Issue #2759275: Cannot delete a config menu link

src/Entity/MenuLinkConfig.php

@@ -16,7 +16,8 @@ use Drupal\menu_link_config\MenuLinkConfigInterface;
  *   handlers = {
  *     "access" = "\Drupal\Core\Entity\EntityAccessControlHandler",
  *     "form" = {
- *       "default" = "\Drupal\menu_link_config\Plugin\Menu\Form\MenuLinkConfigForm"
+ *       "default" =
+ *   "\Drupal\menu_link_config\Plugin\Menu\Form\MenuLinkConfigForm"
  *     }
  *   },
  *   admin_permission = "administer menu link config",
@@ -170,13 +171,44 @@ class MenuLinkConfig extends ConfigEntityBase implements MenuLinkConfigInterface
     /** @var \Drupal\Core\Menu\MenuLinkManagerInterface $menu_link_manager */
     $menu_link_manager = \Drupal::service('plugin.manager.menu.link');
 
-    // The menu link can just be updated if there is already an menu link entry
+    // The menu link can just be updated if there is already a menu link entry
     // on both entity and menu link plugin level.
-    if ($update && $menu_link_manager->getDefinition($this->getPluginId())) {
-      $menu_link_manager->updateDefinition($this->getPluginId(), $this->getPluginDefinition(), FALSE);
+    $definition = $this->getPluginDefinition();
+    // Even when $update is FALSE, for top level links it is possible the link
+    // already is in the storage because of the getPluginDefinition() call
+    // above, see https://www.drupal.org/node/2605684#comment-10515450 for the
+    // call chain. Because of this the $update flag is ignored and only the
+    // existence of the definition (equals to being in the tree storage) is
+    // checked.
+    if ($menu_link_manager->getDefinition($this->getPluginId(), FALSE)) {
+      $menu_link_manager->updateDefinition($this->getPluginId(), $definition, FALSE);
     }
     else {
-      $menu_link_manager->addDefinition($this->getPluginId(), $this->getPluginDefinition());
+      $menu_link_manager->addDefinition($this->getPluginId(), $definition);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function preDelete(EntityStorageInterface $storage, array $entities) {
+    parent::preDelete($storage, $entities);
+
+    /** @var \Drupal\Core\Menu\MenuLinkManagerInterface $menu_link_manager */
+    $menu_link_manager = \Drupal::service('plugin.manager.menu.link');
+
+    /** @var \Drupal\menu_link_config\Entity\MenuLinkConfig $menu_link */
+    foreach ($entities as $menu_link) {
+      /** @var \Drupal\menu_link_content\Entity\MenuLinkContent $menu_link */
+      $menu_link_manager->removeDefinition($menu_link->getPluginId(), FALSE);
+
+      // Children get re-attached to the menu link's parent.
+      $parent_plugin_id = $menu_link->getParent();
+      $children = $storage->loadByProperties(['parent' => $menu_link->getPluginId()]);
+      foreach ($children as $child) {
+        /** @var \Drupal\menu_link_content\Entity\MenuLinkContent $child */
+        $child->set('parent', $parent_plugin_id)->save();
+      }
     }
   }