code: Sink newMessageSink(int formatOpcode, long contentLength) {
      if (activeWriter) {
        throw new IllegalStateException("Another message writer is active. Did you call close()?");
      }
      activeWriter = true;
  
      // Reset FrameSink state for a new writer.
      frameSink.formatOpcode = formatOpcode;
      frameSink.contentLength = contentLength;
      frameSink.isFirstFrame = true;
      frameSink.closed = false;
  
      return frameSink;
    }
docs: Stream a message payload as a series of frames. This allows control frames to be interleaved
  between parts of the message.