Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/system/src/SystemConfigSubscriber.php

@@ -0,0 +1,64 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\SystemConfigSubscriber.
+ */
+
+namespace Drupal\system;
+
+use Drupal\Core\Config\ConfigEvents;
+use Drupal\Core\Config\ConfigImporterEvent;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * System Config subscriber.
+ */
+class SystemConfigSubscriber implements EventSubscriberInterface {
+  use StringTranslationTrait;
+
+  /**
+   * Checks that the configuration synchronization is valid.
+   *
+   * This event listener prevents deleting all configuration. If there is
+   * nothing to import then event propagation is stopped because there is no
+   * config import to validate.
+   *
+   * @param \Drupal\Core\Config\ConfigImporterEvent $event
+   *   The config import event.
+   */
+  public function onConfigImporterValidateNotEmpty(ConfigImporterEvent $event) {
+    $importList = $event->getConfigImporter()->getStorageComparer()->getSourceStorage()->listAll();
+    if (empty($importList)) {
+      $event->getConfigImporter()->logError($this->t('This import is empty and if applied would delete all of your configuration, so has been rejected.'));
+      $event->stopPropagation();
+    }
+  }
+
+  /**
+   * Checks that the configuration synchronization is valid.
+   *
+   * This event listener checks that the system.site:uuid's in the source and
+   * target match.
+   *
+   * @param ConfigImporterEvent $event
+   *   The config import event.
+   */
+  public function onConfigImporterValidateSiteUUID(ConfigImporterEvent $event) {
+    if (!$event->getConfigImporter()->getStorageComparer()->validateSiteUuid()) {
+      $event->getConfigImporter()->logError($this->t('Site UUID in source storage does not match the target storage.'));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    // The empty check has a high priority so that is can stop propagation if
+    // there is no configuration to import.
+    $events[ConfigEvents::IMPORT_VALIDATE][] = array('onConfigImporterValidateNotEmpty', 512);
+    $events[ConfigEvents::IMPORT_VALIDATE][] = array('onConfigImporterValidateSiteUUID', 256);
+    return $events;
+  }
+}