Python 3.12 segfault with mixin classes

[Florian Bruhin]

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
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
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