code: public static void popContext() {
          Deque<RpcInternalContext> deque = DEQUE_LOCAL.get();
          if (deque != null) {
              RpcInternalContext context = deque.peek();
              if (context != null) {
                  LOCAL.set(deque.pop());
              }
          }
      }
docs: 上下文往上取一层（例如服务端B接到A的请求后再作为C的客户端调用，调用完毕后这里就先把放A-B的上下文取起来）