Python 3.12 segfault with mixin classes

[Damon Lynch]

Of interest to those who have not seen it — a discussion of the many
problems around Python's C API, and what folks are doing to ameliorate them:

https://pyfound.blogspot.com/2023/05/the-python-language-summit-2023-three.html

On Fri, 9 Jun 2023 at 09:56, Thomas Caswell <tcaswell at gmail.com> wrote:

> I am also seeing segfaults with the cpython main branch + pyqt6.  I had a
> suspicion it was that change, but had not had time to verify or make a
> minimal example (making a Matplotlib figure segfaulted).
>
> numpy and cython also had issues, see
> https://github.com/python/cpython/issues/104614 , however even with those
> PRs landed I'm still seeing segfaults with PyQt6.
>
> Tom
>
> On Thu, Jun 8, 2023 at 5:22 PM Florian Bruhin <me at the-compiler.org> wrote:
>
>> Hey,
>>
>> When trying PyQt 6.5.1 with Python 3.12 Beta 2, something like:
>>
>>         import sys
>>         from PyQt6.QtWidgets import QWidget, QApplication
>>
>>         class Mixin: pass
>>         class Command(Mixin, QWidget): pass
>>
>>         app = QApplication(sys.argv)
>>         cmd = Command()
>>         cmd.show()
>>         app.exec()
>>
>> segfaults here on the "cmd.Command()" line:
>>
>>         _PyDict_Next (op=0x0, ppos=0x7fffffffbd50, pkey=0x7fffffffbd58,
>> pvalue=0x7fffffffbd60, phash=phash at entry=0x0) at
>> Objects/dictobject.c:2114
>>         2114       if (!PyDict_Check(op))
>>         (gdb) bt
>>         #0  _PyDict_Next (op=0x0, ppos=0x7fffffffbd50,
>> pkey=0x7fffffffbd58,
>> pvalue=0x7fffffffbd60, phash=phash at entry=0x0) at
>> Objects/dictobject.c:2114
>>         #1  0x000055555572a37e in PyDict_Next (op=<optimized out>,
>> ppos=<optimized out>, pkey=<optimized out>, pvalue=<optimized out>) at
>> Objects/dictobject.c:2189
>>         #2  0x00007ffff2436499 in trawl_hierarchy(_typeobject*,
>> qpycore_metaobject*, QMetaObjectBuilder&, QList<EnumFlag>&,
>> QList<_qpycore_pyqtSignal const*>&, QMap<unsigned int, std::pair<_object*,
>> _object*> >&) ()
>>            from .../lib/python3.12/site-packages/PyQt6/QtCore.abi3.so
>>         #3  0x00007ffff2436cc7 in trawl_hierarchy(_typeobject*,
>> qpycore_metaobject*, QMetaObjectBuilder&, QList<EnumFlag>&,
>> QList<_qpycore_pyqtSignal const*>&, QMap<unsigned int, std::pair<_object*,
>> _object*> >&) ()
>>            from .../lib/python3.12/site-packages/PyQt6/QtCore.abi3.so
>>         #4  0x00007ffff2436cc7 in trawl_hierarchy(_typeobject*,
>> qpycore_metaobject*, QMetaObjectBuilder&, QList<EnumFlag>&,
>> QList<_qpycore_pyqtSignal const*>&, QMap<unsigned int, std::pair<_object*,
>> _object*> >&) ()
>>            from .../lib/python3.12/site-packages/PyQt6/QtCore.abi3.so
>>         #5  0x00007ffff24357ba in
>> qpycore_get_dynamic_metaobject(_sipWrapperType*) [clone .part.0] () from
>> .../lib/python3.12/site-packages/PyQt6/QtCore.abi3.so
>>         #6  0x00007ffff24363d8 in
>> qpycore_get_qmetaobject(_sipWrapperType*,
>> _sipTypeDef const*) () from .../lib/python3.12/site-
>> packages/PyQt6/QtCore.abi3.so
>>         #7  0x00007ffff242a4e5 in qpycore_qobject_finalisation(_object*,
>> QObject*, _object*, _object**) () from .../lib/python3.12/site-
>> packages/PyQt6/QtCore.abi3.so
>>         #8  0x00007ffff74efbbc in sipSimpleWrapper_init
>> (self=0x7ffff765ebb0,
>> args=(), kwds=0x0) at sip_core.c:8950
>>         #9  0x0000555555758f0d in type_call (type=<optimized out>,
>> type at entry=0x555555e53e60, args=args at entry=(), kwds=kwds at entry=0x0) at
>> Objects/typeobject.c:1671
>>         [...]
>>
>> (Note the op=0x0).
>>
>> I tried to build a debug PyQt, but it couldn't find Python.h of my
>> (non-systemwide) debug Python, and I couldn't find a way to make
>> sip-install
>> pass a custom -I to gcc.
>>
>> I was able to bisect it in CPython to this commit:
>>
>> https://github.com/python/cpython/commit/de64e7561680fdc5358001e9488091e75d417
>> 4a3
>> <https://github.com/python/cpython/commit/de64e7561680fdc5358001e9488091e75d4174a3>
>> which moves tp_dict/tp_bases/tp_mro to PyInterpreterState.
>>
>> I lack the context to tell more about what's happening here.
>> Is this a CPython bug? If so, is there any chance of reproducing it
>> without
>> PyQt involved?
>>
>> Thanks,
>>
>> Florian
>>
>
>
> --
> Thomas Caswell
> tcaswell at gmail.com
>


-- 
https://damonlynch.net
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