Crash-on-exit after call to app.processEvents()

[Thomas Caswell]

This is reminding me of the issues Armando raised with Matplotlib (
https://github.com/matplotlib/matplotlib/issues/14178 /
https://github.com/matplotlib/matplotlib/pull/14179).  I now understand why
Matplotlib (and IPython) holding references to the qApp can be problematic
(as in this case del ing app just drops your reference to it but does not
actually trigger the `__delete__` code because other parts of the codebase
are keeping it alive) if having a "live" app on process shut down can cause
problems.

My naive guess as to why calling `ProcessEvents` triggers the issue as that
it creates some extra state on at least the c++ side and we end up tearing
down the objects on the Python side in a different order than we need to on
the c++ side.  Oventually something winds up with a null pointer (because
there was a reference at the c++ level but not at the Python level) and
boom.  I stress that while that is where I would start looking to track
this down it is still very much a guess.

Tom

On Mon, May 25, 2020 at 5:36 PM Raymond Osborn <rayosborn at mac.com> wrote:

> Hi Armando,
> Good to hear from you. I thought for a while that I had managed to fix it
> with the following code :
>
>     def main(filename=None):
>         app = NXConsoleApp()
>         app.initialize(filename=filename)
>         app.start()
>         for w in QApplication.topLevelWindows():
>             del w
>         del app.window, app.app
>         sys.exit(0)
>
> It proved to be a false dawn - the problem started to recur again. I’ll
> see if your version catches things that mine missed, and let you know.
>
> I have spent a lot of time trying to cover up PyQt5 bugs in the past year.
> The problem is that I can’t guarantee that my users will have the latest
> version of PyQt, so even if it is fixed in v5.14, I still have to patch the
> code. I just have no idea why simply calling processEvents() triggers the
> bug. If I knew that, perhaps it would give some clue how to work around it.
>
> Ray
>
>
> On May 25, 2020, at 3:38 PM, V. Armando Sole <sole at esrf.fr> wrote:
>
> Hi Ray,
>
> I do not know if it will help your case. I was getting rid of some crashes
> on exit by deleting all widgets that had no parent. For instance, when
> closing the QMainWindow by doing:
>
>     def closeEvent(self, event):
>         if __name__ == "__main__":
>             app = qt.QApplication.instance()
>             allWidgets = app.allWidgets()
>             for widget in allWidgets:
>                 try:
>                     # we cannot afford to crash here
>                     if id(widget) != id(self):
>                         if widget.parent() is None:
>                             widget.close()
>                 except:
>                     _logger.debug("Error closing widget")
>         return qt.QMainWindow.closeEvent(self, event)
>
> Best regards,
>
> Armando
>
> On 25.05.2020 19:48, Raymond Osborn wrote:
>
> I added a call to QtWidgets.QApplication.instance().processEvents() in
> order to force adjustResize() to work when switching tabs (that's another
> story). It succeeded in fixing my resize issue, but now it triggers a
> segfault when I exit the application. This seemed to be related to the bug
> described in
> https://www.riverbankcomputing.com/static/Docs/PyQt5/gotchas.html#crashes-on-exit.
> One fix, suggested by @ekhumoro on stackoverflow (
> https://stackoverflow.com/questions/59120337/why-does-pyqt-sometimes-crash-on-exit)
> was to delete the main window and the app first, but that doesn't seem to
> fix it. I am running PyQt 5.12 because that is the last version supported
> by conda, so I can't test the one-exit fix in v5.13, and my users probably
> wouldn't have it installed anyway. It happens on Macs and linux.
>
> What I am asking is if there are additional things to try in addition to
> @ekhumoro's suggestion? Basically he suggests doing something like:
>
>
> def main():
>     app = QApplication(sys.argv)
>     main_window = MainWindow()
>     main_window.show()
>     app.exec_()
>     # ensure correct deletion order
>     del main_window, app
>
> Does anyone understand why the crash only occurs if I have calls to
> processEvents() in the code? It never happens when I remove them.
>
> Thanks,
> Ray
>
>
>
>
>

-- 
Thomas Caswell
tcaswell at gmail.com
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