_t count = SPECPDL_INDEX ();
-      record_unwind_protect (call_process_kill, fdpid);
+      EMACS_KILLPG (synch_process_pid, SIGINT);
+      record_unwind_protect (call_process_kill, make_number (0));
       message1 ("Waiting for process to die...(type C-g again to kill it instantly)");
       immediate_quit = 1;
       QUIT;
-      wait_for_termination (pid);
+      wait_for_termination (synch_process_pid, 0, 1);
+      synch_process_pid = 0;
       immediate_quit = 0;
       specpdl_ptr = specpdl + count; /* Discard the unwind protect.  */
       message1 ("Waiting for process to die...done");
     }
-  synch_process_alive = 0;
-  emacs_close (fd);
-#endif /* not MSDOS */
+#endif
+
+  if (0 <= synch_process_fd)
+    emacs_close (synch_process_fd);
+
+#ifdef MSDOS
+  /* FILE is "" when we didn't actually create a temporary file in
+     call-process.  */
+  if (!(strcmp (SDATA (file), NULL_DEVICE) == 0 || SREF (file, 0) == '\0'))
+    unlink (SDATA (file));
+#endif
+
   return Qnil;
 }
 
@@ -181,9 +203,10 @@
 usage: (call-process PROGRAM &optional INFILE BUFFER DISPLAY &rest ARGS)  */)
   (ptrdiff_t nargs, Lisp_Object *args)
 {
-  Lisp_Object infile, buffer, current_dir, path, cleanup_info_tail;
+  Lisp_Object infile, buffer, current_dir, path;
   bool display_p;
   int fd0, fd1, filefd;
+  int status;
   ptrdiff_t count = SPECPDL_INDEX ();
   USE_SAFE_ALLOCA;
 
@@ -199,7 +222,7 @@
 #else
   pid_t pid;
 #endif
-  int vfork_errno;
+  int child_errno;
   int fd_output = -1;
   struct coding_system process_coding; /* coding-system of process output */
   struct coding_system argument_coding;	/* coding-system of arguments */
@@ -496,16 +519,6 @@
     if (fd_output >= 0)
       fd1 = fd_output;
 
-    /* Record that we're about to create a synchronous process.  */
-    synch_process_alive = 1;
-
-    /* These vars record information from process termination.
-       Clear them now before process can possibly terminate,
-       to avoid timing error if process terminates soon.  */
-    synch_process_death = 0;
-    synch_process_retcode = 0;
-    synch_process_termsig = 0;
-
     if (NILP (error_file))
       fd_error = emacs_open (NULL_DEVICE, O_WRONLY, 0);
     else if (STRINGP (error_file))
@@ -538,23 +551,21 @@
 
 #ifdef MSDOS /* MW, July 1993 */
     /* Note that on MSDOS `child_setup' actually returns the child process
-       exit status, not its PID, so we assign it to `synch_process_retcode'
-       below.  */
+       exit status, not its PID, so assign it to status below.  */
     pid = child_setup (filefd, outfilefd, fd_error, new_argv, 0, current_dir);
-
-    /* Record that the synchronous process exited and note its
-       termination status.  */
-    synch_process_alive = 0;
-    synch_process_retcode = pid;
-    if (synch_process_retcode < 0)  /* means it couldn't be exec'ed */
-      {
-	synchronize_system_messages_locale ();
-	synch_process_death = strerror (errno);
-      }
+    child_errno = errno;
 
     emacs_close (outfilefd);
     if (fd_error != outfilefd)
       emacs_close (fd_error);
+    if (pid < 0)
+      {
+	synchronize_system_messages_locale ();
+	return
+	  code_convert_string_norecord (build_string (strerror (child_errno)),
+					Vlocale_coding_system, 0);
+      }
+    status = pid;
     fd1 = -1; /* No harm in closing that one!  */
     if (tempfile)
       {
@@ -572,12 +583,21 @@
     else
       fd0 = -1; /* We are not going to read from tempfile.   */
 #else /* not MSDOS */
+
+    /* Do the unwind-protect now, even though the pid is not known, so
+       that no storage allocation is done in the critical section.
+       The actual PID will be filled in during the critical section.  */
+    synch_process_pid = 0;
+    synch_process_fd = fd0;
+    record_unwind_protect (call_process_cleanup, Fcurrent_buffer ());
+
+    block_input ();
+    block_child_signal ();
+
 #ifdef WINDOWSNT
     pid = child_setup (filefd, fd1, fd_error, new_argv, 0, current_dir);
 #else  /* not WINDOWSNT */
 
-    block_input ();
-
     /* vfork, and prevent local vars from being clobbered by the vfork.  */
     {
       Lisp_Object volatile buffer_volatile = buffer;
@@ -597,6 +617,7 @@
       char **volatile new_save_environ = save_environ;
 
       pid = vfork ();
+      child_errno = errno;
 
       buffer = buffer_volatile;
       coding_systems = coding_systems_volatile;
@@ -617,6 +638,8 @@
 
     if (pid == 0)
       {
+	unblock_child_signal ();
+
 	if (fd0 >= 0)
 	  emacs_close (fd0);
 
@@ -628,11 +651,21 @@
 	child_setup (filefd, fd1, fd_error, new_argv, 0, current_dir);
       }
 
-    vfork_errno = errno;
+#endif /* not WINDOWSNT */
+
+    child_errno = errno;
+
+    if (0 < pid)
+      {
+	if (INTEGERP (buffer))
+	  record_deleted_pid (pid);
+	else
+	  synch_process_pid = pid;
+      }
+
+    unblock_child_signal ();
     unblock_input ();
 
-#endif /* not WINDOWSNT */
-
     /* The MSDOS case did this already.  */
     if (fd_error >= 0)
       emacs_close (fd_error);
@@ -651,9 +684,7 @@
 
   if (pid < 0)
     {
-      if (fd0 >= 0)
-	emacs_close (fd0);
-      errno = vfork_errno;
+      errno = child_errno;
       report_file_error ("Doing vfork", Qnil);
     }
 
@@ -664,19 +695,12 @@
       return Qnil;
     }
 
-  /* Enable sending signal if user quits below.  */
-  call_process_exited = 0;
-
 #if defined (MSDOS)
   /* MSDOS needs different cleanup information.  */
-  cleanup_info_tail = build_string (tempfile ? tempfile : "");
-#else
-  cleanup_info_tail = INTEGER_TO_CONS (pid);
-#endif /* not MSDOS */
   record_unwind_protect (call_process_cleanup,
 			 Fcons (Fcurrent_buffer (),
-				Fcons (INTEGER_TO_CONS (fd0),
-				       cleanup_info_tail)));
+				build_string (tempfile ? tempfile : "")));
+#endif
 
   if (BUFFERP (buffer))
     Fset_buffer (buffer);
@@ -863,38 +887,34 @@
 
 #ifndef MSDOS
   /* Wait for it to terminate, unless it already has.  */
-  if (output_to_buffer)
-    wait_for_termination (pid);
-  else
-    interruptible_wait_for_termination (pid);
+  wait_for_termination (pid, &status, !output_to_buffer);
 #endif
 
   immediate_quit = 0;
 
   /* Don't kill any children that the subprocess may have left behind
      when exiting.  */
-  call_process_exited = 1;
+  synch_process_pid = 0;
 
   SAFE_FREE ();
   unbind_to (count, Qnil);
 
-  if (synch_process_termsig)
+  if (WIFSIGNALED (status))
     {
       const char *signame;
 
       synchronize_system_messages_locale ();
-      signame = strsignal (synch_process_termsig);
+      signame = strsignal (WTERMSIG (status));
 
       if (signame == 0)
 	signame = "unknown";
 
-      synch_process_death = signame;
+      return code_convert_string_norecord (build_string (signame),
+					   Vlocale_coding_system, 0);
     }
 
-  if (synch_process_death)
-    return code_convert_string_norecord (build_string (synch_process_death),
-					 Vlocale_coding_system, 0);
-  return make_number (synch_process_retcode);
+  eassert (WIFEXITED (status));
+  return make_number (WEXITSTATUS (status));
 }
 
 static Lisp_Object

=== modified file 'src/process.c'
--- src/process.c	2012-11-27 05:17:07 +0000
+++ src/process.c	2012-11-29 02:24:45 +0000
@@ -777,10 +777,23 @@
 /* Fdelete_process promises to immediately forget about the process, but in
    reality, Emacs needs to remember those processes until they have been
    treated by the SIGCHLD handler and waitpid has been invoked on them;
-   otherwise they might fill up the kernel's process table.  */
+   otherwise they might fill up the kernel's process table.
+
+   Some processes created by call-process are also put onto this list.  */
 static Lisp_Object deleted_pid_list;
 #endif
 
+void
+record_deleted_pid (pid_t pid)
+{
+#ifdef SIGCHLD
+  deleted_pid_list = Fcons (make_fixnum_or_float (pid),
+			    /* GC treated elements set to nil.  */
+			    Fdelq (Qnil, deleted_pid_list));
+
+#endif
+}
+
 DEFUN ("delete-process", Fdelete_process, Sdelete_process, 1, 1, 0,
        doc: /* Delete PROCESS: kill it and forget about it immediately.
 PROCESS may be a process, a buffer, the name of a process or buffer, or
@@ -807,9 +820,7 @@
       pid_t pid = p->pid;
 
       /* No problem storing the pid here, as it is still in Vprocess_alist.  */
-      deleted_pid_list = Fcons (make_fixnum_or_float (pid),
-				/* GC treated elements set to nil.  */
-				Fdelq (Qnil, deleted_pid_list));
+      record_deleted_pid (pid);
       /* If the process has already signaled, remove it from the list.  */
       if (p->raw_status_new)
 	update_status (p);
@@ -6160,35 +6171,37 @@
   return process;
 }
 
-/* If the status of the process DESIRED has changed, return true and
-   set *STATUS to its exit status; otherwise, return false.
-   If HAVE is nonnegative, assume that HAVE = waitpid (HAVE, STATUS, ...)
-   has already been invoked, and do not invoke waitpid again.  */
-
-static bool
-process_status_retrieved (pid_t desired, pid_t have, int *status)
-{
-  if (have < 0)
-    {
-      /* Invoke waitpid only with a known process ID; do not invoke
-	 waitpid with a nonpositive argument.  Otherwise, Emacs might
-	 reap an unwanted process by mistake.  For example, invoking
-	 waitpid (-1, ...) can mess up glib by reaping glib's subprocesses,
-	 so that another thread running glib won't find them.  */
-      do
-	have = waitpid (desired, status, WNOHANG | WUNTRACED);
-      while (have < 0 && errno == EINTR);
-    }
-
-  return have == desired;
-}
-
-/* If PID is nonnegative, the child process PID with wait status W has
-   changed its status; record this and return true.
-
-   If PID is negative, ignore W, and look for known child processes
-   of Emacs whose status have changed.  For each one found, record its new
-   status.
+#ifdef SIGCHLD
+
+/* The main Emacs thread records child processes in three places:
+
+   - Vprocess_alist, for asynchronous subprocesses, which are child
+     processes visible to Lisp.
+
+   - deleted_pid_list, for child processes invisible to Lisp,
+     typically because of delete-process.  These are recorded so that
+     the processes can be reaped when they exit, so that the operating
+     system's process table is not cluttered by zombies.
+
+   - the local variable PID in Fcall_process, call_process_cleanup and
+     call_process_kill, for synchronous subprocesses.
+     record_unwind_protect is used to make sure this process is not
+     forgotten: if the user interrupts call-process and the child
+     process refuses to exit immediately even with two C-g's,
+     call_process_kill adds PID's contents to deleted_pid_list before
+     returning.
+
+   The main Emacs thread invokes waitpid only on child processes that
+   it creates and that have not been reaped.  This avoid races on
+   platforms such as GTK, where other threads create their own
+   subprocesses which the main thread should not reap.  For example,
+   if the main thread attempted to reap an already-reaped child, it
+   might inadvertently reap a GTK-created process that happened to
+   have the same process ID.  */
+
+/* Handle a SIGCHLD signal by looking for known child processes of
+   Emacs whose status have changed.  For each one found, record its
+   new status.
 
    All we do is change the status; we do not run sentinels or print
    notifications.  That is saved for the next time keyboard input is
@@ -6211,20 +6224,15 @@
    ** Malloc WARNING: This should never call malloc either directly or
    indirectly; if it does, that is a bug  */
 
-void
-record_child_status_change (pid_t pid, int w)
+static void
+handle_child_signal (int sig)
 {
-#ifdef SIGCHLD
-
-  /* Record at most one child only if we already know one child that
-     has exited.  */
-  bool record_at_most_one_child = 0 <= pid;
-
   Lisp_Object tail;
 
   /* Find the process that signaled us, and record its status.  */
 
-  /* The process can have been deleted by Fdelete_process.  */
+  /* The process can have been deleted by Fdelete_process, or have
+     been started asynchronously by Fcall_process.  */
   for (tail = deleted_pid_list; CONSP (tail); tail = XCDR (tail))
     {
       bool all_pids_are_fixnums
@@ -6238,12 +6246,8 @@
 	    deleted_pid = XINT (xpid);
 	  else
 	    deleted_pid = XFLOAT_DATA (xpid);
-	  if (process_status_retrieved (deleted_pid, pid, &w))
-	    {
-	      XSETCAR (tail, Qnil);
-	      if (record_at_most_one_child)
-		return;
-	    }
+	  if (child_status_changed (deleted_pid, 0, 0))
+	    XSETCAR (tail, Qnil);
 	}
     }
 
@@ -6252,15 +6256,17 @@
     {
       Lisp_Object proc = XCDR (XCAR (tail));
       struct Lisp_Process *p = XPROCESS (proc);
-      if (p->alive && process_status_retrieved (p->pid, pid, &w))
+      int status;
+
+      if (p->alive && child_status_changed (p->pid, &status, WUNTRACED))
 	{
 	  /* Change the status of the process that was found.  */
 	  p->tick = ++process_tick;
-	  p->raw_status = w;
+	  p->raw_status = status;
 	  p->raw_status_new = 1;
 
 	  /* If process has terminated, stop waiting for its output.  */
-	  if (WIFSIGNALED (w) || WIFEXITED (w))
+	  if (WIFSIGNALED (status) || WIFEXITED (status))
 	    {
 	      int clear_desc_flag = 0;
 	      p->alive = 0;
@@ -6274,44 +6280,8 @@
 		  FD_CLR (p->infd, &non_keyboard_wait_mask);
 		}
 	    }
-
-	  /* Tell wait_reading_process_output that it needs to wake up and
-	     look around.  */
-	  if (input_available_clear_time)
-	    *input_available_clear_time = make_emacs_time (0, 0);
-
-	  if (record_at_most_one_child)
-	    return;
 	}
     }
-
-  if (0 <= pid)
-    {
-      /* The caller successfully waited for a pid but no asynchronous
-	 process was found for it, so this is a synchronous process.  */
-
-      synch_process_alive = 0;
-
-      /* Report the status of the synchronous process.  */
-      if (WIFEXITED (w))
-	synch_process_retcode = WEXITSTATUS (w);
-      else if (WIFSIGNALED (w))
-	synch_process_termsig = WTERMSIG (w);
-
-      /* Tell wait_reading_process_output that it needs to wake up and
-	 look around.  */
-      if (input_available_clear_time)
-	*input_available_clear_time = make_emacs_time (0, 0);
-    }
-#endif
-}
-
-#ifdef SIGCHLD
-
-static void
-handle_child_signal (int sig)
-{
-  record_child_status_change (-1, 0);
 }
 
 static void

=== modified file 'src/process.h'
--- src/process.h	2012-11-27 03:10:32 +0000
+++ src/process.h	2012-11-27 17:08:50 +0000
@@ -185,23 +185,6 @@
 }
 #endif
 
-/* True if we are about to fork off a synchronous process or if we
-   are waiting for it.  */
-extern bool synch_process_alive;
-
-/* Communicate exit status of sync process to from sigchld_handler
-   to Fcall_process.  */
-
-/* Nonzero => this is a string explaining death of synchronous subprocess.  */
-extern const char *synch_process_death;
-
-/* Nonzero => this is the signal number that terminated the subprocess.  */
-extern int synch_process_termsig;
-
-/* If synch_process_death is zero,
-   this is exit code of synchronous subprocess.  */
-extern int synch_process_retcode;
-
 /* Nonzero means don't run process sentinels.  This is used
    when exiting.  */
 extern int inhibit_sentinels;

=== modified file 'src/sysdep.c'
--- src/sysdep.c	2012-11-27 03:10:32 +0000
+++ src/sysdep.c	2012-11-29 01:11:49 +0000
@@ -266,45 +266,71 @@
 
 #ifndef MSDOS
 
-static void
-wait_for_termination_1 (pid_t pid, int interruptible)
+/* Wait for the subprocess with process id CHILD to terminate or change status.
+   CHILD must be a child process that has not been reaped.
+   If STATUS is non-null, store the waitpid-style exit status into *STATUS
+   and tell wait_reading_process_output that it needs to look around.
+   Use waitpid-style OPTIONS when waiting.
+   If INTERRUPTIBLE, this function is interruptible by a signal.
+
+   Return CHILD if successful, 0 if no status is available;
+   the latter is possible only when options & NOHANG.  */
+static pid_t
+get_child_status (pid_t child, int *status, int options, bool interruptible)
 {
-  while (1)
+  pid_t pid;
+
+  /* Invoke waitpid only with a known process ID; do not invoke
+     waitpid with a nonpositive argument.  Otherwise, Emacs might
+     reap an unwanted process by mistake.  For example, invoking
+     waitpid (-1, ...) can mess up glib by reaping glib's subprocesses,
+     so that another thread running glib won't find them.  */
+  eassert (0 < child);
+
+  while ((pid = waitpid (child, status, options)) < 0)
     {
-      int status;
-      int wait_result = waitpid (pid, &status, 0);
-      if (wait_result < 0)
-	{
-	  if (errno != EINTR)
-	    break;
-	}
-      else
-	{
-	  record_child_status_change (wait_result, status);
-	  break;
-	}
+      /* CHILD must be a child process that has not been reaped, and
+	 STATUS and OPTIONS must be valid.  */
+      eassert (errno == EINTR);
 
       /* Note: the MS-Windows emulation of waitpid calls QUIT
 	 internally.  */
       if (interruptible)
 	QUIT;
     }
-}
-
-/* Wait for subprocess with process id `pid' to terminate and
-   make sure it will get eliminated (not remain forever as a zombie) */
-
-void
-wait_for_termination (pid_t pid)
-{
-  wait_for_termination_1 (pid, 0);
-}
-
-/* Like the above, but allow keyboard interruption. */
-void
-interruptible_wait_for_termination (pid_t pid)
-{
-  wait_for_termination_1 (pid, 1);
+
+  /* If successful and status is requested, tell wait_reading_process_output
+     that it needs to wake up and look around.  */
+  if (pid && status && input_available_clear_time)
+    *input_available_clear_time = make_emacs_time (0, 0);
+
+  return pid;
+}
+
+/* Wait for the subprocess with process id CHILD to terminate.
+   CHILD must be a child process that has not been reaped.
+   If STATUS is non-null, store the waitpid-style exit status into *STATUS
+   and tell wait_reading_process_output that it needs to look around.
+   If INTERRUPTIBLE, this function is interruptible by a signal.  */
+void
+wait_for_termination (pid_t child, int *status, bool interruptible)
+{
+  get_child_status (child, status, 0, interruptible);
+}
+
+/* Report whether the subprocess with process id CHILD has changed status.
+   Termination counts as a change of status.
+   CHILD must be a child process that has not been reaped.
+   If STATUS is non-null, store the waitpid-style exit status into *STATUS
+   and tell wait_reading_process_output that it needs to look around.
+   Use waitpid-style OPTIONS to check status, but do not wait.
+
+   Return CHILD if successful, 0 if no status is available because
+   the process's state has not changed.  */
+pid_t
+child_status_changed (pid_t child, int *status, int options)
+{
+  return get_child_status (child, status, WNOHANG | options, 0);
 }
 
 /*
@@ -454,6 +480,7 @@
   char oldwd[MAXPATHLEN+1]; /* Fixed length is safe on MSDOS.  */
 #endif
   pid_t pid;
+  int status;
   struct save_signal saved_handlers[5];
   Lisp_Object dir;
   unsigned char *volatile str_volatile = 0;
@@ -491,7 +518,6 @@
 #ifdef DOS_NT
   pid = 0;
   save_signal_handlers (saved_handlers);
-  synch_process_alive = 1;
 #else
   pid = vfork ();
   if (pid == -1)
@@ -560,14 +586,12 @@
   /* Do this now if we did not do it before.  */
 #ifndef MSDOS
   save_signal_handlers (saved_handlers);
-  synch_process_alive = 1;
 #endif
 
 #ifndef DOS_NT
-  wait_for_termination (pid);
+  wait_for_termination (pid, &status, 0);
 #endif
   restore_signal_handlers (saved_handlers);
-  synch_process_alive = 0;
 }
 
 static void

=== modified file 'src/syswait.h'
--- src/syswait.h	2012-09-23 22:25:22 +0000
+++ src/syswait.h	2012-11-29 01:11:49 +0000
@@ -23,6 +23,7 @@
 #ifndef EMACS_SYSWAIT_H
 #define EMACS_SYSWAIT_H
 
+#include <stdbool.h>
 #include <sys/types.h>
 
 #ifdef HAVE_SYS_WAIT_H	/* We have sys/wait.h with POSIXoid definitions. */
@@ -52,10 +53,10 @@
 #endif
 
 /* Defined in process.c.  */
-extern void record_child_status_change (pid_t, int);
+extern void record_deleted_pid (pid_t);
 
 /* Defined in sysdep.c.  */
-extern void wait_for_termination (pid_t);
-extern void interruptible_wait_for_termination (pid_t);
+extern void wait_for_termination (pid_t, int *, bool);
+extern pid_t child_status_changed (pid_t, int *, int);
 
 #endif /* EMACS_SYSWAIT_H */

=== modified file 'src/w32proc.c'
--- src/w32proc.c	2012-11-27 03:10:32 +0000
+++ src/w32proc.c	2012-11-29 01:11:49 +0000
@@ -1274,33 +1274,7 @@
 #endif
 
   if (status)
-    {
-      *status = retval;
-    }
-  else if (synch_process_alive)
-    {
-      synch_process_alive = 0;
-
-      /* Report the status of the synchronous process.  */
-      if (WIFEXITED (retval))
-	synch_process_retcode = WEXITSTATUS (retval);
-      else if (WIFSIGNALED (retval))
-	{
-	  int code = WTERMSIG (retval);
-	  const char *signame;
-
-	  synchronize_system_messages_locale ();
-	  signame = strsignal (code);
-
-	  if (signame == 0)
-	    signame = "unknown";
-
-	  synch_process_death = signame;
-	}
-
-      reap_subprocess (cp);
-    }
-
+    *status = retval;
   reap_subprocess (cp);
 
   return pid;