override fun writeTo(sink: BufferedSink) {
    requestBodySinks.add(sink)
  }

  override fun isDuplex(): Boolean = true

  @Throws(InterruptedException::class)
  fun takeSink(): BufferedSink = requestBodySinks.poll(5, SECONDS) ?: throw AssertionError("no sink to take")

  fun assertNoMoreSinks() {
    assertTrue(requestBodySinks.isEmpty())
  }