Account for marked objects in candidates.

Author: brandtbucher

Python/gc.c

@@ -1667,6 +1667,7 @@ gc_collect_increment(PyThreadState *tstate, struct gc_collection_stats *stats)
         Py_ssize_t objects_marked = mark_at_start(tstate);
         GC_STAT_ADD(1, objects_transitively_reachable, objects_marked);
         gcstate->work_to_do -= objects_marked;
+        stats->candidates += objects_marked;
         validate_spaces(gcstate);
         return;
     }

Python/gc_free_threading.c

@@ -982,18 +982,18 @@ update_refs(const mi_heap_t *heap, const mi_heap_area_t *area,
         return true;
     }
 
-    if (gc_is_alive(op)) {
-        return true;
-    }
-
     // Exclude immortal objects from garbage collection
     if (_Py_IsImmortal(op)) {
         op->ob_tid = 0;
         _PyObject_GC_UNTRACK(op);
         gc_clear_unreachable(op);
         return true;
     }
+    // Marked objects count as candidates, immortals don't:
     state->candidates++;
+    if (gc_is_alive(op)) {
+        return true;
+    }
 
     Py_ssize_t refcount = Py_REFCNT(op);
     if (_PyObject_HasDeferredRefcount(op)) {