// a tensor with the result.
  AbstractTensorHandle* AggregateGradients(
      gtl::ArraySlice<AbstractTensorHandle*> 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,

  /**
   * The map does not throw {@code NullPointerException} on calls such as {@code containsKey(null)},
   * {@code get(null)}, {@code keySet().contains(null)} or {@code remove(null)}.
   */
  ALLOWS_NULL_KEY_QUERIES,
  ALLOWS_NULL_KEYS(ALLOWS_NULL_KEY_QUERIES),
  /**
   * The map does not throw {@code NullPointerException} on calls such as {@code

/**
 * A list iterator which forwards all its method calls to another list iterator. Subclasses should
 * override one or more methods to modify the behavior of the backing iterator as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>{@code default} method warning:</b> This class forwards calls to <i>only some</i> {@code

    source: BufferedSource,
    length: Int,
  ) {
    lock.assertNotHeld()

    this.source.receive(source, length.toLong())
  }

  /** Accept headers from the network and store them until the client calls [takeHeaders]. */
  fun receiveHeaders(
    headers: Headers,
    inFinished: Boolean,
  ) {
    lock.assertNotHeld()

    val open: Boolean
    this.withLock {
      if (!hasResponseHeaders ||

   * timeout. Because networks can fail during an exchange, it is possible that the remote server
   * accepted the request before the failure.
   */
  fun onFailure(
    call: Call,
    e: IOException,
  )

  /**
   * Called when the HTTP response was successfully returned by the remote server. The callback may

      address = address,
      proxy = proxy,
      socketAddress = socketAddress,
    )
  }

  fun newChain(call: RealCall): RealInterceptorChain {
    return RealInterceptorChain(
      call = call,
      interceptors = listOf(),
      index = 0,
      exchange = null,
      request = call.request(),
      connectTimeoutMillis = 10_000,
      readTimeoutMillis = 10_000,
      writeTimeoutMillis = 10_000,
    )

            } catch( IOException ioe ) {
            }
        }
    }

    public void destroy() {
    }

    /**
     * This method simply calls <tt>negotiate( req, resp, false )</tt>
     * and then <tt>chain.doFilter</tt>. You can override and call
     * negotiate manually to achive a variety of different behavior.
     */
    public void doFilter( ServletRequest request,
                ServletResponse response,

        )
        .dns(dns)
        .build()

    server.enqueue(MockResponse())

    val url = server.url("/").newBuilder().host("localhost.localdomain").build()
    val call = client.newCall(Request(url = url))
    val response = call.execute()
    assertThat(response.isSuccessful).isTrue()

    val recordedRequest = server.takeRequest()
    assertThat(recordedRequest.handshakeServerNames).containsExactly(url.host)
  }

  ) : CallEvent()

  data class SecureConnectEnd(
    override val timestampNs: Long,
    override val call: Call,
    val handshake: Handshake?,
  ) : CallEvent() {
    override fun closes(event: CallEvent): Boolean = event is SecureConnectStart && call == event.call
  }

  data class ConnectionAcquired(
    override val timestampNs: Long,
    override val call: Call,
    val connection: Connection,

    // Service b startup takes at least 353 millis, but starting the timer is delayed by at least
    // 150 milliseconds. so in a perfect world the timing would be 353-150=203ms, but since either
    // of our sleep calls can be arbitrarily delayed we should just assert that there is a time
    // recorded.
    assertThat(startupTimes.get(b)).isNotNull();
  }

  public void testServiceStartStop() {
    Service a = new NoOpService();