gh-118331: Handle errors in _PyObject_SetManagedDict by colesbury · Pull Request #118334 · python/cpython

If we're no longer assigning this before calling _PyDict_DetachFromObject we can bring back some of the assertions in _PyDict_DetachFromObject, specifically:

    assert(_PyObject_ManagedDictPointer(obj)->dict == mp);
    assert(_PyObject_InlineValuesConsistencyCheck(obj));

I think assigning after should be fine after all, because the object and the dict are locked, so we'd only see reads which are proceeding against the values which are being copied. Once the values are invalidated we'd go to the dict which is being detached, and then we'd finally start seeing the new dict.