gh-118789: Add `PyUnstable_Object_ClearWeakRefsNoCallbacks` (#118807)

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.

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

1 files changed, 28 insertions, 0 deletions

diff --git a/Lib/test/test_capi/test_object.py b/Lib/test/test_capi/test_object.py
index fa23bff..cc9c9b6 100644
--- a/Lib/test/test_capi/test_object.py
+++ b/Lib/test/test_capi/test_object.py
@@ -103,5 +103,33 @@ class PrintTest(unittest.TestCase):
         with self.assertRaises(OSError):
             _testcapi.pyobject_print_os_error(output_filename)
 
+
+class ClearWeakRefsNoCallbacksTest(unittest.TestCase):
+    """Test PyUnstable_Object_ClearWeakRefsNoCallbacks"""
+    def test_ClearWeakRefsNoCallbacks(self):
+        """Ensure PyUnstable_Object_ClearWeakRefsNoCallbacks works"""
+        import weakref
+        import gc
+        class C:
+            pass
+        obj = C()
+        messages = []
+        ref = weakref.ref(obj, lambda: messages.append("don't add this"))
+        self.assertIs(ref(), obj)
+        self.assertFalse(messages)
+        _testcapi.pyobject_clear_weakrefs_no_callbacks(obj)
+        self.assertIsNone(ref())
+        gc.collect()
+        self.assertFalse(messages)
+
+    def test_ClearWeakRefsNoCallbacks_no_weakref_support(self):
+        """Don't fail on objects that don't support weakrefs"""
+        import weakref
+        obj = object()
+        with self.assertRaises(TypeError):
+            ref = weakref.ref(obj)
+        _testcapi.pyobject_clear_weakrefs_no_callbacks(obj)
+
+
 if __name__ == "__main__":
     unittest.main()