https://github.com/python/cpython/commit/2500eb96b260b05387d4ab1063fcfafebf37f1a4
commit: 2500eb96b260b05387d4ab1063fcfafebf37f1a4
branch: main
author: andrewreds <[email protected]>
committer: colesbury <[email protected]>
date: 2025-07-15T11:26:16-04:00
summary:
gh-135909: Assert incoming `refcnt != 0` for the free threaded GC (GH-136009)
This helps catch double deallocation bugs and is similar to the
assertion in the GIL-enabled build. The call to `validate_refcounts`
is moved up to start of the GC because `queue_untracked_obj_decref()`
creates it own zero reference count garbage.
files:
M Python/gc_free_threading.c
diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index d46598b23b3b2f..0b0ddf227e4952 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -1073,6 +1073,14 @@ validate_refcounts(const mi_heap_t *heap, const
mi_heap_area_t *area,
return true;
}
+ // This assert mirrors the one in Python/gc.c:update_refs(). There must be
+ // no tracked objects with a reference count of 0 when the cyclic
+ // collector starts. If there is, then the collector will double dealloc
+ // the object. The likely cause for hitting this is a faulty .tp_dealloc.
+ // Also see the comment in `update_refs()`.
+ _PyObject_ASSERT_WITH_MSG(op, Py_REFCNT(op) > 0,
+ "tracked objects must have a reference count >
0");
+
_PyObject_ASSERT_WITH_MSG(op, !gc_is_unreachable(op),
"object should not be marked as unreachable
yet");
@@ -1422,13 +1430,6 @@ static int
deduce_unreachable_heap(PyInterpreterState *interp,
struct collection_state *state)
{
-
-#ifdef GC_DEBUG
- // Check that all objects are marked as unreachable and that the computed
- // reference count difference (stored in `ob_tid`) is non-negative.
- gc_visit_heaps(interp, &validate_refcounts, &state->base);
-#endif
-
// Identify objects that are directly reachable from outside the GC heap
// by computing the difference between the refcount and the number of
// incoming references.
@@ -2158,6 +2159,13 @@ gc_collect_internal(PyInterpreterState *interp, struct
collection_state *state,
state->gcstate->old[i-1].count = 0;
}
+#ifdef GC_DEBUG
+ // Before we start, check that the heap is in a good condition. There must
+ // be no objects with a zero reference count. And `ob_tid` must only have a
+ // thread if the refcount is unmerged.
+ gc_visit_heaps(interp, &validate_refcounts, &state->base);
+#endif
+
_Py_FOR_EACH_TSTATE_BEGIN(interp, p) {
_PyThreadStateImpl *tstate = (_PyThreadStateImpl *)p;
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