[PyQt] New crasher

Giovanni Bajo rasky at develer.com
Mon Nov 2 10:32:28 GMT 2009

On Tue, 2009-10-20 at 09:41 +0100, Phil Thompson wrote:
> On Fri, 16 Oct 2009 19:42:20 +0200, Giovanni Bajo <rasky at develer.com>
> wrote:
> > On 10/16/2009 3:38 PM, Phil Thompson wrote:
> >> On Fri, 16 Oct 2009 15:31:08 +0200, Giovanni Bajo <rasky at develer.com>
> >> wrote:
> >>> On 10/16/2009 10:10 AM, Phil Thompson wrote:
> >>>> All outstanding bug reports against PyQt and SIP have been resolved -
> >>>> if
> >>>> you disagree then let me know.
> >>>>
> >>>> I plan to release PyQt v4.6.1 and SIP v4.9.1 early next week.
> >>>>
> >>>> BTW, the current PyQt snapshot will build against Qt v4.6-beta1.
> >>> Hi Phil,
> >>>
> >>> I got some GC-related random crashes in a testsuite run after you fixed
> >>> the QApplication destruction. I investigated them enough to see that 
> >>> it's related to invokation of focusPolicy() or other focus-related 
> >>> virtual methods on an already-deleted object, but they are hard to
> track
> >>>
> >>> down and I am not sure whenever I will have a reproduction recipe
> ready.
> >>>
> >>> Is there anything specific to focus stuff that rings a bell to you?
> >> 
> >> No. The obvious source of the problem is the QApplication being deleted
> >> too
> >> soon in relation to some other object.
> > 
> > I delete all top-level widgets, flush all the events 
> > (qApp.processEvents()), and run an explicit gc.collect() pass. I 
> > actually don't know what else to do...
> > 
> > I start to believe that it's a Qt problem rather than a PyQt's one; eg: 
> > some globals somewhere in the focus code that is not reset by 
> > QApplication's destructor and still points to some destroyed object.
> > 
> > I'll have a further look.
> Any progress? I'd prefer to revert to the existing broken behaviour (which
> affects almost nobody) than introduce the potential for random
> crash-on-exit "bugs".


after having updated to the latest SIP/PyQt versions and having worked
on this crash, I found out that it is actually unrelated to
QApplication's destruction. I managed to find a reproduction recipe:

#!/usr/bin/env python
import sip
from PyQt4.Qt import *
app = QApplication([])

class MyWidget(QWidget):
    def __init__(self, parent):
        QWidget.__init__(self, parent)
        self.cb = QComboBox(self)
        self.ed = QLineEdit(self)
        QObject.connect(self.ed, SIGNAL("XXX"), self.update)
        self.setStyleSheet("UNKNOWN {}")

for i in range(100):
    print i

    dw = QWidget(None)
    vl = QVBoxLayout(dw)
    for e in range(10):
        tp = MyWidget(None)

    for w in dw.findChildren(QWidget):


I think this wins the prize as weirdest bug ever. It randomically
segfaults on me after a few loops. Remove any line, including the
setStyleSheet() and it does not crash anymore. The backtrace starts

0xb70d353a in QWidget::focusPolicy () from _qt.so

I'm on Linux right now, in case it matters, but I first saw this bug on
Windows on the real application, so I'm positive that this crasher
crashes on Windows too.

The latest funny bit I want to share is that it does not segfault nor
gives any error under valgrind :)
Giovanni Bajo
Develer S.r.l.

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