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Fix a race condition with glib (Bug#8855).

Browse source 
This is a backport from the trunk, consisting of:

2012-11-17  Eli Zaretskii  <eliz@gnu.org>

* nt/inc/sys/wait.h: New file, with prototype of waitpid and
definitions of macros it needs.
* nt/inc/ms-w32.h (wait): Don't define, 'wait' is not used anymore.
(sys_wait): Remove prototype.
* nt/config.nt (HAVE_SYS_WAIT_H): Define to 1.
* src/w32proc.c (create_child): Don't clip the PID of the child
process to fit into an Emacs integer, as this is no longer a
restriction.
(waitpid): Rename from sys_wait.  Emulate a Posix 'waitpid' by
reaping only the process specified by PID argument, if that is
positive.  Use PID instead of dead_child to know which process to
reap.  Wait for the child to die only if WNOHANG is not in
OPTIONS.
(sys_select): Don't set dead_child.
* src/sysdep.c (wait_for_termination_1): Remove the WINDOWSNT portion,
as it is no longer needed.
* src/process.c (waitpid, WUNTRACED) [!WNOHANG]: Remove definitions,
no longer needed.
(record_child_status_change): Remove the setting of
record_at_most_one_child for the !WNOHANG case.

2012-11-03  Paul Eggert  <eggert@cs.ucla.edu>

Fix a race condition that causes Emacs to mess up glib (Bug#8855).
This is a backport from the trunk.
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.
* src/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.
* src/process.h (Lisp_Process): New boolean member 'alive'.
This commit is contained in:
Paul Eggert 2012-11-23 14:20:31 -08:00
parent 002c019c34
commit 6d4e8f62e9
9 changed files with 290 additions and 152 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

13
nt/ChangeLog
View file

@ -1,3 +1,16 @@
2012-11-23 Paul Eggert <eggert@cs.ucla.edu>
Fix a race condition with glib (Bug#8855).
This is a backport from the trunk, consisting of:
2012-11-17 Eli Zaretskii <eliz@gnu.org>
* inc/sys/wait.h: New file, with prototype of waitpid and
definitions of macros it needs.
* inc/ms-w32.h (wait): Don't define, 'wait' is not used anymore.
(sys_wait): Remove prototype.
* config.nt (HAVE_SYS_WAIT_H): Define to 1.
2012-11-20 Eli Zaretskii <eliz@gnu.org>
* nmake.defs: Use !if, not !ifdef. See

2
nt/config.nt
View file

@ -963,7 +963,7 @@ along with GNU Emacs. If not, see <http://www.gnu.org/licenses/>. */
#undef HAVE_SYS_VLIMIT_H
/* Define to 1 if you have <sys/wait.h> that is POSIX.1 compatible. */
#undef HAVE_SYS_WAIT_H
#define HAVE_SYS_WAIT_H 1
/* Define to 1 if you have the <term.h> header file. */
#undef HAVE_TERM_H

3
nt/inc/ms-w32.h
View file

@ -185,15 +185,12 @@ extern char *getenv ();
/* Subprocess calls that are emulated. */
#define spawnve sys_spawnve
#define wait sys_wait
#define kill sys_kill
#define signal sys_signal
/* Internal signals. */
#define emacs_raise(sig) emacs_abort()
extern int sys_wait (int *);
/* termcap.c calls that are emulated. */
#define tputs sys_tputs
#define tgetstr sys_tgetstr

33
nt/inc/sys/wait.h Normal file
View file

@ -0,0 +1,33 @@
/* A limited emulation of sys/wait.h on Posix systems.
Copyright (C) 2012 Free Software Foundation, Inc.
This file is part of GNU Emacs.
GNU Emacs is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
GNU Emacs is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with GNU Emacs. If not, see <http://www.gnu.org/licenses/>. */
#ifndef INC_SYS_WAIT_H_
#define INC_SYS_WAIT_H_
#define WNOHANG 1
#define WUNTRACED 2
#define WSTOPPED 2 /* same as WUNTRACED */
#define WEXITED 4
#define WCONTINUED 8
/* The various WIF* macros are defined in src/syswait.h. */
extern pid_t waitpid (pid_t, int *, int);
#endif /* INC_SYS_WAIT_H_ */

57
src/ChangeLog
View file

@ -1,3 +1,60 @@
2012-11-23 Paul Eggert <eggert@cs.ucla.edu>
Fix a race condition with glib (Bug#8855).
This is a backport from the trunk, consisting of:
2012-11-17 Eli Zaretskii <eliz@gnu.org>
* w32proc.c (create_child): Don't clip the PID of the child
process to fit into an Emacs integer, as this is no longer a
restriction.
(waitpid): Rename from sys_wait. Emulate a Posix 'waitpid' by
reaping only the process specified by PID argument, if that is
positive. Use PID instead of dead_child to know which process to
reap. Wait for the child to die only if WNOHANG is not in
OPTIONS.
(sys_select): Don't set dead_child.
* sysdep.c (wait_for_termination_1): Remove the WINDOWSNT portion,
as it is no longer needed.
* process.c (waitpid, WUNTRACED) [!WNOHANG]: Remove definitions,
no longer needed.
(record_child_status_change): Remove the setting of
record_at_most_one_child for the !WNOHANG case.
2012-11-03 Paul Eggert <eggert@cs.ucla.edu>
Fix a race condition that causes Emacs to mess up glib (Bug#8855).
This is a backport from the trunk.
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'.
2012-11-23 Eli Zaretskii <eliz@gnu.org>
* xdisp.c (set_cursor_from_row): Skip step 2 only if point is not

197
src/process.c
View file

@ -130,14 +130,6 @@ extern int sys_select (int, SELECT_TYPE *, SELECT_TYPE *, SELECT_TYPE *,
EMACS_TIME *, void *);
#endif
#ifndef WNOHANG
# undef waitpid
# define waitpid(pid, status, options) wait (status)
#endif
#ifndef WUNTRACED
# define WUNTRACED 0
#endif
/* Work around GCC 4.7.0 bug with strict overflow checking; see
<http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52904>.
These lines can be removed once the GCC bug is fixed. */
@ -795,9 +787,8 @@ get_process (register Lisp_Object name)
#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 +1695,7 @@ create_process (Lisp_Object process, char **new_argv, Lisp_Object current_dir)
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);
@ -1880,6 +1862,8 @@ create_process (Lisp_Object process, char **new_argv, Lisp_Object current_dir)
#endif
XPROCESS (process)->pid = pid;
if (0 <= pid)
XPROCESS (process)->alive = 1;
/* Stop blocking signals in the parent. */
#ifdef SIGCHLD
@ -6273,9 +6257,35 @@ process has been transmitted to the serial port. */)
return process;
}
/* 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)
{
/* 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.
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
@ -6298,13 +6308,15 @@ process has been transmitted to the serial port. */)
** 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
record_child_status_change (pid_t pid, int w)
{
#ifdef SIGCHLD
Lisp_Object proc;
struct Lisp_Process *p;
/* On POSIXish hosts, 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. */
@ -6312,68 +6324,69 @@ record_child_status_change (pid_t pid, int w)
/* The process can have been deleted by Fdelete_process. */
for (tail = deleted_pid_list; CONSP (tail); tail = XCDR (tail))
{
bool all_pids_are_fixnums
= (MOST_NEGATIVE_FIXNUM <= TYPE_MINIMUM (pid_t)
&& TYPE_MAXIMUM (pid_t) <= MOST_POSITIVE_FIXNUM);
Lisp_Object xpid = XCAR (tail);
if ((INTEGERP (xpid) && pid == XINT (xpid))
|| (FLOATP (xpid) && pid == XFLOAT_DATA (xpid)))
if (all_pids_are_fixnums ? INTEGERP (xpid) : NUMBERP (xpid))
{
XSETCAR (tail, Qnil);
return;
pid_t deleted_pid;
if (INTEGERP (xpid))
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;
}
}
}
/* Otherwise, if it is asynchronous, it is in Vprocess_alist. */
p = 0;
for (tail = Vprocess_alist; CONSP (tail); tail = XCDR (tail))
{
proc = XCDR (XCAR (tail));
p = XPROCESS (proc);
if (EQ (p->type, Qreal) && p->pid == pid)
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)
{
int clear_desc_flag = 0;
p->tick = ++process_tick;
p->raw_status = w;
p->raw_status_new = 1;
/* If process has terminated, stop waiting for its output. */
if ((WIFSIGNALED (w) || WIFEXITED (w))
&& p->infd >= 0)
clear_desc_flag = 1;
/* We use clear_desc_flag to avoid a compiler bug in Microsoft C. */
if (clear_desc_flag)
Lisp_Object proc = XCDR (XCAR (tail));
struct Lisp_Process *p = XPROCESS (proc);
if (p->alive && process_status_retrieved (p->pid, pid, &w))
{
FD_CLR (p->infd, &input_wait_mask);
FD_CLR (p->infd, &non_keyboard_wait_mask);
}
/* Change the status of the process that was found. */
p->tick = ++process_tick;
p->raw_status = w;
p->raw_status_new = 1;
/* 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 process has terminated, stop waiting for its output. */
if (WIFSIGNALED (w) || WIFEXITED (w))
{
int clear_desc_flag = 0;
p->alive = 0;
if (p->infd >= 0)
clear_desc_flag = 1;
/* clear_desc_flag avoids a compiler bug in Microsoft C. */
if (clear_desc_flag)
{
FD_CLR (p->infd, &input_wait_mask);
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;
}
}
/* There was no asynchronous process found for that pid: we have
a synchronous process. */
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. */
@ -6392,38 +6405,10 @@ record_child_status_change (pid_t pid, int w)
#ifdef SIGCHLD
/* On some systems, the SIGCHLD handler must return right away. If
any more processes want to signal us, we will get another signal.
Otherwise, loop around to use up all the processes that have
something to tell us. */
#if (defined WINDOWSNT \
|| (defined USG && !defined GNU_LINUX \
&& !(defined HPUX && defined WNOHANG)))
enum { CAN_HANDLE_MULTIPLE_CHILDREN = 0 };
#else
enum { CAN_HANDLE_MULTIPLE_CHILDREN = 1 };
#endif
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);
record_child_status_change (-1, 0);
}
static void

3
src/process.h
View file

@ -142,6 +142,9 @@ struct Lisp_Process
/* Flag to set coding-system of the process buffer from the
coding_system used to decode process output. */
unsigned int inherit_coding_system_flag : 1;
/* Whether the process is alive, i.e., can be waited for. Running
processes can be waited for, but exited and fake processes cannot. */
unsigned int alive : 1;
/* Record the process status in the raw form in which it comes from `wait'.
This is to avoid consing in a signal handler. The `raw_status_new'
flag indicates that `raw_status' contains a new status that still

7
src/sysdep.c
View file

@ -289,10 +289,6 @@ wait_for_termination_1 (pid_t pid, int interruptible)
{
while (1)
{
#ifdef WINDOWSNT
wait (0);
break;
#else /* not WINDOWSNT */
int status;
int wait_result = waitpid (pid, &status, 0);
if (wait_result < 0)
@ -306,7 +302,8 @@ wait_for_termination_1 (pid_t pid, int interruptible)
break;
}
#endif /* not WINDOWSNT */
/* Note: the MS-Windows emulation of waitpid calls QUIT
internally. */
if (interruptible)
QUIT;
}

127
src/w32proc.c
View file

@ -783,7 +783,6 @@ alarm (int seconds)
/* Child process management list. */
int child_proc_count = 0;
child_process child_procs[ MAX_CHILDREN ];
child_process *dead_child = NULL;
static DWORD WINAPI reader_thread (void *arg);
@ -1036,9 +1035,6 @@ create_child (char *exe, char *cmdline, char *env, int is_gui_app,
if (cp->pid < 0)
cp->pid = -cp->pid;
/* pid must fit in a Lisp_Int */
cp->pid = cp->pid & INTMASK;
*pPid = cp->pid;
return TRUE;
@ -1114,55 +1110,110 @@ reap_subprocess (child_process *cp)
delete_child (cp);
}
/* Wait for any of our existing child processes to die
When it does, close its handle
Return the pid and fill in the status if non-NULL. */
/* Wait for a child process specified by PID, or for any of our
existing child processes (if PID is nonpositive) to die. When it
does, close its handle. Return the pid of the process that died
and fill in STATUS if non-NULL. */
int
sys_wait (int *status)
pid_t
waitpid (pid_t pid, int *status, int options)
{
DWORD active, retval;
int nh;
int pid;
child_process *cp, *cps[MAX_CHILDREN];
HANDLE wait_hnd[MAX_CHILDREN];
DWORD timeout_ms;
int dont_wait = (options & WNOHANG) != 0;
nh = 0;
if (dead_child != NULL)
/* According to Posix:
PID = -1 means status is requested for any child process.
PID > 0 means status is requested for a single child process
whose pid is PID.
PID = 0 means status is requested for any child process whose
process group ID is equal to that of the calling process. But
since Windows has only a limited support for process groups (only
for console processes and only for the purposes of passing
Ctrl-BREAK signal to them), and since we have no documented way
of determining whether a given process belongs to our group, we
treat 0 as -1.
PID < -1 means status is requested for any child process whose
process group ID is equal to the absolute value of PID. Again,
since we don't support process groups, we treat that as -1. */
if (pid > 0)
{
/* We want to wait for a specific child */
wait_hnd[nh] = dead_child->procinfo.hProcess;
cps[nh] = dead_child;
if (!wait_hnd[nh]) emacs_abort ();
nh++;
active = 0;
goto get_result;
int our_child = 0;
/* We are requested to wait for a specific child. */
for (cp = child_procs + (child_proc_count-1); cp >= child_procs; cp--)
{
/* Some child_procs might be sockets; ignore them. Also
ignore subprocesses whose output is not yet completely
read. */
if (CHILD_ACTIVE (cp)
&& cp->procinfo.hProcess
&& cp->pid == pid)
{
our_child = 1;
break;
}
}
if (our_child)
{
if (cp->fd < 0 || (fd_info[cp->fd].flags & FILE_AT_EOF) != 0)
{
wait_hnd[nh] = cp->procinfo.hProcess;
cps[nh] = cp;
nh++;
}
else if (dont_wait)
{
/* PID specifies our subprocess, but its status is not
yet available. */
return 0;
}
}
if (nh == 0)
{
/* No such child process, or nothing to wait for, so fail. */
errno = ECHILD;
return -1;
}
}
else
{
for (cp = child_procs + (child_proc_count-1); cp >= child_procs; cp--)
/* some child_procs might be sockets; ignore them */
if (CHILD_ACTIVE (cp) && cp->procinfo.hProcess
&& (cp->fd < 0 || (fd_info[cp->fd].flags & FILE_AT_EOF) != 0))
{
wait_hnd[nh] = cp->procinfo.hProcess;
cps[nh] = cp;
nh++;
}
{
if (CHILD_ACTIVE (cp)
&& cp->procinfo.hProcess
&& (cp->fd < 0 || (fd_info[cp->fd].flags & FILE_AT_EOF) != 0))
{
wait_hnd[nh] = cp->procinfo.hProcess;
cps[nh] = cp;
nh++;
}
}
if (nh == 0)
{
/* Nothing to wait on, so fail. */
errno = ECHILD;
return -1;
}
}
if (nh == 0)
{
/* Nothing to wait on, so fail */
errno = ECHILD;
return -1;
}
if (dont_wait)
timeout_ms = 0;
else
timeout_ms = 1000; /* check for quit about once a second. */
do
{
/* Check for quit about once a second. */
QUIT;
active = WaitForMultipleObjects (nh, wait_hnd, FALSE, 1000);
active = WaitForMultipleObjects (nh, wait_hnd, FALSE, timeout_ms);
} while (active == WAIT_TIMEOUT);
if (active == WAIT_FAILED)
@ -1192,8 +1243,10 @@ sys_wait (int *status)
}
if (retval == STILL_ACTIVE)
{
/* Should never happen */
/* Should never happen. */
DebPrint (("Wait.WaitForMultipleObjects returned an active process\n"));
if (pid > 0 && dont_wait)
return 0;
errno = EINVAL;
return -1;
}
@ -1207,6 +1260,8 @@ sys_wait (int *status)
else
retval <<= 8;
if (pid > 0 && active != 0)
emacs_abort ();
cp = cps[active];
pid = cp->pid;
#ifdef FULL_DEBUG
@ -1995,9 +2050,7 @@ sys_select (int nfds, SELECT_TYPE *rfds, SELECT_TYPE *wfds, SELECT_TYPE *efds,
DebPrint (("select calling SIGCHLD handler for pid %d\n",
cp->pid));
#endif
dead_child = cp;
sig_handlers[SIGCHLD] (SIGCHLD);
dead_child = NULL;
}
}
else if (fdindex[active] == -1)