Allow objects to decide if they can be collected by GC

Modules/gcmodule.c

 /*
 
   Reference Cycle Garbage Collection
   ==================================
 
   Neil Schemenauer <nas@arctrix.com>
 
   Based on a post on the python-dev list.  Ideas from Guido van Rossum,
   Eric Tiedemann, and various others.
 
@@ skipping 445 matching lines @@
         gc = next;
     }
 }
 
 /* Return true if object has a finalization method, or signals
  * itself as not being collectable at this time
  */
 static int
 has_finalizer(PyObject *op)
 {
-    if (Py_TYPE(op)->tp_is_gc != NULL && \
-        (Py_TYPE(op)->tp_is_gc(op) & PyGC_UNCOLLECTABLE))
-    {
-        return 1;
+    if (Py_TYPE(op)->tp_is_gc != NULL) {
+        /* the flags returned by tp_is_gc override the simple
+         * method of detecting a finalizer, based on the
+         * current state of the instance
+         */
+        int flag = Py_TYPE(op)->tp_is_gc(op);
+        if (flag & PyGC_CANNOT_COLLECT)
+            return 1;  /* even if no tp_del */
+        if (flag & PyGC_CAN_COLLECT)
+            return 0;  /* even if there is a tp_del */
     }
     return op->ob_type->tp_del != NULL;
 }
 
 /* Move the objects in unreachable with __del__ methods into `finalizers`.
  * Objects moved into `finalizers` have gc_refs set to GC_REACHABLE; the
  * objects remaining in unreachable are left at GC_TENTATIVELY_UNREACHABLE.
  */
 static void
 move_finalizers(PyGC_Head *unreachable, PyGC_Head *finalizers)
@@ skipping 1033 matching lines @@
 PyObject_GC_Del(void *op)
 {
     PyGC_Head *g = AS_GC(op);
     if (IS_TRACKED(op))
         gc_list_remove(g);
     if (generations[0].count > 0) {
         generations[0].count--;
     }
     PyObject_FREE(g);
 }