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.
diff --git a/Include/weakrefobject.h b/Include/weakrefobject.h
index 7258387..537e7eb 100644
--- a/Include/weakrefobject.h
+++ b/Include/weakrefobject.h
@@ -70,7 +70,17 @@
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
diff --git a/Lib/test/test_weakref.py b/Lib/test/test_weakref.py
index 92a4713..571e33f 100644
--- a/Lib/test/test_weakref.py
+++ b/Lib/test/test_weakref.py
@@ -776,6 +776,27 @@
self.assertEqual(hash(a), hash(42))
self.assertRaises(TypeError, hash, b)
+ def test_trashcan_16602(self):
+ # Issue #16602: when a weakref's target was part of a long
+ # deallocation chain, the trashcan mechanism could delay clearing
+ # of the weakref and make the target object visible from outside
+ # code even though its refcount had dropped to 0. A crash ensued.
+ class C:
+ def __init__(self, parent):
+ if not parent:
+ return
+ wself = weakref.ref(self)
+ def cb(wparent):
+ o = wself()
+ self.wparent = weakref.ref(parent, cb)
+
+ d = weakref.WeakKeyDictionary()
+ root = c = C(None)
+ for n in range(100):
+ d[c] = c = C(c)
+ del root
+ gc.collect()
+
class SubclassableWeakrefTestCase(TestBase):
diff --git a/Misc/ACKS b/Misc/ACKS
index 9e662f6..4500d1a 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1067,6 +1067,7 @@
Frank J. Tobin
R Lindsay Todd
Bennett Todd
+Eugene Toder
Erik Tollerud
Matias Torchinsky
Sandro Tosi
diff --git a/Misc/NEWS b/Misc/NEWS
index 3c9f382..5152c03 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,10 @@
Core and Builtins
-----------------
+- 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.
+
- Issue #16416: On Mac OS X, operating system data are now always
encoded/decoded to/from UTF-8/surrogateescape, instead of the locale encoding
(which may be ASCII if no locale environment variable is set), to avoid
diff --git a/Objects/weakrefobject.c b/Objects/weakrefobject.c
index dae3c24..8d571e6 100644
--- a/Objects/weakrefobject.c
+++ b/Objects/weakrefobject.c
@@ -52,9 +52,8 @@
{
PyObject *callback = self->wr_callback;
- if (PyWeakref_GET_OBJECT(self) != Py_None) {
- PyWeakReference **list = GET_WEAKREFS_LISTPTR(
- PyWeakref_GET_OBJECT(self));
+ if (self->wr_object != Py_None) {
+ PyWeakReference **list = GET_WEAKREFS_LISTPTR(self->wr_object);
if (*list == self)
/* If 'self' is the end of the list (and thus self->wr_next == NULL)