From: Paul Eggert <eggert@cs.ucla.edu>
To: Eli Zaretskii <eliz@gnu.org>
Cc: 8855@debbugs.gnu.org
Subject: bug#8855: dbus error at startup
Date: Sun, 28 Oct 2012 18:38:18 -0700 [thread overview]
Message-ID: <508DDE0A.9020903@cs.ucla.edu> (raw)
In-Reply-To: <83y5iua59n.fsf@gnu.org>
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Attached is a slightly less-intrusive version of the patch.
It omits the removal of "#ifdef SIGCHLD", which isn't
strictly needed to fix the bug. I don't see any further
simplifications, unfortunately. The patch fixes a bug that's
been reported multiple times so I'm thinking it may be
worthwhile to install now, even though there's a feature
freeze.
[-- Attachment #2: g_spawn_sync.txt --]
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=== modified file 'src/ChangeLog'
--- src/ChangeLog 2012-10-28 17:42:52 +0000
+++ src/ChangeLog 2012-10-29 01:25:53 +0000
@@ -1,3 +1,31 @@
+2012-10-28 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.
+ Perhaps this bug fix can be made more efficient by invoking
+ waitpid once on a process group instead of invoking it on each
+ known child, but at least this code fixes the bug.
+ * 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.
+ (process_status_retrieved): New function.
+ (record_child_status_change): Use it. Return bool.
+ Accept negative 1st argument, which means to wait for one of
+ the processes that Emacs already knows about.
+ (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.
+ * syswait.h: Adjust to above API changes.
+
2012-10-28 Eli Zaretskii <eliz@gnu.org>
* w32proc.c (TIMER_TICKS_PER_SEC): New macro.
=== modified file 'src/process.c'
--- src/process.c 2012-10-19 19:25:18 +0000
+++ src/process.c 2012-10-29 01:25:53 +0000
@@ -795,9 +795,8 @@
#ifdef SIGCHLD
/* 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; otherwise this handler would consider the
- process as being synchronous and say that the synchronous process is
- dead. */
+ treated by the SIGCHLD handler and waitpid has been invoked on them;
+ otherwise they might fill up the kernel's process table. */
static Lisp_Object deleted_pid_list;
#endif
@@ -1704,16 +1703,7 @@
if (inchannel > max_process_desc)
max_process_desc = inchannel;
- /* Until we store the proper pid, enable the SIGCHLD handler
- to recognize an unknown pid as standing for this process.
- It is very important not to let this `marker' value stay
- in the table after this function has returned; if it does
- it might cause call-process to hang and subsequent asynchronous
- processes to get their return values scrambled. */
- XPROCESS (process)->pid = -1;
-
- /* This must be called after the above line because it may signal an
- error. */
+ /* This may signal an error. */
setup_process_coding_systems (process);
encoded_current_dir = ENCODE_FILE (current_dir);
@@ -6279,9 +6269,30 @@
return process;
}
\f
-/* On receipt of a signal that a child status has changed, loop asking
- about children with changed statuses until the system says there
- are no more.
+/* 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)
+ {
+ 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 a known child process
+ of Emacs whose status has changed. If one is found, record its new
+ status and return true; otherwise, return false.
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
@@ -6304,8 +6315,7 @@
** Malloc WARNING: This should never call malloc either directly or
indirectly; if it does, that is a bug */
-/* Record the changed status of the child process PID with wait status W. */
-void
+bool
record_child_status_change (pid_t pid, int w)
{
#ifdef SIGCHLD
@@ -6319,11 +6329,18 @@
for (tail = deleted_pid_list; CONSP (tail); tail = XCDR (tail))
{
Lisp_Object xpid = XCAR (tail);
- if ((INTEGERP (xpid) && pid == XINT (xpid))
- || (FLOATP (xpid) && pid == XFLOAT_DATA (xpid)))
+ bool all_pids_are_fixnums
+ = (MOST_NEGATIVE_FIXNUM <= TYPE_MINIMUM (pid_t)
+ && TYPE_MAXIMUM (pid_t) <= MOST_POSITIVE_FIXNUM);
+ pid_t deleted_pid;
+ if (all_pids_are_fixnums || INTEGERP (xpid))
+ deleted_pid = XINT (xpid);
+ else
+ deleted_pid = XFLOAT_DATA (xpid);
+ if (process_status_retrieved (deleted_pid, pid, &w))
{
XSETCAR (tail, Qnil);
- return;
+ return 1;
}
}
@@ -6333,23 +6350,11 @@
{
proc = XCDR (XCAR (tail));
p = XPROCESS (proc);
- if (EQ (p->type, Qreal) && p->pid == pid)
+ if (EQ (p->type, Qreal) && process_status_retrieved (p->pid, pid, &w))
break;
p = 0;
}
- /* Look for an asynchronous process whose pid hasn't been filled
- in yet. */
- if (! p)
- for (tail = Vprocess_alist; CONSP (tail); tail = XCDR (tail))
- {
- proc = XCDR (XCAR (tail));
- p = XPROCESS (proc);
- if (p->pid == -1)
- break;
- p = 0;
- }
-
/* Change the status of the process that was found. */
if (p)
{
@@ -6375,11 +6380,14 @@
look around. */
if (input_available_clear_time)
*input_available_clear_time = make_emacs_time (0, 0);
+
+ return 1;
}
- /* There was no asynchronous process found for that pid: we have
- a synchronous process. */
- else
+ else 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. */
@@ -6392,8 +6400,12 @@
look around. */
if (input_available_clear_time)
*input_available_clear_time = make_emacs_time (0, 0);
+
+ return 1;
}
#endif
+
+ return 0;
}
#ifdef SIGCHLD
@@ -6413,23 +6425,8 @@
static void
handle_child_signal (int sig)
{
- do
- {
- pid_t pid;
- int status;
-
- do
- pid = waitpid (-1, &status, WNOHANG | WUNTRACED);
- while (pid < 0 && errno == EINTR);
-
- /* PID == 0 means no processes found, PID == -1 means a real failure.
- Either way, we have done all our job. */
- if (pid <= 0)
- break;
-
- record_child_status_change (pid, status);
- }
- while (CAN_HANDLE_MULTIPLE_CHILDREN);
+ while (record_child_status_change (-1, 0) && CAN_HANDLE_MULTIPLE_CHILDREN)
+ continue;
}
static void
=== modified file 'src/syswait.h'
--- src/syswait.h 2012-09-23 22:25:22 +0000
+++ src/syswait.h 2012-10-25 05:29:40 +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. */
@@ -52,7 +53,7 @@
#endif
/* Defined in process.c. */
-extern void record_child_status_change (pid_t, int);
+extern bool record_child_status_change (pid_t, int);
/* Defined in sysdep.c. */
extern void wait_for_termination (pid_t);
next prev parent reply other threads:[~2012-10-29 1:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-13 17:49 bug#8855: dbus error at startup Dan Nicolaescu
2011-06-26 10:22 ` Michael Albinus
2012-10-25 5:35 ` Paul Eggert
2012-10-25 16:24 ` Eli Zaretskii
2012-10-25 20:57 ` Paul Eggert
2012-10-29 1:38 ` Paul Eggert [this message]
2012-10-29 9:24 ` Chong Yidong
2012-10-29 17:06 ` Eli Zaretskii
2012-10-31 7:36 ` Paul Eggert
2012-10-29 6:27 ` bug#8855: confirmation from glib side Paul Eggert
2012-11-03 18:34 ` bug#8855: installed patch into trunk Paul Eggert
2012-11-27 2:32 ` bug#8855: Fix backported to Emacs 24 Paul Eggert
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