fix: call 'PyObject_ClearWeakRefs' unconditionally

CHANGES.rst

@@ -5,6 +5,8 @@
 7.1.1 (unreleased)
 ==================
 
+- Fix segmentation faults in `weakrefobject.c` on Python 3.12 and 3.13.
+  (`#323 <https://github.com/zopefoundation/zope.interface/issues/323>`_)
 
 7.1.0 (2024-10-10)
 ==================

src/zope/interface/_zope_interface_coptimizations.c

@@ -49,6 +49,11 @@
 #define USE_HEAP_TYPES 1
 #endif
 
+#define BASETYPE_FLAGS \
+    Py_TPFLAGS_DEFAULT | \
+    Py_TPFLAGS_BASETYPE | \
+    Py_TPFLAGS_HAVE_GC
+
 #if PY_VERSION_HEX >= 0x030c0000
 /* Add MANAGED_WEAKREF flag for Python >= 3.12, and don't define
  * the '.tp_weaklistoffset' slot.
@@ -57,11 +62,7 @@
  *      #c.PyTypeObject.tp_weaklistoffset
  */
 #define USE_EXPLICIT_WEAKREFLIST 0
-#define BASETYPE_FLAGS \
-    Py_TPFLAGS_DEFAULT | \
-    Py_TPFLAGS_BASETYPE | \
-    Py_TPFLAGS_MANAGED_WEAKREF | \
-    Py_TPFLAGS_HAVE_GC
+#define WEAKREFTYPE_FLAGS BASETYPE_FLAGS | Py_TPFLAGS_MANAGED_WEAKREF
 #else
 /* No MANAGED_WEAKREF flag for Python < 3.12, and therefore define
  * the '.tp_weaklistoffset' slot, and the member whose offset it holds.
@@ -70,10 +71,7 @@
  *      #c.PyTypeObject.tp_weaklistoffset
  */
 #define USE_EXPLICIT_WEAKREFLIST 1
-#define BASETYPE_FLAGS \
-    Py_TPFLAGS_DEFAULT | \
-    Py_TPFLAGS_BASETYPE | \
-    Py_TPFLAGS_HAVE_GC
+#define WEAKREFTYPE_FLAGS BASETYPE_FLAGS
 #endif
 
 /* Static strings, used to invoke PyObject_GetAttr (only in hot paths) */
@@ -276,12 +274,8 @@ static void
 SB_dealloc(SB* self)
 {
     PyObject_GC_UnTrack((PyObject*)self);
+    PyObject_ClearWeakRefs(OBJECT(self));
     PyTypeObject* tp = Py_TYPE(self);
-#if USE_EXPLICIT_WEAKREFLIST
-    if (self->weakreflist != NULL) {
-        PyObject_ClearWeakRefs(OBJECT(self));
-    }
-#endif
     SB_clear(self);
     tp->tp_free(OBJECT(self));
 #if USE_HEAP_TYPES
@@ -419,7 +413,7 @@ static PyTypeObject SB_type_def = {
     .tp_name           = SB__name__,
     .tp_doc            = SB__doc__,
     .tp_basicsize      = sizeof(SB),
-    .tp_flags          = BASETYPE_FLAGS,
+    .tp_flags          = WEAKREFTYPE_FLAGS,
     .tp_call           = (ternaryfunc)SB__call__,
     .tp_traverse       = (traverseproc)SB_traverse,
     .tp_clear          = (inquiry)SB_clear,
@@ -450,7 +444,7 @@ static PyType_Slot SB_type_slots[] = {
 static PyType_Spec SB_type_spec = {
     .name               = SB__name__,
     .basicsize          = sizeof(SB),
-    .flags              = BASETYPE_FLAGS,
+    .flags              = WEAKREFTYPE_FLAGS,
     .slots              = SB_type_slots
 };
 
@@ -569,8 +563,7 @@ CPB_clear(CPB* self)
 {
     Py_CLEAR(self->_cls);
     Py_CLEAR(self->_implements);
-    SB_clear((SB*)self);
-    return 0;
+    return SB_clear((SB*)self);
 }
 
 static void