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gh-135552: Don't clear weakref if wr_object is a type. #136151

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gh-135552: Don't clear weakref if wr_object is a type. #136151

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34 changes: 25 additions & 9 deletions Python/gc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -858,6 +858,19 @@ move_legacy_finalizer_reachable(PyGC_Head *finalizers)
}
}

static bool
should_clear_weakref(PyWeakReference *wr)
{
if (wr->wr_callback == NULL && PyType_Check(wr->wr_object)) {
/* This is intended to match the values of the tp_subclasses dict.
* It matches weakrefs to types in other places as well, which is
* an unintended side effect.
*/
return false;
}
return true;
}

/* Clear all weakrefs to unreachable objects, and if such a weakref has a
* callback, invoke it if necessary. Note that it's possible for such
* weakrefs to be outside the unreachable set -- indeed, those are precisely
Expand Down Expand Up @@ -895,7 +908,7 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
op = FROM_GC(gc);
next = GC_NEXT(gc);

if (PyWeakref_Check(op)) {
if (PyWeakref_Check(op) && should_clear_weakref((PyWeakReference *)op)) {
/* A weakref inside the unreachable set must be cleared. If we
* allow its callback to execute inside delete_garbage(), it
* could expose objects that have tp_clear already called on
Expand Down Expand Up @@ -925,16 +938,19 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
* all the weakrefs, and move the weakrefs with callbacks
* that must be called into wrcb_to_call.
*/
for (wr = *wrlist; wr != NULL; wr = *wrlist) {
PyWeakReference *next_wr;
for (wr = *wrlist; wr != NULL; wr = next_wr) {
next_wr = wr->wr_next;
PyGC_Head *wrasgc; /* AS_GC(wr) */

/* _PyWeakref_ClearRef clears the weakref but leaves
* the callback pointer intact. Obscure: it also
* changes *wrlist.
*/
_PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
_PyWeakref_ClearRef(wr);
_PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
if (should_clear_weakref(wr)) {
/* _PyWeakref_ClearRef clears the weakref but leaves
* the callback pointer intact. Obscure: it also
* changes *wrlist.
*/
_PyWeakref_ClearRef(wr);
_PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
}
if (wr->wr_callback == NULL) {
/* no callback */
continue;
Expand Down
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