/**
   * Returns a sink that can be used to write data to the peer.
   *
   * @throws IllegalStateException if this stream was initiated by the peer and a [writeHeaders] has
   *     not yet been sent.
   */
  fun getSink(): Sink {
    this.withLock {
      check(hasResponseHeaders || isLocallyInitiated) {
        "reply before requesting the sink"
      }
    }
    return sink
  }