Python 3.11 and QObject.destroyed signals: _PyThreadState_PopFrame: Assertion `tstate->datastack_top >= base' failed

[Florian Bruhin]

On Fri, Jul 08, 2022 at 05:30:27PM +0200, Florian Bruhin wrote:
> when testing my application with the current Python 3.11 beta, I've
> noticed a bug which seems to be triggered somehow by something PyQt
> does: https://github.com/python/cpython/issues/93252
> 
> This is the most trivial reproducer I could find:
> 
>     from PyQt5.QtCore import QObject
> 
>     def run():
>         obj = QObject()
>         for _ in range(202):
>             obj.destroyed.connect(lambda: None)
> 
>     run()
> 
> Which crashes CPython with:
> 
>     python: Python/pystate.c:2201: _PyThreadState_PopFrame: Assertion `tstate->datastack_top >= base' failed.

FWIW, we ended up finding a much simpler reproducer, not involving PyQt:

    def f():
        pass

    for _ in range(203):
        try:
            f(None)
        except:
            pass

Thus this happened when a stack frame can't be built up - Python never
cleaned it properly in that case. It's been fixed in 3.11.0b4:
https://github.com/python/cpython/pull/94708/commits/be41a146c6595200107cb3ece97809cb506dffba

I don't quite understand why/how the connection to QObject.destroyed
exhibits the same behavior, but in any case, the patch fixes it.

Florian

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