connection.calls.add(CallReference(this, callStackTrace))
  }

  /**
   * Releases resources held with the request or response of [exchange]. This should be called when
   * the request completes normally or when it fails due to an exception, in which case [e] should
   * be non-null.
   *
   * If the exchange was canceled or timed out, this will wrap [e] in an exception that provides

      ) ?: return null

    // If we coalesced our connection, remember the replaced connection's route. That way if the
    // coalesced connection later fails we don't waste a valid route.
    if (planToReplace != null) {
      nextRouteToTry = planToReplace.route
      planToReplace.closeQuietly()
    }

    connectionUser.connectionAcquired(result)

    had a severe bug where web sockets were closed on the wrong thread, which caused
    `NullPointerException` crashes in `Deflater`.

 *  Fix: Don't crash after a web socket fails its connection upgrade. We incorrectly released
    the web socket's connections back to the pool before their resources were cleaned up.

    assertThat(rstStream.streamId).isEqualTo(2)
    assertThat(rstStream.errorCode).isEqualTo(ErrorCode.CANCEL)
  }

  /**
   * When writing a set of headers fails due to an `IOException`, make sure the writer is left
   * in a consistent state so the next writer also gets an `IOException` also instead of
   * something worse (like an [IllegalStateException].
   *
   *