* proxy server can issue an auth challenge and then close the connection.
   *
   * @return the next plan to attempt, or null if no further attempt should be made either because
   *     we've successfully connected or because no further attempts should be made.
   */
  @Throws(IOException::class)
  internal fun connectTunnel(): ConnectResult {
    val nextTunnelRequest =
      createTunnel()

     * Multiple calls to this method may be made concurrently.
     */
    @Throws(IOException::class)
    abstract fun onStream(stream: Http2Stream)

    /**
     * Notification that the connection's peer's settings may have changed to [settings].
     * Implementations should take appropriate action to handle the updated settings.
     *

    backwards-incompatible changes in forthcoming releases.
 *  Fix: Don't crash loading the public suffix database resource in obfuscated builds.
 *  Fix: Don't silently ignore calls to `EventSource.cancel()` made from
    `EventSourceListener.onOpen()`.
 *  Fix: Enforce the max intermediates constraint when using pinned certificates with Conscrypt.