[3.15] gh-123619: Fix PyUnstable_Object_EnableDeferredRefcount() (GH-151260) (#151326)

miss-islington · vstinner · web-flow · commit dc5ebe3da4de · 2026-06-11T11:21:26.000Z
gh-123619: Fix PyUnstable_Object_EnableDeferredRefcount() (GH-151260) Return 0 if the object is not tracked by the GC. (cherry picked from commit 72e7edd) Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Lib/test/test_capi/test_object.py b/Lib/test/test_capi/test_object.py
@@ -178,11 +178,17 @@ class EnableDeferredRefcountingTest(unittest.TestCase):
     @support.requires_resource("cpu")
     def test_enable_deferred_refcount(self):
         from threading import Thread
+        import gc
 
         self.assertEqual(_testcapi.pyobject_enable_deferred_refcount("not tracked"), 0)
         foo = []
         self.assertEqual(_testcapi.pyobject_enable_deferred_refcount(foo), int(support.Py_GIL_DISABLED))
 
+        # The object must be tracked by the GC
+        not_gc_tracked = tuple([1, 2])
+        self.assertFalse(gc.is_tracked(not_gc_tracked))
+        self.assertEqual(_testcapi.pyobject_enable_deferred_refcount(not_gc_tracked), 0)
+
         # Make sure reference counting works on foo now
         self.assertEqual(foo, [])
         if support.Py_GIL_DISABLED:
diff --git a/Misc/NEWS.d/next/C_API/2026-06-10-16-43-37.gh-issue-123619.dV82r6.rst b/Misc/NEWS.d/next/C_API/2026-06-10-16-43-37.gh-issue-123619.dV82r6.rst
@@ -0,0 +1,3 @@
+:c:func:`PyUnstable_Object_EnableDeferredRefcount` now returns ``0`` if the
+object is not tracked by the garbage collector: if :func:`gc.is_tracked` is
+false. Patch by Victor Stinner.
diff --git a/Objects/object.c b/Objects/object.c
@@ -2821,6 +2821,13 @@ PyUnstable_Object_EnableDeferredRefcount(PyObject *op)
         return 0;
     }
 
+    if (!PyObject_GC_IsTracked(op)) {
+        // When deferred refcount is enabled, the object will only be
+        // deallocated by the tracing garbage collector. So it must be tracked
+        // by the garbage collector.
+        return 0;
+    }
+
     uint8_t bits = _Py_atomic_load_uint8(&op->ob_gc_bits);
     if ((bits & _PyGC_BITS_DEFERRED) != 0)
     {

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+:c:func:`PyUnstable_Object_EnableDeferredRefcount` now returns ``0`` if the
	`2`	+object is not tracked by the garbage collector: if :func:`gc.is_tracked` is
	`3`	`+false. Patch by Victor Stinner.`
Original file line number	Diff line number	Diff line change
`@@ -2821,6 +2821,13 @@ PyUnstable_Object_EnableDeferredRefcount(PyObject *op)`
`2821`	`2821`	`return 0;`
`2822`	`2822`	`}`
`2823`	`2823`
	`2824`	`+ if (!PyObject_GC_IsTracked(op)) {`
	`2825`	`+ // When deferred refcount is enabled, the object will only be`
	`2826`	`+ // deallocated by the tracing garbage collector. So it must be tracked`
	`2827`	`+ // by the garbage collector.`
	`2828`	`+ return 0;`
	`2829`	`+ }`
	`2830`	`+`
`2824`	`2831`	`uint8_t bits = _Py_atomic_load_uint8(&op->ob_gc_bits);`
`2825`	`2832`	`if ((bits & _PyGC_BITS_DEFERRED) != 0)`
`2826`	`2833`	`{`