override val timestampNs: Long,
    val route: Route,
    val call: Call,
  ) : ConnectionEvent()

  data class ConnectFailed(
    override val timestampNs: Long,
    val route: Route,
    val call: Call,
    val exception: IOException,
  ) : ConnectionEvent() {
    override fun closes(event: ConnectionEvent): Boolean = event is ConnectStart && call == event.call && route == event.route
  }

  data class ConnectEnd(

    check(rawSocket == null) { "TCP already connected" }

    var success = false

    // Tell the call about the connecting call so async cancels work.
    call.plansToCancel += this
    try {
      call.eventListener.connectStart(call, route.socketAddress, route.proxy)
      connectionPool.connectionListener.connectStart(route, call)

      connectSocket()
      success = true
      return ConnectResult(plan = this)

        .build()

    val startNanos = System.nanoTime()
    val call = client.newCall(request)

    // Schedule a job to cancel the call in 1 second.
    executor.schedule({
      System.out.printf("%.2f Canceling call.%n", (System.nanoTime() - startNanos) / 1e9f)
      call.cancel()
      System.out.printf("%.2f Canceled call.%n", (System.nanoTime() - startNanos) / 1e9f)
    }, 1, TimeUnit.SECONDS)

        localhostIpv4,
        localhostIpv6,
      )
    serverIpv4.enqueue(
      MockResponse(body = "unexpected call to IPv4"),
    )
    serverIpv6.enqueue(
      MockResponse(body = "hello from IPv6"),
    )

    val call = client.newCall(Request(url))
    val response = call.execute()
    assertThat(response.body.string()).isEqualTo("hello from IPv6")

        override fun requestHeadersEnd(
          call: Call,
          request: Request,
        ) = TODO()

        override fun requestBodyStart(call: Call) = TODO()

        override fun requestBodyEnd(
          call: Call,
          byteCount: Long,
        ) = TODO()

        override fun requestFailed(
          call: Call,
          ioe: IOException,

      .tag(Integer.class, 5)
      .tag(String.class, "hello")
      .build();
    Call call = client.newCall(request);

    assertEquals(5, call.tag(Integer.class));
    assertEquals("hello", call.tag(String.class));
    assertEquals(null, call.tag(Boolean.class));
    assertEquals(null, call.tag(Object.class));
  }

  @Test
  public void tagsCanBeComputed() {
    Request request = new Request.Builder()

        .build();

    final long startNanos = System.nanoTime();
    final Call call = client.newCall(request);

    // Schedule a job to cancel the call in 1 second.
    executor.schedule(() -> {
      System.out.printf("%.2f Canceling call.%n", (System.nanoTime() - startNanos) / 1e9f);
      call.cancel();
      System.out.printf("%.2f Canceled call.%n", (System.nanoTime() - startNanos) / 1e9f);
    }, 1, TimeUnit.SECONDS);

  }

  @Test
  fun successfulCallEventSequence() {
    server.enqueue(MockResponse(body = "abc"))
    val call =
      client.newCall(
        Request
          .Builder()
          .url(server.url("/"))
          .build(),
      )
    val response = call.execute()
    assertThat(response.code).isEqualTo(200)
    assertThat(response.body.string()).isEqualTo("abc")
    response.body.close()

) : EventSource,
  ServerSentEventReader.Callback,
  Callback {
  private var call: Call? = null

  @Volatile private var canceled = false

  fun connect(callFactory: Call.Factory) {
    call =
      callFactory.newCall(request).apply {
        enqueue(this@RealEventSource)
      }
  }

  override fun onResponse(
    call: Call,
    response: Response,
  ) {
    processResponse(response)
  }

  // a tensor with the result.
  AbstractTensorHandle* AggregateGradients(
      absl::Span<AbstractTensorHandle* const> gradient_tensors) const override;

  // Calls the passed-in backward function.
  // op_type is the op's name provided in RecordOperation.
  absl::Status CallBackwardFunction(
      const string& op_type, GradientFunction* gradient_function,