gh-158194: Pin the type lock in PyType_Freeze()

PyType_Freeze() stopped the world under the type lock without pinning
it, so the lock could be released while waiting for the pause.  Pin it
with type_lock_prevent_release() like the other type-lock callers of
types_stop_world().

Author: nascheme

Misc/NEWS.d/next/Core_and_Builtins/2026-10-02-12-30-00.gh-issue-158194.Rk7vWp.rst

@@ -0,0 +1,5 @@
+Fix :c:func:`PyType_Freeze` releasing the type lock while waiting for a
+stop-the-world pause in the :term:`free-threaded build`. Another thread could
+modify the type before the frozen type's caches were invalidated, and the
+release could deadlock against a thread waiting for the pause with the type
+lock held.

Objects/typeobject.c

@@ -12463,10 +12463,13 @@ PyType_Freeze(PyTypeObject *type)
         return -1;
     }
 
+    pinned_mutexes_t pinned;
     BEGIN_TYPE_LOCK();
+    type_lock_prevent_release(&pinned);
     types_stop_world();
     type_add_flags(type, Py_TPFLAGS_IMMUTABLETYPE);
     types_start_world();
+    type_lock_allow_release(&pinned);
     ASSERT_TYPE_LOCK_HELD();
     _PyType_Modified_Unlocked(type);
     END_TYPE_LOCK();