code: public static void pushContext() {
          RpcInternalContext context = LOCAL.get();
          if (context != null) {
              Deque<RpcInternalContext> deque = DEQUE_LOCAL.get();
              if (deque == null) {
                  deque = new ArrayDeque<RpcInternalContext>();
                  DEQUE_LOCAL.set(deque);
              }
              deque.push(context);
              LOCAL.set(null);
          }
      }
docs: 上下文往下放一层（例如服务端B接到A的请求后再作为C的客户端调用，调用前这里就先把放A-B的上下文存起来）