bpo-38400 Don't check for NULL linked list pointers in _PyObject_IsFreed (GH-16630)

Some objects like Py_None are not initialized with conventional means
that prepare the circular linked list pointers, leaving them unlinked
from the rest of the objects. For those objects, NULL pointers does
not mean that they are freed, so we need to skip the check in those
cases.
(cherry picked from commit 36e33c360ed7716a2b5ab2b53210da81f8ce1295)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

diff --git a/Objects/object.c b/Objects/object.c
index 566593a9c67cc35d2b824183aeb554a99208fae8..74b1b15d30bbf57254907d064807e79d2ed103f8 100644 (file)
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -454,9 +454,12 @@ _PyObject_IsFreed(PyObject *op)
     /* ignore op->ob_ref: its value can have be modified
        by Py_INCREF() and Py_DECREF(). */
 #ifdef Py_TRACE_REFS
-    if (_PyMem_IsPtrFreed(op->_ob_next) || _PyMem_IsPtrFreed(op->_ob_prev)) {
+    if (op->_ob_next != NULL && _PyMem_IsPtrFreed(op->_ob_next)) {
         return 1;
     }
+    if (op->_ob_prev != NULL && _PyMem_IsPtrFreed(op->_ob_prev)) {
+         return 1;
+     }
 #endif
     return 0;
 }