[3.13] gh-118789: Add `PyUnstable_Object_ClearWeakRefsNoCallbacks` (GH-118807) (#120695)

This exposes `PyUnstable_Object_ClearWeakRefsNoCallbacks` as an unstable
C-API function to provide a thread-safe mechanism for clearing weakrefs
without executing callbacks.

Some C-API extensions need to clear weakrefs without calling callbacks,
such as after running finalizers like we do in subtype_dealloc.
Previously they could use `_PyWeakref_ClearRef` on each weakref, but
that's not thread-safe in the free-threaded build.

(cherry picked from commit e8752d7b80775ec2a348cd4bf38cbe26a4a07615)

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>

2 files changed, 11 insertions, 3 deletions

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 5c490e8..5bf2bc2 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2369,7 +2369,7 @@ subtype_dealloc(PyObject *self)
            finalizers since they might rely on part of the object
            being finalized that has already been destroyed. */
         if (type->tp_weaklistoffset && !base->tp_weaklistoffset) {
-            _PyWeakref_ClearWeakRefsExceptCallbacks(self);
+            _PyWeakref_ClearWeakRefsNoCallbacks(self);
         }
     }
 
diff --git a/Objects/weakrefobject.c b/Objects/weakrefobject.c
index 3b027e1..0fcd37d 100644
--- a/Objects/weakrefobject.c
+++ b/Objects/weakrefobject.c
@@ -1016,7 +1016,7 @@ PyObject_ClearWeakRefs(PyObject *object)
     PyObject *exc = PyErr_GetRaisedException();
     PyObject *tuple = PyTuple_New(num_weakrefs * 2);
     if (tuple == NULL) {
-        _PyWeakref_ClearWeakRefsExceptCallbacks(object);
+        _PyWeakref_ClearWeakRefsNoCallbacks(object);
         PyErr_WriteUnraisable(NULL);
         PyErr_SetRaisedException(exc);
         return;
@@ -1057,6 +1057,14 @@ PyObject_ClearWeakRefs(PyObject *object)
     PyErr_SetRaisedException(exc);
 }
 
+void
+PyUnstable_Object_ClearWeakRefsNoCallbacks(PyObject *obj)
+{
+    if (_PyType_SUPPORTS_WEAKREFS(Py_TYPE(obj))) {
+        _PyWeakref_ClearWeakRefsNoCallbacks(obj);
+    }
+}
+
 /* This function is called by _PyStaticType_Dealloc() to clear weak references.
  *
  * This is called at the end of runtime finalization, so we can just
@@ -1076,7 +1084,7 @@ _PyStaticType_ClearWeakRefs(PyInterpreterState *interp, PyTypeObject *type)
 }
 
 void
-_PyWeakref_ClearWeakRefsExceptCallbacks(PyObject *obj)
+_PyWeakref_ClearWeakRefsNoCallbacks(PyObject *obj)
 {
     /* Modeled after GET_WEAKREFS_LISTPTR().