/**
   * Invoked as soon as a call is enqueued or executed by a client. In case of thread or stream
   * limits, this call may be executed well before processing the request is able to begin.
   *
   * This will be invoked only once for a single [Call]. Retries of different routes or redirects
   * will be handled within the boundaries of a single [callStart] and [callEnd]/[callFailed] pair.
   */
  open fun callStart(call: Call) {
  }

  /**

 *  Fix: Don't leak a connection when a call is canceled immediately preceding the `onFailure()`
    callback.

 *  Fix: Apply call timeouts when connecting duplex calls, web sockets, and server-sent events.
    Once the streams are established no further timeout is enforced.

 *  Fix: Retain the `Route` when a connection is reused on a redirect or other follow-up. This was

    builder.interceptors.toImmutableList()

  /**
   * Returns an immutable list of interceptors that observe a single network request and response.
   * These interceptors must call [Interceptor.Chain.proceed] exactly once: it is an error for
   * a network interceptor to short-circuit or repeat a network request.
   */
  @get:JvmName("networkInterceptors")
  val networkInterceptors: List<Interceptor> =

        return headersQueue.removeFirst()
      }
      throw errorException ?: StreamResetException(errorCode!!)
    }
  }

  /**
   * Returns the trailers. It is only safe to call this once the source stream has been completely
   * exhausted.
   */
  @Throws(IOException::class)
  fun trailers(): Headers {
    this.withLock {

    cache.close()
    createNewCache()

    // The journal will have no record that 'a' was removed. It will have an entry for 'a', but when
    // it tries to read the cache files, it will find they were deleted. Once it encounters an entry
    // with missing cache files, it should remove it from the cache entirely.
    assertThat(cache.size()).isEqualTo(4)
    assertThat(cache["a"]).isNull()
    assertThat(cache.size()).isEqualTo(2)

    checkEntry(entry1, "custom-bar", "custom-header", 55)
    val entry2 = hpackReader!!.dynamicTable[readerHeaderTableLength() - 2]!!
    checkEntry(entry2, "custom-baz", "custom-header", 55)

    // Once a header field is decoded and added to the reconstructed header
    // list, it cannot be removed from it. Hence, foo is here.
    assertThat(hpackReader!!.getAndResetHeaderList()).isEqualTo(headerBlock)

 *  New: Kotlin-specific APIs for request tags. Kotlin language users can lookup tags with a type
    parameter only, like `request.tag<MyTagClass>()`.
 *  New: MockWebServer has improved support for HTTP/1xx responses. Once you've migrated to the new
    `mockwebserver3` package, there's a new field, `MockResponse.informationalResponses`.
 *  Fix: Don't interpret trailers as headers after an HTTP/100 response. This was a bug only when

        set(Settings.INITIAL_WINDOW_SIZE, OKHTTP_CLIENT_WINDOW_SIZE)
      }
    }

  /**
   * Settings we receive from the peer. Changes to the field are guarded by this. The instance is
   * never mutated once it has been assigned.
   */
  var peerSettings = DEFAULT_SETTINGS

  /** The bytes consumed and acknowledged by the application. */
  val readBytes: WindowCounter = WindowCounter(streamId = 0)

 * `::1` will reach the IPv6 server.
 *
 * By orchestrating two different servers with the same port but different IP addresses, we can
 * test what OkHttp does when both are reachable, or if only one is reachable.
 *
 * This test only runs on host machines that have both IPv4 and IPv6 addresses for localhost.
 */
@Timeout(30)
class FastFallbackTest {
  @RegisterExtension
  val clientTestRule = OkHttpClientTestRule()

## Retrying Requests

Sometimes connections fail: either a pooled connection was stale and disconnected, or the webserver itself couldn’t be reached. OkHttp will retry the request with a different route if one is available.