final Executor executor() {
    return executor;
  }

  /** Handles the given exception thrown by a subscriber with the given context. */
  void handleSubscriberException(Throwable e, SubscriberExceptionContext context) {
    checkNotNull(e);
    checkNotNull(context);
    try {
      exceptionHandler.handleException(e, context);
    } catch (Throwable e2) {