bpo-38070: Enhance visit_decref() debug trace (GH-16631)

subtract_refs() now pass the parent object to visit_decref() which
pass it to _PyObject_ASSERT(). So if the "is freed" assertion fails,
the parent is used in debug trace, rather than the freed object. The
parent object is more likely to contain useful information. Freed
objects cannot be inspected are are displayed as "<object at xxx is
freed>" with no other detail.

1 files changed, 5 insertions, 4 deletions

diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 2cbf573cf4..2b47abae1a 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -373,9 +373,9 @@ update_refs(PyGC_Head *containers)
 
 /* A traversal callback for subtract_refs. */
 static int
-visit_decref(PyObject *op, void *data)
+visit_decref(PyObject *op, void *parent)
 {
-    _PyObject_ASSERT(op, !_PyObject_IsFreed(op));
+    _PyObject_ASSERT(_PyObject_CAST(parent), !_PyObject_IsFreed(op));
 
     if (PyObject_IS_GC(op)) {
         PyGC_Head *gc = AS_GC(op);
@@ -401,10 +401,11 @@ subtract_refs(PyGC_Head *containers)
     traverseproc traverse;
     PyGC_Head *gc = GC_NEXT(containers);
     for (; gc != containers; gc = GC_NEXT(gc)) {
-        traverse = Py_TYPE(FROM_GC(gc))->tp_traverse;
+        PyObject *op = FROM_GC(gc);
+        traverse = Py_TYPE(op)->tp_traverse;
         (void) traverse(FROM_GC(gc),
                        (visitproc)visit_decref,
-                       NULL);
+                       op);
     }
 }