Segfault starting with 2.16.1-0ubuntu2 using Python 2.5

[Severin H]

A few days ago, the development version of the PyGTK application LottaNZB 0.5 (https://code.edge.launchpad.net/~lottanzb/lottanzb/main) stopped working because of a segmentation fault during startup. I was able to track down the problem:

2.16.1-0ubuntu1: Works fine both using Python 2.5 and Python 2.6.
2.16.1-0ubuntu2 when launching the application using Python 2.5: Segmentation fault.
2.16.1-0ubuntu2 when launching the application using Python 2.6: Works fine.

I wasn't able to produce a simple demonstration of the Python 2.5 problem yet, but in our code the situation is as follows:

class GObjectSingletonMeta(GObjectMeta):
    """GObject singleton metaclass"""

    def __init__(mcs, name, bases, _dict):
        # THIS IS WHERE THE APPLICATION SEGFAULTS.
        GObjectMeta.__init__(mcs, name, bases, _dict)
        mcs.__instance = None

    def __call__(mcs, *args, **kwargs):
        """Creates an instance of mcs if it doesn't exist yet"""

        if mcs.__instance is None:
            mcs.__instance = GObjectMeta.__call__(mcs, *args, **kwargs)

        return mcs.__instance

class App(GObject, kiwi.environ.Application):
    __metaclass__ = GObjectSingletonMeta

    lock = property(type=object)

    [...]

It's possible to prevent the application from segfaulting in the singleton metaclass by *either* removing the Application parent class from the App class or by removing all GObject properties from the App class.

I'm using an up-to-date Jaunty amd64 machine. I only exchanged the python-gobject package for the tests.

I'll try to provide further information about the problem and I wonder if someone else can reproduce this. You can try to reproduce the problem by branching the project and running ./run-lottanzb.sh (which will run the application using Python 2.5 by default).

[Severin H (severinh)]

Changelog of python-gobject_2.16.1-0ubuntu2: https://edge.launchpad.net/ubuntu/jaunty/+source/pygobject/2.16.1-0ubuntu2

  * Remove explicit dependencies on python2.5.
  * Rename -dbg extensions for python2.6 as well. LP: #333557.

[Severin H (severinh)]

I seem to have found a solution. Right now, the GObjectSingletonMeta metaclass is part of the util module. Moving it to the core module where it is actually used by the App class solved the problem. This is totally weird.

I was able to create a simple script that demonstrates the problem. demo.py and demo_util.py. Install the python-kiwi package (!). Run demo.py using Python 2.5 and the latest python-gobject package (2.16.1-0ubuntu2). Make sure that the demo_util.py file is in the same directory.

[Severin H (severinh)]

Forgot to remove unnecessary stuff from the demo.py file. Download this one and not the one above.

[Severin H (severinh)]

...and the demo_util.py file.

[Matthias Klose (doko)]

2.16.1-0ubuntu1 and 2.16.1-0ubuntu2 don't have any code changes; are you sure that you didn't use 2.15.x before?

[Severin H (severinh)]

The most interesting thing about it is that the order of the three import statements in demo.py has an effect on what happens:

# Finally leads to the segfault described in this bug report.
from kiwi.environ import Application
from gobject import property, GObject
from demo_util import GObjectSingletonMeta

# Everything works fine
from gobject import property, GObject
from kiwi.environ import Application
from demo_util import GObjectSingletonMeta

# Raises
# Exception exceptions.TypeError: 'could not get typecode from object' in <module 'threading' from '/usr/lib/python2.5/threading.pyc'> ignored
from gobject import property, GObject
from demo_util import GObjectSingletonMeta
from kiwi.environ import Application

Can anyone confirm this behaviour?

[Severin H (severinh)]

@Matthias:

I've downloaded the required files from https://edge.launchpad.net/ubuntu/jaunty/+source/pygobject/2.16.1-0ubuntu1, built it using dpkg-source -x and installed the package using dpkg --install --force-all (which replaced the ubuntu2 version). Everything was fine after this. Upgrading back to ubuntu2 using apt-get upgrade reintroduced the bug.

[Severin H (severinh)]

lantash@gauss:~/Desktop$ python demo.py # Using 2.16.1-0ubuntu2.
Segmentation fault (core dumped)

lantash@gauss:~/Desktop$ sudo dpkg --install --force-all python-gobject_2.16.1-0ubuntu1_amd64.deb
[...]
Vorbereiten zum Ersetzen von python-gobject 2.16.1-0ubuntu2 (durch python-gobject_2.16.1-0ubuntu1_amd64.deb) ...
[...]

lantash@gauss:~/Desktop$ python demo.py

lantash@gauss:~/Desktop$ sudo apt-get upgrade
[...]
Vorbereiten zum Ersetzen von python-gobject 2.16.1-0ubuntu1 (durch .../python-gobject_2.16.1-0ubuntu2_amd64.deb) ...
[...]

lantash@gauss:~/Desktop$ python demo.py
Segmentation fault (core dumped)

[Severin H (severinh)]

Sorry for bothering you once more. I'm not quite sure what to think about it. In addition to the ubuntu1, I built the ubuntu2 pacakge by myself a few minutes ago, and now it looks as if the application segfaults if either the self-built ubuntu1 package (why did this work before?!) or the official ubuntu2 package is used. Running the application using my own ubuntu2 build works fine.

I just commited a "fix" described above for this problem to the LottaNZB development branch, so that at least this application can be started again on Jaunty.

If noone can reproduce this bug using the demo files one might consider marking this bug as invalid or incomplete for the moment.

[Severin H (severinh)]

According to a message by Matthias Klose in ubuntu-devel, the Python interpreter will be updated from 2.5.4 to 2.6 within the next days and numerous packages will be rebuilt for that purpose. Since the bug only affects Python 2.5 I assume that everything will be fine after the transition. I'll problably make a clean install a couple of days after Alpha 5.

[Sebastien Bacher (seb128)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. Please try to obtain a backtrace following the instructions at http://wiki.ubuntu.com/DebuggingProgramCrash and upload the backtrace (as an attachment) to the bug report. This will greatly help us in tracking down your problem.

[Severin H (severinh)]

This bug has somehow been fixed. As I mentioned above, LottaNZB works just fine using Python 2.6.