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gh-135909: Assert incoming refcnt != 0 for the free threaded GC (GH-136009)

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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.
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andrewreds 2025-07-16 01:26:16 +10:00 • committed by GitHub
parent be02e68158
commit 2500eb96b2
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1 changed files with 15 additions and 7 deletions
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22
Python/gc_free_threading.c
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@ -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;