final class ExportStorageManager

final class ExportStorageManager implements StorageManagerInterface {
  use StorageCopyTrait;

  /**
   * The name used to identify the lock.
   */
  const LOCK_NAME = 'config_storage_export_manager';

  /**
   * The active configuration storage.
   *
   * @var \Drupal\Core\Config\StorageInterface
   */
  protected $active;

  /**
   * The database storage.
   *
   * @var \Drupal\Core\Config\DatabaseStorage
   */
  protected $storage;

  /**
   * The event dispatcher.
   *
   * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
   */
  protected $eventDispatcher;

  /**
   * The used lock backend instance.
   *
   * @var \Drupal\Core\Lock\LockBackendInterface
   */
  protected $lock;

  /**
   * ExportStorageManager constructor.
   *
   * @param \Drupal\Core\Config\StorageInterface $active
   *   The active config storage to prime the export storage.
   * @param \Drupal\Core\Database\Connection $connection
   *   The database connection.
   * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $event_dispatcher
   *   The event dispatcher.
   * @param \Drupal\Core\Lock\LockBackendInterface $lock
   *   The used lock backend instance.
   */
  public function __construct(StorageInterface $active, Connection $connection, EventDispatcherInterface $event_dispatcher, LockBackendInterface $lock) {
    $this->active = $active;
    $this->eventDispatcher = $event_dispatcher;
    $this->lock = $lock;

    // The point of this service is to provide the storage and dispatch the
    // event when needed, so the storage itself can not be a service.
    $this->storage = new DatabaseStorage($connection, 'config_export');
  }

  /**
   * {@inheritdoc}
   */
  public function getStorage() {

    // Acquire a lock for the request to assert that the storage does not change
    // when a concurrent request transforms the storage.
    if (!$this->lock
      ->acquire(self::LOCK_NAME)) {
      $this->lock
        ->wait(self::LOCK_NAME);
      if (!$this->lock
        ->acquire(self::LOCK_NAME)) {
        throw new StorageTransformerException("Cannot acquire config export transformer lock.");
      }
    }
    self::replaceStorageContents($this->active, $this->storage);
    $this->eventDispatcher
      ->dispatch(new StorageTransformEvent($this->storage), ConfigEvents::STORAGE_TRANSFORM_EXPORT);
    return new ReadOnlyStorage($this->storage);
  }

}