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Date:	Thu, 19 Jun 2014 09:11:37 -0400
From:	Ken Brown <kbrown AT cornell DOT edu>
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To:	cygwin AT cygwin DOT com
Subject:	Re: bzr emacs(24.4.50.1) crashes when using auto-revert-tail-mode
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On 6/18/2014 1:46 PM, Ken Brown wrote:
> On 6/18/2014 11:06 AM, Filipp Gunbin wrote:
>> I'm not sure whether this is Cygwin-specific, but I'm not able to test
>> it on other OS, so here are the steps to reproduce:
>>
>> emacs -Q
>> C-x C-r <some_file>
> 
> You mean C-x C-f
> 
>> M-x auto-revert-tail-mode
>> wait for few seconds -> emacs crashes
> 
> I can confirm this, but on 32-bit Cygwin only; there's no crash on 
> 64-bit Cygwin.  (This is a refreshing change from the emacs crashes 
> people have been reporting on 64-bit Cygwin.)
> 
> Here's the backtrace:
> 
> #0  0x00000000 in ?? ()
> No symbol table info available.
> #1  0x610f842f in pthread_mutex::lock (this=0x0)
>      at /usr/src/debug/cygwin-1.7.30-1/winsup/cygwin/thread.cc:1745
>          self = <optimized out>
>          result = <optimized out>
>          __PRETTY_FUNCTION__ = "int pthread_mutex::lock()"
> #2  0x00000201 in ?? ()
> No symbol table info available.
> 
> Lisp Backtrace:
> "gfile-add-watch" (0x288dc8)
> "file-notify-add-watch" (0x2890c8)
> "byte-code" (0x289340)
> "auto-revert-notify-add-watch" (0x289778)
> "auto-revert-buffers" (0x289b4c)
> "apply" (0x289b48)
> "byte-code" (0x289dc0)
> "timer-event-handler" (0x28a1fc)
> 
> I'll look into this.  In the meantime, you can work around it by 
> configuring --with-file-notification=no.

I'm afraid I ran into a brick wall trying to debug this.  I wanted to see what gfile-add-watch was doing, so I ran emacs under gdb with a breakpoint at Fgfile_add_watch and then a breakpoint at g_file_monitor (a Glib function called by Fgfile_add_watch).  When I tried to step through this function, I hit an assertion violation in gdb.  This is repeatable.  A log of the gdb session is appended below.

The problem occurs only on 32-bit Cygwin.  On 64-bit Cygwin, I can step through Fgfile_add_watch without a problem.

Filipp, do you want to try debugging this yourself?  Maybe your debugging skills are better than mine.  (You'll need to install glib2.0-debuginfo.)

Ken

$ gdb /usr/bin/emacs-w32.exe
GNU gdb (GDB) 7.6.50.20130728-cvs (cygwin-special)
[...]
Reading symbols from /usr/bin/emacs-w32.exe...Reading symbols from /usr/lib/debug/usr/bin/emacs-w32.exe.dbg...done.
done.
(gdb) b Fgfile_add_watch
Breakpoint 1 at 0x5f7a3f: file /usr/src/debug/emacs-24.3.91-2/src/gfilenotify.c, line 170.
(gdb) r -Q
Starting program: /usr/bin/emacs-w32.exe -Q
[...]
Breakpoint 1, Fgfile_add_watch (file=-2145740495, flags=13798510,
    callback=-2146880150)
    at /usr/src/debug/emacs-24.3.91-2/src/gfilenotify.c:170
170       GFileMonitorFlags gflags = G_FILE_MONITOR_NONE;
(gdb) b g_file_monitor
Breakpoint 2 at 0x6a357e50: file /usr/src/debug/glib2.0-2.38.2-2/gio/gfile.c, line 5338.
(gdb) c
Continuing.
[...]
Breakpoint 2, g_file_monitor (file=0x80061530,
    flags=(G_FILE_MONITOR_WATCH_MOUNTS | G_FILE_MONITOR_SEND_MOVED),
    cancellable=0x0, error=0x0)
    at /usr/src/debug/glib2.0-2.38.2-2/gio/gfile.c:5338
5338    {
(gdb) n
5339      if (g_file_query_file_type (file, 0, cancellable) == G_FILE_TYPE_DIRECTORY)
(gdb) n
5338    {
(gdb) n
5339      if (g_file_query_file_type (file, 0, cancellable) == G_FILE_TYPE_DIRECTORY)
(gdb) n
5340        return g_file_monitor_directory (file,
(gdb) n
5339      if (g_file_query_file_type (file, 0, cancellable) == G_FILE_TYPE_DIRECTORY)
(gdb) n
5341                                         flags & ~G_FILE_MONITOR_WATCH_HARD_LINKS,
(gdb) n
5340        return g_file_monitor_directory (file,
(gdb) n
5345    }
(gdb) n
5340        return g_file_monitor_directory (file,
(gdb) n
g_file_monitor_directory (file=0x80061530,
    flags=(G_FILE_MONITOR_WATCH_MOUNTS | G_FILE_MONITOR_SEND_MOVED),
    cancellable=0x0, error=0x0)
    at /usr/src/debug/glib2.0-2.38.2-2/gio/gfile.c:5235
5235    {
(gdb) n
5238      g_return_val_if_fail (G_IS_FILE (file), NULL);
(gdb) n
/netrel/src/gdb-7.6.50-4/gdb/infrun.c:1942: internal-error: resume: Assertion `pc_in_thread_step_range (pc, tp)' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) y



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