Re: (call-process ...) hangs in emacs

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Aug  4 09:34, Ken Brown wrote:
> On 8/4/2014 4:00 AM, Corinna Vinschen wrote:
> >On Aug  3 21:02, Ken Brown wrote:
> >>On 8/1/2014 9:32 AM, Corinna Vinschen wrote:
> >>>It could be a problem with the new default pthread mutexes being
> >>>NORMAL, rather then ERRORCHECK mutexes.
> >>
> >>That does seem to be the problem, since I can reproduce the bug starting
> >>with the 2014-07-14 snapshot.  More precisely, I can reproduce it using
> >>emacs-nox (which is what the OP was using according to his cygcheck output)
> >>but not using emacs-X11 or emacs-w32.
> >>
> >>I tried running emacs under gdb with a breakpoint at call_process, but all I
> >>could see from that is that emacs tries to fork a subprocess, but the call
> >>to fork() never returns.  I also tried running it under strace, but again
> >>all I can see is that fork() is called and then everything seems to be at a
> >>standstill.
> >>
> >>Corinna, if you want to take a look, here's the precise recipe:
> >>
> >>1. emacs-nox -Q [This should start emacs and put you in the *scratch*
> >>buffer.]
> >>
> >>2. Enter the following text into the buffer:
> >>
> >>   (call-process "pwd" nil t)
> >>
> >>3. Position the cursor at the end of the line and type Ctrl-j.
> >>
> >>What should happen, and what does happen prior to the 2014-07-14 snapshot,
> >>is that the current directory is displayed, followed by the exit code of 0.
> >>What happens instead is that emacs appears to hang.
> >
> >How does emacs start a process?  Does it create a thread and then
> >forks and execs from the thread?  Does it use its own pthread_mutex
> >to control the job?  Is there a chance to create an STC of this
> >process?
> 
> emacs does some bookkeeping and then calls vfork.  It does not create a new
> thread, nor does it create a pthread_mutex.  The only pthread_mutexes
> created anywhere in the emacs source code are in its implementation of
> malloc and friends, not in anything directly related to controlling
> subprocesses.  (FWIW, this malloc implementation is used in the Cygwin build
> of emacs but not in the Linux build.)

Can you take a close look here?  This malloc will be used by Cygwin
as well if it's implemented in the usual way and...

> I did think about trying to create an STC, but I'm stymied because the
> problem depends so strongly on how emacs is run:
> 
>  - If emacs is run interactively, the problem only occurs with emacs-nox,
> not with emacs-X11 or emacs-w32.
> 
>  - If emacs is run non-interactively (i.e., in batch mode), the problem
> occurs with emacs-w32 and emacs-X11 too, as Angelo and Katsumi pointed out
> earlier in the thread.
> 
> I can't think of any way to capture these peculiarities in an STC.

...this, and the fact that fork/exec (vfork == fork on Cygwin) still
works nicely in other scenarios points to some problem with the usage of
pthread_mutexes in the application may be the culprit.

For instance, is it possible that emacs expects the pthread_mutexes
in malloc to be ERRORCHECK mutexes?  What if you explicitely set
them to ERRORCHECK at creation time?


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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