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.

diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 2cbf573cf46ecaf9780f2a42dcf8a6cdbe25d670..2b47abae1ae4d0bdeb55b286495ca7d8eb1f44a5 100644 (file)
--- 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);
     }
 }