code: ClientStream returnStream() {
      synchronized (lock) {
        if (returnedStream == null) {
          // apply() has not been called, needs to buffer the requests.
          delayedStream = new DelayedStream();
          return returnedStream = delayedStream;
        } else {
          return returnedStream;
        }
      }
    }
docs: Return a stream on which the RPC will run on.