Issue #16602: When a weakref's target was part of a long deallocation chain, the object could remain reachable through its weakref even though its refcount had dropped to zero.

Thanks to Eugene Toder for diagnosing and reporting the issue.

1 files changed, 11 insertions, 1 deletions

diff --git a/Include/weakrefobject.h b/Include/weakrefobject.h
index 725838726d..537e7ebf28 100644
--- a/Include/weakrefobject.h
+++ b/Include/weakrefobject.h
@@ -70,7 +70,17 @@ PyAPI_FUNC(Py_ssize_t) _PyWeakref_GetWeakrefCount(PyWeakReference *head);
 PyAPI_FUNC(void) _PyWeakref_ClearRef(PyWeakReference *self);
 #endif
 
-#define PyWeakref_GET_OBJECT(ref) (((PyWeakReference *)(ref))->wr_object)
+/* Explanation for the Py_REFCNT() check: when a weakref's target is part
+   of a long chain of deallocations which triggers the trashcan mechanism,
+   clearing the weakrefs can be delayed long after the target's refcount
+   has dropped to zero.  In the meantime, code accessing the weakref will
+   be able to "see" the target object even though it is supposed to be
+   unreachable.  See issue #16602. */
+
+#define PyWeakref_GET_OBJECT(ref)                           \
+    (Py_REFCNT(((PyWeakReference *)(ref))->wr_object) > 0   \
+     ? ((PyWeakReference *)(ref))->wr_object                \
+     : Py_None)
 
 
 #ifdef __cplusplus