[pygtk] Problem with the GC starting in the middle of _wrap_gtk_window_list_toplevels

[Tim Evans]

Margarita Manterola wrote:
> Hi!
> 
> I've already submitted a bug, with the whole information about this,
> but I'm writting also to the list to see if someone has any idea of
> how to fix it.
> 
> This is a problem with the C wrapper of gtk_window_list_toplevels for PyGTK.
> 
> This is the reported bug:
> http://bugzilla.gnome.org/show_bug.cgi?id=574259 (it has stacktraces
> and the like)
> 
> The problem is that in this code for _wrap_gtk_window_list_toplevels:
> 
> static PyObject *
> _wrap_gtk_window_list_toplevels(PyGObject *self)
> {
>    GList *list, *tmp;
>    PyObject *py_list;
>    PyObject *gtk_obj;
> 
>    list = gtk_window_list_toplevels();
> 
>    if ((py_list = PyList_New(0)) == NULL) {
>        g_list_free(list);
>        return NULL;
>    }
>    for (tmp = list; tmp != NULL; tmp = tmp->next) {
>        gtk_obj = pygobject_new(G_OBJECT(tmp->data));
>        if (gtk_obj == NULL) {
>            g_list_free(list);
>            Py_DECREF(py_list);
>            return NULL;
>        }
>        PyList_Append(py_list, gtk_obj);
>        Py_DECREF(gtk_obj);
>    }
>    g_list_free(list);
>    return py_list;
> }
> 
> The garbage collector is called while the code is in the for loop, it
> removes one of the toplevel windows which is not in use anywhere,
> except that it's already on the list, thus after the GC returns and
> the loop tries to access that removed window, it triggers a segfault.
> 
> I can think of one ugly solution, but no correct solution.  The ugly
> solution would be to add extra checks everywhere, so that if a window
> was destroyed in the middle of the function, everything would still
> work.  The correct solution would be to somehow prevent the GC from
> destroying the unused window which is still in the list, but I have no
> idea how.
> 
> Any ideas on how to fix it?

For a rather brute-force hack, you can do this:

   import gtk, gobject
   import gc

   gc.disable()
   def collect():
       with gtk.gdk.lock:
           gc.collect()
           return True
   gobject.timeout_add_seconds(10, collect)

Not a long-term solution, but it gets your software working.

-- 
Tim Evans
Applied Research Associates NZ
http://www.aranz.com/