GH-117108: Set the "old space bit" to "visited" for all young objects (#117213)

Change old space bit of young objects from 0 to gcstate->visited_space.
This ensures that any object created *and* collected during cycle GC has the bit set correctly.

2 files changed, 15 insertions, 3 deletions

diff --git a/Include/internal/pycore_gc.h b/Include/internal/pycore_gc.h
index e729616..c4482c4 100644
--- a/Include/internal/pycore_gc.h
+++ b/Include/internal/pycore_gc.h
@@ -113,7 +113,19 @@ static inline void _PyObject_GC_SET_SHARED_INLINE(PyObject *op) {
 /* Bit 1 is set when the object is in generation which is GCed currently. */
 #define _PyGC_PREV_MASK_COLLECTING 2
 
-/* Bit 0 is set if the object belongs to old space 1 */
+/* Bit 0 in _gc_next is the old space bit.
+ * It is set as follows:
+ * Young: gcstate->visited_space
+ * old[0]: 0
+ * old[1]: 1
+ * permanent: 0
+ *
+ * During a collection all objects handled should have the bit set to
+ * gcstate->visited_space, as objects are moved from the young gen
+ * and the increment into old[gcstate->visited_space].
+ * When object are moved from the pending space, old[gcstate->visited_space^1]
+ * into the increment, the old space bit is flipped.
+*/
 #define _PyGC_NEXT_MASK_OLD_SPACE_1    1
 
 #define _PyGC_PREV_SHIFT           2
diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index 13fe543..0b17ddf 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -318,8 +318,8 @@ static inline void _PyObject_GC_TRACK(
     PyGC_Head *last = (PyGC_Head*)(generation0->_gc_prev);
     _PyGCHead_SET_NEXT(last, gc);
     _PyGCHead_SET_PREV(gc, last);
-    _PyGCHead_SET_NEXT(gc, generation0);
-    assert((gc->_gc_next & _PyGC_NEXT_MASK_OLD_SPACE_1) == 0);
+    /* Young objects will be moved into the visited space during GC, so set the bit here */
+    gc->_gc_next = ((uintptr_t)generation0) | interp->gc.visited_space;
     generation0->_gc_prev = (uintptr_t)gc;
 #endif
 }