threading and iterator crashing interpreter

[Janto Dreijer]

On Mar 11, 1:46 pm, "Gabriel Genellina" <gagsl-... at yahoo.com.ar>
wrote:
> En Sun, 11 Mar 2007 07:32:04 -0300, Janto Dreijer <jan... at gmail.com>
> escribió:
>
>
>
> > I have been having problems with the Python 2.4 and 2.5 interpreters
> > on both Linux and Windows crashing on me. Unfortunately it's rather
> > complex code and difficult to pin down the source.
>
> > So I've been trying to reduce the code. In the process it's started to
> > crash in different ways. I'm not sure if any of it is related. The
> > following is only crashing Python 2.5 (r25:51908, Sep 19 2006,
> > 09:52:17) [MSC v.1310 32 bit (Intel)] on win32 in two different(?)
> > ways.
>
> > ====================
>
> > Using the login1() function will throw a bunch of exceptions the most
> > interesting of which is:
>
> > Exception in thread Thread-34:
> > Traceback (most recent call last):
> >   File "C:\Python25\lib\threading.py", line 460, in __bootstrap
> >     self.run()
> >   File "C:\Python25\lib\threading.py", line 440, in run
> >     self.__target(*self.__args, **self.__kwargs)
> >   File "Copy of scratchpad.py", line 20, in login1
> >     random.choice(System.sessions).session_iter.next()
> > RuntimeError: instance.__dict__ not accessible in restricted mode
>
>  From the error message, you appear to be using some form of restricted
> execution - RExec or similar. I didn't try that way, but using the normal
> mode, I got this different exception instead (using login1):
>
> Exception in thread Thread-85:
> Traceback (most recent call last):
>    File "c:\apps\python\lib\threading.py", line 460, in __bootstrap
>      self.run()
>    File "c:\apps\python\lib\threading.py", line 440, in run
>      self.__target(*self.__args, **self.__kwargs)
>    File "crash.py", line 20, in login1
>      random.choice(System.sessions).session_iter.next()
> ValueError: generator already executing
>
> It appears to indicate that you must syncronize the generators.
> Adding a Lock object to Session appears to work OK:
>
> class Session:
>          def __init__(self):
>                  self.lock = Lock()
>                  self.session_iter = session_iter(self)
>
> def login1():
>          # first interpreter
>          System.sessions.append(Session())
>          while 1:
>                  session = random.choice(System.sessions)
>                  session.lock.acquire()
>                  session.session_iter.next()
>                  session.lock.release()
>
> Your login2 version does not generate any error on my PC.
>
> --
> Gabriel Genellina

As far as I can tell I'm not running it from restricted mode
explicitly.

The reason I'm doing the random.choice() is so I don't have a handle
("session" in this case) that would prevent it from being garbage
collected. I am not bothered by the Python-level Exceptions. I am
bothered by the interpreter throwing a segfault. What I suspect is
happening is the Session object is deallocated, while it still has a
reference from within the iterator.

It's quite difficult to reproduce these bugs consistently. You might
need to run it a few times.