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gh-135228: When @dataclass(slots=True) replaces a dataclass, make the original class collectible (take 2) #137047

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gh-135228: When @dataclass(slots=True) replaces a dataclass, make the original class collectible (take 2) #137047

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916ce1b
gh-135228: When @dataclass(slots=True) replaces a dataclass, make the…
JelleZijlstra Jul 22, 2025
6dc0f48
Use a new private sys function instead
JelleZijlstra Jul 23, 2025
b9fb155
obsolete comment
JelleZijlstra Jul 23, 2025
52c7d2b
restrict to heap types
JelleZijlstra Jul 23, 2025
250f3e9
more paranoid
JelleZijlstra Jul 25, 2025
a9483b4
Apply suggestions from code review
JelleZijlstra Jul 29, 2025
8c2c1e1
Update Python/sysmodule.c
JelleZijlstra Jul 30, 2025
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more paranoid
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@JelleZijlstra
JelleZijlstra committed Jul 25, 2025
commit 250f3e9935d8188c27dcd91e6cb61293166475c9
13 changes: 11 additions & 2 deletions Python/sysmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2666,13 +2666,22 @@ sys__clear_type_descriptors(PyObject *module, PyObject *type)
return NULL;
}
PyObject *dict = _PyType_GetDict(typeobj);
if (PyDict_PopString(dict, "__dict__", NULL) < 0) {
PyObject *dunder_dict = NULL;
if (PyDict_PopString(dict, "__dict__", &dunder_dict) < 0) {
return NULL;
}
if (PyDict_PopString(dict, "__weakref__", NULL) < 0) {
PyObject *dunder_weakref = NULL;
if (PyDict_PopString(dict, "__weakref__", &dunder_weakref) < 0) {
PyType_Modified(typeobj);
Py_XDECREF(dunder_dict);
return NULL;
}
PyType_Modified(typeobj);
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This seems to be technically not thread-safe, because somebody in another thread may be working with the type between the PopString calls and the PyType_Modified call. However, this is also true for the various functions in typeobject.c that modify the type object.

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Could anything be deallocated as a result of these pops? Could have a reentrancy problem even without threads present.

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I guess to prevent that we can hold onto a reference to the old value (by passing the third arg to PyDict_PopString) and DECREF it only after we call PyType_Modified.

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Yeah, that seems safer.

// We try to hold onto a reference to these until after we call
// PyType_Modified(), in case their deallocation triggers somer user code
// that tries to do something to the type.
Py_XDECREF(dunder_dict);
Py_XDECREF(dunder_weakref);
Py_RETURN_NONE;
}

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