[Python-Dev] Strange segfault in Python threads and linux kernel 2.6

[Donovan Baarda]

On Wed, 2005-01-19 at 13:37 +0000, Michael Hudson wrote:
> Donovan Baarda <abo at minkirri.apana.org.au> writes:
[...]
> You've left out a very important piece of information: which version
> of Python you are using.  I'm guessing 2.3.4.  Can you try 2.4?

Debian Python2.3 (2.3.4-18), Debian kernel-image-2.6.8-1-686 (2.6.8-10),
and Debian kernel-image-2.4.27-1-686 (2.4.27-6)

> I'd be astonished if this is the same bug.
> 
> The main oddness about python threads (before 2.3) is that they run
> with all signals masked.  You could play with a C wrapper (call
> setprocmask, then exec fop) to see if this is what is causing the
> problem.  But please try 2.4.

Python 2.4 does indeed fix the problem. Unfortunately we are using Zope
2.7.4, and I'm a bit wary of attempting to migrate it all from 2.3 to
2.4. Is there any way this "Fix" can be back-ported to 2.3?

Note that this problem is being triggered when using 
Popen3() in a thread. Popen3() simply uses os.fork() and os.execvp().
The segfault is occurring in the excecvp'ed process. I'm sure there must
be plenty of cases where this could happen. I think most people manage
to avoid it because the processes they are popen'ing or exec'ing happen
to not use signals.

After testing a bit, it seems the fork() in Popen3 is not a contributing
factor. The problem occurs whenever os.execvp() is executed in a thread.
It looks like the exec'ed command inherits the masked signals from the
thread.

I'm not sure what the correct behaviour should be. The fact that it
works in python2.4 feels more like a byproduct of the thread mask change
than correct behaviour. To me it seems like execvp() should be setting
the signal mask back to defaults or at least the mask of the main
process before doing the exec.

> > BTW, built in file objects really could use better non-blocking
> > support... I've got a half-drafted PEP for it... anyone interested in
> > it?
> 
> Err, this probably should be in a different mail :)

The verboseness of the attached test code because of this issue prompted
that comment... so vaguely related :-)

-- 
Donovan Baarda <abo at minkirri.apana.org.au>
http://minkirri.apana.org.au/~abo/