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

[Donovan Baarda]

G'day,

From: "Anthony Baxter" <anthony at interlink.com.au>
> On Thursday 20 January 2005 12:43, Donovan Baarda wrote:
> > On Wed, 2005-01-19 at 13:37 +0000, Michael Hudson wrote:
> > > 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 wa  this "Fix" can be back-ported to 2.3?
>
> It's extremely unlikely - I couldn't make myself comfortable with it
> when attempting to figure out it's backportedness. While the current
> behaviour on 2.3.4 is broken in some cases, I fear very much that
> the new behaviour will break other (working) code - and this is
> something I try very hard to avoid in a bugfix release, particularly
> in one that's probably the final one of a series.
>
> Fundamentally, the answer is "don't do signals+threads, you will
> get burned". For your application, you might want to instead try

In this case it turns out to be "don't do exec() in a thread, because what
you exec can have all it's signals masked". That turns out to be a hell of a
lot of things; popen, os.command, etc. They all only work OK in a threaded
application if what you are exec'ing doesn't use any signals.

> something where you write requests to a file in a spool directory,
> and have a python script that loops looking for requests, and
> generates responses. This is likely to be much simpler to debug
> and work with.

Hmm, interprocess communications; great fun :-) And no spawning the process
from within the zope application; it's gotta be a separate daemon.

Actually, I've noticed that zope often has a sorta zombie "which" process
which it spawns. I wonder it this is a stuck thread waiting for some
signal...

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Donovan Baarda                http://minkirri.apana.org.au/~abo/
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