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From:	Jean-Pierre Flori <jpflori AT gmail DOT com>
Subject:	Re: Problem with multiprocessing module from Python
Date:	Thu, 31 Oct 2013 20:47:58 +0000 (UTC)
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Le Thu, 31 Oct 2013 14:41:14 -0400, Christopher Faylor a écrit :

> On Thu, Oct 31, 2013 at 06:27:39PM +0000, Jean-Pierre Flori wrote:
>>Le Wed, 30 Oct 2013 10:33:13 +0100, Corinna Vinschen a ??crit??:
>>
>>> On Oct 29 21:22, Jean-Pierre Flori wrote:
>>>> Le Tue, 29 Oct 2013 21:19:14 +0000, Jean-Pierre Flori a ??crit??:
>>>> 
>>>> > Le Tue, 29 Oct 2013 16:59:35 -0400, Christopher Faylor a ??crit??:
>>>> >> If you want this fixed, the easiest way to get that to happen is
>>>> >> to post a simple test case which reproduces the problem.  That is
>>>> >> not the code snippet that you sent.  A real working example would
>>>> >> be required.
>>>> > Sorry about that.
>>>> > 
>>>> > Here you go:
>>>> > """
>>>> > from multiprocessing import Pool
>>>> > 
>>>> > def f(x): return x
>>>> > 
>>>> > p = pool(2)
>>>> > 
>>>> > p.map(f, [1, 2])
>>>> > """
>>>> And I managed to introduce a typo. The third line should read Pool,
>>>> so it is:
>>>> """
>>>> from multiprocessing import Pool
>>>> 
>>>> def f(x): return x
>>>> 
>>>> p = Pool(2)
>>>> 
>>>> p.map(f, [1, 2])
>>>> """
>>> 
>>> Works for me.  I guess.  At least, if I run the script, nothing
>>> happens:
>>> 
>>>   $ python x.py $
>>> 
>>> Same on 32 and 64 bit Cygwin.
>>> 
>>> 
>>> Corinna
>>
>>I think I got to the bottom of this.
>>It seems the new implem of sem_getvalue in cgwin1.dll is the cause, see:
>>http://cygwin.com/ml/cygwin-patches/2013-q3/msg00006.html It may also
>>explain the random reproducibility if sval stays uninitialized or
>>something like that (I did not check it is the case though).
> 
> I doubt that was the problem.  More likely it is something related to
> the changes in thread.cc that followed that change.
> 
> cgf

I must admit that was just a guess.
The thread.cc code changed whereas python code in Modules/
_multiprocessing/semaphore.c did not.
The python multiprocessing module (file semaphore.c) contains:
"""
        if (sem_getvalue(self->handle, &sval) < 0) {
            return PyErr_SetFromErrno(PyExc_OSError);
        } else if (sval >= self->maxvalue) {
            PyErr_SetString(PyExc_ValueError, "semaphore or lock "
                            "released too many times");
            return NULL;
        }
"""
Changing this to
"""
        sval = sem_getvalue(self->handle, &sval);
        if (sem_getvalue(self->handle, &sval) < 0) {
            return PyErr_SetFromErrno(PyExc_OSError);
        } else if (sval >= self->maxvalue) {
            PyErr_SetString(PyExc_ValueError, "semaphore or lock "
                            "released too many times");
            return NULL;
        }
"""
to emulate the previous behavior of sem_getvalue seems to solve my problem 
(and was easier than rebuilding cygwin1.dll).

Commit http://cygwin.com/cgi-bin/cvsweb.cgi/src/winsup/cygwin/thread.cc?
rev=1.286&content-type=text/x-cvsweb-markup&cvsroot=src
would be exactly what I need.
I'll just be waiting for 1.7.26.

Hopefully this will get sorted out.

Best,
JP


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