const FunctionBody* fbody = lib_runtime->GetFunctionBody(handle);
    TF_RETURN_IF_ERROR(GetNodesRelatedToRefVariablesInDirection(
        *fbody->graph, lib_runtime, direction, depth + 1, &callee_ref_nodes));

    // We could possibly use something cheaper than
    // GetNodesRelatedToRefVariablesInDirection since we only care about the
    // size of `callee_ref_nodes` but for now we don't ceare.