gh-144054: no deferred refcount for untracked (gh-144081)

This reverts gh-144055 and fixes the bug in a different way.  Deferred
reference counting relies on the object being tracked by the GC,
otherwise the object will live until interpreter shutdown.  So, take
care that we do not enable deferred reference counting for objects that
are untracked.  Also, if a tuple has deferred reference counting
enabled, don't untrack it.

Author: nascheme

Python/gc_free_threading.c

@@ -308,18 +308,17 @@ disable_deferred_refcounting(PyObject *op)
         // should also be disabled when we turn off deferred refcounting.
         _PyObject_DisablePerThreadRefcounting(op);
     }
-    if (_PyObject_GC_IS_TRACKED(op)) {
-        // Generators and frame objects may contain deferred references to other
-        // objects. If the pointed-to objects are part of cyclic trash, we may
-        // have disabled deferred refcounting on them and need to ensure that we
-        // use strong references, in case the generator or frame object is
-        // resurrected by a finalizer.
-        if (PyGen_CheckExact(op) || PyCoro_CheckExact(op) || PyAsyncGen_CheckExact(op)) {
-            frame_disable_deferred_refcounting(&((PyGenObject *)op)->gi_iframe);
-        }
-        else if (PyFrame_Check(op)) {
-            frame_disable_deferred_refcounting(((PyFrameObject *)op)->f_frame);
-        }
+
+    // Generators and frame objects may contain deferred references to other
+    // objects. If the pointed-to objects are part of cyclic trash, we may
+    // have disabled deferred refcounting on them and need to ensure that we
+    // use strong references, in case the generator or frame object is
+    // resurrected by a finalizer.
+    if (PyGen_CheckExact(op) || PyCoro_CheckExact(op) || PyAsyncGen_CheckExact(op)) {
+        frame_disable_deferred_refcounting(&((PyGenObject *)op)->gi_iframe);
+    }
+    else if (PyFrame_Check(op)) {
+        frame_disable_deferred_refcounting(((PyFrameObject *)op)->f_frame);
     }
 }
 
@@ -507,6 +506,10 @@ gc_visit_thread_stacks(PyInterpreterState *interp, struct collection_state *stat
 static bool
 gc_maybe_untrack(PyObject *op)
 {
+    if (_PyObject_HasDeferredRefcount(op)) {
+        // deferred refcounting only works if the object is tracked
+        return false;
+    }
     // Currently we only check for tuples containing only non-GC objects.  In
     // theory we could check other immutable objects that contain references
     // to non-GC objects.
@@ -1019,7 +1022,7 @@ update_refs(const mi_heap_t *heap, const mi_heap_area_t *area,
     }
     _PyObject_ASSERT(op, refcount >= 0);
 
-    if (refcount > 0 && !_PyObject_HasDeferredRefcount(op)) {
+    if (refcount > 0) {
         if (gc_maybe_untrack(op)) {
             gc_restore_refs(op);
             return true;
@@ -1241,30 +1244,19 @@ scan_heap_visitor(const mi_heap_t *heap, const mi_heap_area_t *area,
         return true;
     }
 
+    if (state->reason == _Py_GC_REASON_SHUTDOWN) {
+        // Disable deferred refcounting for reachable objects as well during
+        // interpreter shutdown. This ensures that these objects are collected
+        // immediately when their last reference is removed.
+        disable_deferred_refcounting(op);
+    }
+
     // object is reachable, restore `ob_tid`; we're done with these objects
     gc_restore_tid(op);
     gc_clear_alive(op);
     return true;
 }
 
-// Disable deferred refcounting for reachable objects during interpreter
-// shutdown. This ensures that these objects are collected immediately when
-// their last reference is removed. This needs to consider both tracked and
-// untracked GC objects, since either might have deferred refcounts enabled.
-static bool
-scan_heap_disable_deferred(const mi_heap_t *heap, const mi_heap_area_t *area,
-        void *block, size_t block_size, void *args)
-{
-    PyObject *op = op_from_block_all_gc(block, args);
-    if (op == NULL) {
-        return true;
-    }
-    if (!_Py_IsImmortal(op)) {
-        disable_deferred_refcounting(op);
-    }
-    return true;
-}
-
 static int
 move_legacy_finalizer_reachable(struct collection_state *state);
 
@@ -1499,10 +1491,6 @@ deduce_unreachable_heap(PyInterpreterState *interp,
     // Restores ob_tid for reachable objects.
     gc_visit_heaps(interp, &scan_heap_visitor, &state->base);
 
-    if (state->reason == _Py_GC_REASON_SHUTDOWN) {
-        gc_visit_heaps(interp, &scan_heap_disable_deferred, &state->base);
-    }
-
     if (state->legacy_finalizers.head) {
         // There may be objects reachable from legacy finalizers that are in
         // the unreachable set. We need to mark them as reachable.

Python/specialize.c

@@ -362,7 +362,7 @@ static uint32_t function_get_version(PyObject *o, int opcode);
 static void
 maybe_enable_deferred_ref_count(PyObject *op)
 {
-    if (!_Py_IsOwnedByCurrentThread(op)) {
+    if (!_Py_IsOwnedByCurrentThread(op) && _PyObject_GC_IS_TRACKED(op)) {
         // For module level variables that are heavily used from multiple
         // threads, deferred reference counting provides good scaling
         // benefits.  The downside is that the object will only be deallocated