gh-159437: Support frozen objects in combined GC reference passes

Include/internal/pycore_gc.h

@@ -118,7 +118,15 @@ static inline void _PyObject_GC_SET_SHARED(PyObject *op) {
 /* Bit 1 is set when the object is in generation which is GCed currently. */
 #define _PyGC_PREV_MASK_COLLECTING ((uintptr_t)2)
 
-#define _PyGC_PREV_SHIFT           2
+#if !defined(Py_GIL_DISABLED) && SIZEOF_VOID_P > 4
+// Pointer alignment reserves bit 2 for permanent ownership, including queued weakrefs.
+#  define _PyGC_PREV_MASK_PERM     ((uintptr_t)4)
+#  define _PyGC_PREV_SHIFT         3
+#else
+// 32-bit and free-threaded builds retain the existing two-bit header layout.
+#  define _PyGC_PREV_SHIFT         2
+#endif
+// The remaining high bits contain the predecessor address or gc_refs.
 #define _PyGC_PREV_MASK            (((uintptr_t) -1) << _PyGC_PREV_SHIFT)
 
 /* set for debugging information */
@@ -151,7 +159,7 @@ static inline void _PyGCHead_SET_NEXT(PyGC_Head *gc, PyGC_Head *next) {
     gc->_gc_next = (uintptr_t)next;
 }
 
-// Lowest two bits of _gc_prev is used for _PyGC_PREV_MASK_* flags.
+// Low bits of _gc_prev store _PyGC_PREV_MASK_* flags.
 static inline PyGC_Head* _PyGCHead_PREV(PyGC_Head *gc) {
     uintptr_t prev = (gc->_gc_prev & _PyGC_PREV_MASK);
     return (PyGC_Head*)prev;

Include/internal/pycore_interp_structs.h

@@ -163,7 +163,7 @@ typedef struct {
     _Py_ALIGNED_DEF(_PyObject_MIN_ALIGNMENT, uintptr_t) _gc_next;
 
     // Tagged pointer to previous object in the list.
-    // Lowest two bits are used for flags documented later.
+    // Low bits store the flags documented in pycore_gc.h.
     // Those bits are made available by the struct's minimum alignment.
     uintptr_t _gc_prev;
 } PyGC_Head;

Include/internal/pycore_pystate.h

@@ -62,6 +62,18 @@ _Py_IsMainInterpreter(PyInterpreterState *interp)
     return (interp == _PyInterpreterState_Main());
 }
 
+/* Return whether tracked objects may belong to another interpreter lifetime.
+ *
+ * Result:
+ * - false before any subinterpreter reservation or runtime reinitialization.
+ * - true after either event, including across Py_Finalize()/Py_Initialize().
+ *
+ * Extension-owned objects may outlive the interpreter whose GC list held them,
+ * so callers must not assume every tracked object belongs to the current
+ * runtime when this returns true.
+ */
+PyAPI_FUNC(bool) _PyInterpreterState_MayHaveForeignGcObjects(void);
+
 extern int _Py_IsMainInterpreterFinalizing(PyInterpreterState *interp);
 
 // Export for _interpreters module.

Include/object.h

@@ -112,9 +112,19 @@ whose size is determined when the object is allocated.
 
 /* PyObjects are given a minimum alignment so that the least significant bits
  * of an object pointer become available for other purposes.
- * This must be an integer literal with the value (1 << _PyGC_PREV_SHIFT), number of bytes.
+ * For exposed layouts, this must be an integer literal with the value (1 << _PyGC_PREV_SHIFT), in bytes.
+ * Opaque headers retain 4 for source compatibility.
  */
-#define _PyObject_MIN_ALIGNMENT 4
+// ABI3T test headers may poison Py_GIL_DISABLED.
+#ifndef _Py_OPAQUE_PYOBJECT
+#  if !defined(Py_GIL_DISABLED) && SIZEOF_VOID_P > 4
+#    define _PyObject_MIN_ALIGNMENT 8
+#  else
+#    define _PyObject_MIN_ALIGNMENT 4
+#  endif
+#else
+#  define _PyObject_MIN_ALIGNMENT 4
+#endif
 
 /* Nothing is actually declared to be a PyObject, but every pointer to
  * a Python object can be cast to a PyObject*.  This is inheritance built

InternalDocs/garbage_collector.md

@@ -660,11 +660,15 @@ of `PyGC_Head` discussed in the `Memory layout and object structure`_ section:
 
 - The `_gc_prev` field is normally used as the "previous" pointer to maintain the
   doubly linked list but its lowest two bits are used to keep the flags
-  `PREV_MASK_COLLECTING` and `_PyGC_PREV_MASK_FINALIZED`. Between collections,
-  the only flag that can be present is `_PyGC_PREV_MASK_FINALIZED` that indicates
-  if an object has been already finalized. During collections `_gc_prev` is
-  temporarily used for storing a copy of the reference count (`gc_ref`), in
-  addition to two flags, and the GC linked list becomes a singly linked list until
+  `PREV_MASK_COLLECTING` and `_PyGC_PREV_MASK_FINALIZED`. On GIL builds where `SIZEOF_VOID_P > 4`,
+  bit 2 is also used for `_PyGC_PREV_MASK_PERM` to record permanent-generation
+  ownership, including weakrefs temporarily queued for callbacks. Unfreeze clears
+  this flag on objects in the permanent generation. Surviving queued weakrefs
+  have it cleared before moving to the old generation. Between collections,
+  `_PyGC_PREV_MASK_FINALIZED` indicates if an object has been already finalized,
+  and `_PyGC_PREV_MASK_PERM` can also remain set. During collections `_gc_prev`
+  is temporarily used for storing a copy of the reference count (`gc_ref`), in
+  addition to these flags, and the GC linked list becomes a singly linked list until
   `_gc_prev` is restored.
 
 - The `_gc_next` field is used as the "next" pointer to maintain the doubly linked