Revert overridden configuration on rebuild().

Inspired by ConfigUpdateUiCliService.

modules/core/update/farm_update.info.yml

@@ -3,3 +3,5 @@ description: Automatically update farmOS configuration. Turn this off if configu
 type: module
 package: farmOS
 core_version_requirement: ^9
+dependencies:
+  - config_update:config_update

modules/core/update/farm_update.services.yml

@@ -1,3 +1,4 @@
 services:
   farm.update:
     class: Drupal\farm_update\FarmUpdate
+    arguments: [ '@entity_type.manager', '@config_update.config_diff', '@config_update.config_list', '@config_update.config_update' ]

modules/core/update/src/FarmUpdate.php

@@ -2,16 +2,155 @@
 
 namespace Drupal\farm_update;
 
+use Drupal\config_update\ConfigDiffer;
+use Drupal\config_update\ConfigListerWithProviders;
+use Drupal\config_update\ConfigReverter;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+
 /**
  * Farm update service.
  */
 class FarmUpdate implements FarmUpdateInterface {
 
+  /**
+   * The entity manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityManager;
+
+  /**
+   * The config differ.
+   *
+   * @var \Drupal\config_update\ConfigDiffer
+   */
+  protected $configDiff;
+
+  /**
+   * The config lister.
+   *
+   * @var \Drupal\config_update\ConfigListerWithProviders
+   */
+  protected $configList;
+
+  /**
+   * The config reverter.
+   *
+   * @var \Drupal\config_update\ConfigReverter
+   */
+  protected $configUpdate;
+
+  /**
+   * Constructs a FarmUpdate object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_manager
+   *   The entity type manager.
+   * @param \Drupal\config_update\ConfigDiffer $config_diff
+   *   The config differ.
+   * @param \Drupal\config_update\ConfigListerWithProviders $config_list
+   *   The config lister.
+   * @param \Drupal\config_update\ConfigReverter $config_update
+   *   The config reverter.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_manager, ConfigDiffer $config_diff, ConfigListerWithProviders $config_list, ConfigReverter $config_update) {
+    $this->entityManager = $entity_manager;
+    $this->configDiff = $config_diff;
+    $this->configList = $config_list;
+    $this->configUpdate = $config_update;
+  }
+
   /**
    * {@inheritdoc}
    */
   public function rebuild(): void {
 
+    // Build a list of config to revert.
+    $revert_config = $this->getDifferentItems('type', 'system.all');
+
+    // Iterate through config items and revert them.
+    foreach ($revert_config as $name) {
+
+      // Get the config type.
+      // The lister gives NULL if simple configuration, but the reverter
+      // expects 'system.simple' so we convert it.
+      $type = $this->configList->getTypeNameByConfigName($name);
+      if ($type === NULL) {
+        $type = 'system.simple';
+      }
+
+      // Get the config short name.
+      $shortname = $this->getConfigShortname($type, $name);
+
+      // Perform the operation.
+      $this->configUpdate->revert($type, $shortname);
+    }
+  }
+
+  /**
+   * Lists differing config items.
+   *
+   * Lists config items that differ from the versions provided by your
+   * installed modules, themes, or install profile. See config-diff to show
+   * what the differences are.
+   *
+   * This method is copied directly from ConfigUpdateUiCliService.
+   *
+   * @param string $type
+   *   Run the report for: module, theme, profile, or "type" for config entity
+   *   type.
+   * @param string $name
+   *   The machine name of the module, theme, etc. to report on. See
+   *   config-list-types to list types for config entities; you can also use
+   *   system.all for all types, or system.simple for simple config.
+   *
+   * @return array
+   *   An array of differing configuration items.
+   *
+   * @see \Drupal\config_update_ui\ConfigUpdateUiCliService::getDifferentItems()
+   */
+  protected function getDifferentItems($type, $name) {
+    [$activeList, $installList, $optionalList] = $this->configList->listConfig($type, $name);
+    $addedItems = array_diff($activeList, $installList, $optionalList);
+    $activeAndAddedItems = array_diff($activeList, $addedItems);
+    $differentItems = [];
+    foreach ($activeAndAddedItems as $name) {
+      $active = $this->configUpdate->getFromActive('', $name);
+      $extension = $this->configUpdate->getFromExtension('', $name);
+      if (!$this->configDiff->same($active, $extension)) {
+        $differentItems[] = $name;
+      }
+    }
+    sort($differentItems);
+
+    return $differentItems;
+  }
+
+  /**
+   * Gets the config item shortname given the type and name.
+   *
+   * This method is copied directly from ConfigUpdateUiCliService.
+   *
+   * @param string $type
+   *   The type of the config item.
+   * @param string $name
+   *   The name of the config item.
+   *
+   * @return string
+   *   The shortname for the configuration item.
+   *
+   * @see \Drupal\config_update_ui\ConfigUpdateUiCliService::getConfigShortname()
+   */
+  protected function getConfigShortname($type, $name) {
+    $shortname = $name;
+    if ($type != 'system.simple') {
+      $definition = $this->entityManager->getDefinition($type);
+      $prefix = $definition->getConfigPrefix() . '.';
+      if (strpos($name, $prefix) === 0) {
+        $shortname = substr($name, strlen($prefix));
+      }
+    }
+
+    return $shortname;
   }
 
 }