class MaintenanceModeSubscriber

1. 8.x drupal/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php \Drupal\Core\EventSubscriber\MaintenanceModeSubscriber
2. 8.x drupal/core/modules/user/lib/Drupal/user/EventSubscriber/MaintenanceModeSubscriber.php \Drupal\user\EventSubscriber\MaintenanceModeSubscriber
3. 8.x drupal/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/EventSubscriber/MaintenanceModeSubscriber.php \Drupal\menu_test\EventSubscriber\MaintenanceModeSubscriber

user.services.yml in drupal/core/modules/user/user.services.yml

drupal/core/modules/user/user.services.yml

user_maintenance_mode_subscriber in drupal/core/modules/user/user.services.yml

Drupal\user\EventSubscriber\MaintenanceModeSubscriber

class MaintenanceModeSubscriber implements EventSubscriberInterface {

  /**
   * Determine whether the page is configured to be offline.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   The event to process.
   */
  public function onKernelRequestMaintenance(GetResponseEvent $event) {
    $request = $event
      ->getRequest();
    $site_status = $request->attributes
      ->get('_maintenance');
    $path = $request->attributes
      ->get('system_path');
    if ($site_status == MENU_SITE_OFFLINE) {

      // If the site is offline, log out unprivileged users.
      if (user_is_logged_in() && !user_access('access site in maintenance mode')) {
        user_logout();

        // Redirect to homepage.
        $event
          ->setResponse(new RedirectResponse(url('<front>', array(
          'absolute' => TRUE,
        ))));
        return;
      }
      if (user_is_anonymous()) {
        switch ($path) {
          case 'user':

            // Forward anonymous user to login page.
            $event
              ->setResponse(new RedirectResponse(url('user/login', array(
              'absolute' => TRUE,
            ))));
            return;
          case 'user/login':
          case 'user/password':

            // Disable offline mode.
            $request->attributes
              ->set('_maintenance', MENU_SITE_ONLINE);
            break;
          default:
            if (strpos($path, 'user/reset/') === 0) {

              // Disable offline mode.
              $request->attributes
                ->set('_maintenance', MENU_SITE_ONLINE);
            }
            break;
        }
      }
    }
    if (user_is_logged_in()) {
      if ($path == 'user/login') {

        // If user is logged in, redirect to 'user' instead of giving 403.
        $event
          ->setResponse(new RedirectResponse(url('user', array(
          'absolute' => TRUE,
        ))));
        return;
      }
      if ($path == 'user/register') {

        // Authenticated user should be redirected to user edit page.
        $event
          ->setResponse(new RedirectResponse(url('user/' . $GLOBALS['user']->uid . '/edit', array(
          'absolute' => TRUE,
        ))));
        return;
      }
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = array(
      'onKernelRequestMaintenance',
      35,
    );
    return $events;
  }

}