class ImportStorageTransformer

final class ImportStorageTransformer {
    use StorageCopyTrait;
    
    /**
     * The name used to identify the lock.
     */
    const LOCK_NAME = 'config_import_transformer';
    
    /**
     * The event dispatcher to get changes to the configuration.
     *
     * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
     */
    protected $eventDispatcher;
    
    /**
     * The drupal database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * The normal lock for the duration of the request.
     *
     * @var \Drupal\Core\Lock\LockBackendInterface
     */
    protected $requestLock;
    
    /**
     * The persistent lock which the config importer uses across requests.
     *
     * @see \Drupal\Core\Config\ConfigImporter::alreadyImporting()
     *
     * @var \Drupal\Core\Lock\LockBackendInterface
     */
    protected $persistentLock;
    
    /**
     * ImportStorageTransformer constructor.
     *
     * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $event_dispatcher
     *   The event dispatcher.
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection.
     * @param \Drupal\Core\Lock\LockBackendInterface $requestLock
     *   The lock for the request.
     * @param \Drupal\Core\Lock\LockBackendInterface $persistentLock
     *   The persistent lock used by the config importer.
     */
    public function __construct(EventDispatcherInterface $event_dispatcher, Connection $connection, LockBackendInterface $requestLock, LockBackendInterface $persistentLock) {
        $this->eventDispatcher = $event_dispatcher;
        $this->connection = $connection;
        $this->requestLock = $requestLock;
        $this->persistentLock = $persistentLock;
    }
    
    /**
     * Transform the storage to be imported from.
     *
     * An import transformation is done before the config importer uses the
     * storage to synchronize the configuration. The transformation is also
     * done for displaying differences to review imports.
     * Importing in this context means the active drupal configuration is changed
     * with the ConfigImporter which may or may not be as part of the config
     * synchronization.
     *
     * @param \Drupal\Core\Config\StorageInterface $storage
     *   The storage to transform for importing from it.
     *
     * @return \Drupal\Core\Config\StorageInterface
     *   The transformed storage ready to be imported from.
     *
     * @throws \Drupal\Core\Config\StorageTransformerException
     *   Thrown when the lock could not be acquired.
     */
    public function transform(StorageInterface $storage) {
        // We use a database storage to reduce the memory requirement.
        $mutable = new DatabaseStorage($this->connection, 'config_import');
        if (!$this->persistentLock
            ->lockMayBeAvailable(ConfigImporter::LOCK_NAME)) {
            // If the config importer is already importing, the transformation will
            // always be the one the config importer is already using. This makes sure
            // that even if the storage changes the importer continues importing the
            // same configuration.
            return $mutable;
        }
        // Acquire a lock to ensure that the storage is not changed when a
        // concurrent request tries to transform the storage. The lock will be
        // released at the end of the request.
        if (!$this->requestLock
            ->acquire(self::LOCK_NAME)) {
            $this->requestLock
                ->wait(self::LOCK_NAME);
            if (!$this->requestLock
                ->acquire(self::LOCK_NAME)) {
                throw new StorageTransformerException("Cannot acquire config import transformer lock.");
            }
        }
        // Copy the sync configuration to the created mutable storage.
        self::replaceStorageContents($storage, $mutable);
        // Dispatch the event so that event listeners can alter the configuration.
        $this->eventDispatcher
            ->dispatch(new StorageTransformEvent($mutable), ConfigEvents::STORAGE_TRANSFORM_IMPORT);
        // Return the storage with the altered configuration.
        return $mutable;
    }

}