From 3ae3e525b003fec2a2d98b3005ee685a86c49498 Mon Sep 17 00:00:00 2001
From: Alex Pott <alex.a.pott@googlemail.com>
Date: Thu, 11 Jul 2019 08:16:32 +0100
Subject: [PATCH] =?UTF-8?q?Issue=20#3047812=20by=20bircher,=20Krzysztof=20?=
 =?UTF-8?q?Doma=C5=84ski,=20alexpott,=20ricardoamaro,=20larowlan,=20boriss?=
 =?UTF-8?q?on=5F,=20mpotter:=20Add=20a=20Config=20Transformation=20event?=
 =?UTF-8?q?=20dispatching=20during=20config=20import=20and=20export?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../config_environment.module                 |   9 +
 .../config_environment.routing.yml            |  31 ++++
 .../config_environment.services.yml           |  13 ++
 .../src/Controller/ConfigController.php       |  47 ++++++
 .../src/Core/Config/ConfigEvents.php          |  79 +++++++++
 .../src/Core/Config/ExportStorageManager.php  | 115 +++++++++++++
 .../Core/Config/ImportStorageTransformer.php  |  82 +++++++++
 .../src/Core/Config/ManagedStorage.php        | 157 ++++++++++++++++++
 .../Core/Config/StorageManagerInterface.php   |  24 +++
 .../src/Core/Config/StorageTransformEvent.php |  52 ++++++
 .../src/Form/ConfigSync.php                   |  48 ++++++
 .../config_transformer_test.info.yml          |  10 ++
 .../config_transformer_test.services.yml      |   6 +
 .../src/EventSubscriber.php                   | 102 ++++++++++++
 .../TransformedConfigExportImportUITest.php   | 100 +++++++++++
 .../Core/Config/ExportStorageManagerTest.php  |  62 +++++++
 .../Config/ImportStorageTransformerTest.php   |  63 +++++++
 .../Config/Storage/ManagedStorageTest.php     |  77 +++++++++
 18 files changed, 1077 insertions(+)
 create mode 100644 core/modules/config_environment/config_environment.routing.yml
 create mode 100644 core/modules/config_environment/config_environment.services.yml
 create mode 100644 core/modules/config_environment/src/Controller/ConfigController.php
 create mode 100644 core/modules/config_environment/src/Core/Config/ConfigEvents.php
 create mode 100644 core/modules/config_environment/src/Core/Config/ExportStorageManager.php
 create mode 100644 core/modules/config_environment/src/Core/Config/ImportStorageTransformer.php
 create mode 100644 core/modules/config_environment/src/Core/Config/ManagedStorage.php
 create mode 100644 core/modules/config_environment/src/Core/Config/StorageManagerInterface.php
 create mode 100644 core/modules/config_environment/src/Core/Config/StorageTransformEvent.php
 create mode 100644 core/modules/config_environment/src/Form/ConfigSync.php
 create mode 100644 core/modules/config_environment/tests/config_transformer_test/config_transformer_test.info.yml
 create mode 100644 core/modules/config_environment/tests/config_transformer_test/config_transformer_test.services.yml
 create mode 100644 core/modules/config_environment/tests/config_transformer_test/src/EventSubscriber.php
 create mode 100644 core/modules/config_environment/tests/src/Functional/TransformedConfigExportImportUITest.php
 create mode 100644 core/modules/config_environment/tests/src/Kernel/Core/Config/ExportStorageManagerTest.php
 create mode 100644 core/modules/config_environment/tests/src/Kernel/Core/Config/ImportStorageTransformerTest.php
 create mode 100644 core/modules/config_environment/tests/src/Kernel/Core/Config/Storage/ManagedStorageTest.php

diff --git a/core/modules/config_environment/config_environment.module b/core/modules/config_environment/config_environment.module
index b54b3dc632f1..4737f74dba99 100644
--- a/core/modules/config_environment/config_environment.module
+++ b/core/modules/config_environment/config_environment.module
@@ -5,6 +5,15 @@
  * Allows site administrators to modify environment configuration.
  */
 
+// Set class aliases for the classes that will go into core when we are in beta.
+// See the experimental modules policy https://www.drupal.org/core/experimental
+// @todo: remove class aliases in #2991683
+@class_alias('Drupal\config_environment\Core\Config\StorageTransformEvent', 'Drupal\Core\Config\StorageTransformEvent');
+@class_alias('Drupal\config_environment\Core\Config\ManagedStorage', 'Drupal\Core\Config\ManagedStorage');
+@class_alias('Drupal\config_environment\Core\Config\StorageManagerInterface', 'Drupal\Core\Config\StorageManagerInterface');
+@class_alias('Drupal\config_environment\Core\Config\ExportStorageManager', 'Drupal\Core\Config\ExportStorageManager');
+@class_alias('Drupal\config_environment\Core\Config\ImportStorageTransformer', 'Drupal\Core\Config\ImportStorageTransformer');
+
 use Drupal\Core\Routing\RouteMatchInterface;
 
 /**
diff --git a/core/modules/config_environment/config_environment.routing.yml b/core/modules/config_environment/config_environment.routing.yml
new file mode 100644
index 000000000000..7c9edeac2126
--- /dev/null
+++ b/core/modules/config_environment/config_environment.routing.yml
@@ -0,0 +1,31 @@
+# @todo: Stop taking over config module routes in #2991683
+config.sync:
+  path: '/admin/config/development/configuration'
+  defaults:
+    _form: '\Drupal\config_environment\Form\ConfigSync'
+    _title: 'Synchronize'
+  requirements:
+    _permission: 'synchronize configuration'
+
+config.diff:
+  path: '/admin/config/development/configuration/sync/diff/{source_name}/{target_name}'
+  defaults:
+    _controller: '\Drupal\config_environment\Controller\ConfigController::diff'
+    target_name: NULL
+  requirements:
+    _permission: 'synchronize configuration'
+
+config.diff_collection:
+  path: '/admin/config/development/configuration/sync/diff_collection/{collection}/{source_name}/{target_name}'
+  defaults:
+    _controller: '\Drupal\config_environment\Controller\ConfigController::diff'
+    target_name: NULL
+  requirements:
+    _permission: 'synchronize configuration'
+
+config.export_download:
+  path: '/admin/config/development/configuration/full/export-download'
+  defaults:
+    _controller: '\Drupal\config_environment\Controller\ConfigController::downloadExport'
+  requirements:
+    _permission: 'export configuration'
diff --git a/core/modules/config_environment/config_environment.services.yml b/core/modules/config_environment/config_environment.services.yml
new file mode 100644
index 000000000000..6243c14ee1c5
--- /dev/null
+++ b/core/modules/config_environment/config_environment.services.yml
@@ -0,0 +1,13 @@
+services:
+  # @todo: Move this back to core services in #2991683
+  config.import_transformer:
+    class: Drupal\config_environment\Core\Config\ImportStorageTransformer
+    arguments: ['@event_dispatcher', '@database']
+  config.storage.export:
+    class: Drupal\config_environment\Core\Config\ManagedStorage
+    arguments: ['@config.storage.export.manager']
+  config.storage.export.manager:
+    class: Drupal\config_environment\Core\Config\ExportStorageManager
+    arguments: ['@config.storage', '@state', '@database', '@event_dispatcher']
+    tags:
+      - { name: event_subscriber }
diff --git a/core/modules/config_environment/src/Controller/ConfigController.php b/core/modules/config_environment/src/Controller/ConfigController.php
new file mode 100644
index 000000000000..92cb3fa9a612
--- /dev/null
+++ b/core/modules/config_environment/src/Controller/ConfigController.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Drupal\config_environment\Controller;
+
+use Drupal\config\Controller\ConfigController as OriginalConfigController;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Returns responses for config module routes.
+ */
+class ConfigController extends OriginalConfigController {
+
+  /**
+   * The import transformer service.
+   *
+   * @var \Drupal\Core\Config\ImportStorageTransformer
+   */
+  protected $importTransformer;
+
+  /**
+   * The sync storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $syncStorage;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    $controller = parent::create($container);
+    $controller->importTransformer = $container->get('config.import_transformer');
+    $controller->syncStorage = $container->get('config.storage.sync');
+
+    return $controller;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function diff($source_name, $target_name = NULL, $collection = NULL) {
+    $this->sourceStorage = $this->importTransformer->transform($this->syncStorage);
+
+    return parent::diff($source_name, $target_name, $collection);
+  }
+
+}
diff --git a/core/modules/config_environment/src/Core/Config/ConfigEvents.php b/core/modules/config_environment/src/Core/Config/ConfigEvents.php
new file mode 100644
index 000000000000..bc91eec462bd
--- /dev/null
+++ b/core/modules/config_environment/src/Core/Config/ConfigEvents.php
@@ -0,0 +1,79 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Core\Config\ConfigEvents in #2991683.
+// @codingStandardsIgnoreEnd
+namespace Drupal\config_environment\Core\Config;
+
+/**
+ * Defines events for the configuration transform system.
+ *
+ * The constants in this class will be moved back into ConfigEvents.
+ * But due to the fact that the config_environment is not in beta we save their
+ * definitions here and use the literal strings in the mean time.
+ *
+ * @internal
+ *
+ * @deprecated The class will be merged with Drupal\Core\Config\ConfigEvents.
+ */
+final class ConfigEvents {
+
+  /**
+   * Name of the event fired just before importing configuration.
+   *
+   * This event allows subscribers to modify the configuration which is about to
+   * be imported. The event listener method receives a
+   * \Drupal\Core\Config\StorageTransformEvent instance. This event contains a
+   * config storage which subscribers can interact with and which will finally
+   * be used to import the configuration from.
+   * Together with \Drupal\Core\Config\ConfigEvents::STORAGE_TRANSFORM_EXPORT
+   * subscribers can alter the active configuration in a config sync workflow
+   * instead of just overriding at runtime via the config-override system.
+   * This allows a complete customisation of the workflow including additional
+   * modules and editable configuration in different environments.
+   *
+   * @code
+   *   $storage = $event->getStorage();
+   * @endcode
+   *
+   * This event is also fired when just viewing the difference of configuration
+   * to be imported independently of whether the import takes place or not.
+   * Use the \Drupal\Core\Config\ConfigEvents::IMPORT event to subscribe to the
+   * import having taken place.
+   *
+   * @Event
+   *
+   * @see \Drupal\Core\Config\StorageTransformEvent
+   * @see \Drupal\Core\Config\ConfigEvents::STORAGE_TRANSFORM_EXPORT
+   *
+   * @var string
+   */
+  const STORAGE_TRANSFORM_IMPORT = 'config.transform.import';
+
+  /**
+   * Name of the event fired when the export storage is used.
+   *
+   * This event allows subscribers to modify the configuration which is about to
+   * be exported. The event listener method receives a
+   * \Drupal\Core\Config\StorageTransformEvent instance. This event contains a
+   * config storage which subscribers can interact with and which will finally
+   * be used to export the configuration from.
+   *
+   * @code
+   *   $storage = $event->getStorage();
+   * @endcode
+   *
+   * Typically subscribers will want to perform the reverse operation on the
+   * storage than for \Drupal\Core\Config\ConfigEvents::STORAGE_TRANSFORM_IMPORT
+   * to make sure successive exports and imports yield no difference.
+   *
+   * @Event
+   *
+   * @see \Drupal\Core\Config\StorageTransformEvent
+   * @see \Drupal\Core\Config\ConfigEvents::STORAGE_TRANSFORM_IMPORT
+   *
+   * @var string
+   */
+  const STORAGE_TRANSFORM_EXPORT = 'config.transform.export';
+
+}
diff --git a/core/modules/config_environment/src/Core/Config/ExportStorageManager.php b/core/modules/config_environment/src/Core/Config/ExportStorageManager.php
new file mode 100644
index 000000000000..3385b84d39ca
--- /dev/null
+++ b/core/modules/config_environment/src/Core/Config/ExportStorageManager.php
@@ -0,0 +1,115 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Core\Config in #2991683.
+// Use this class with its class alias Drupal\Core\Config\ExportStorageManager
+// @codingStandardsIgnoreEnd
+namespace Drupal\config_environment\Core\Config;
+
+use Drupal\Core\Config\ConfigEvents;
+use Drupal\Core\Config\DatabaseStorage;
+use Drupal\Core\Config\ReadOnlyStorage;
+use Drupal\Core\Config\StorageCopyTrait;
+use Drupal\Core\Config\StorageInterface;
+use Drupal\Core\Database\Connection;
+use Drupal\Core\State\StateInterface;
+use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * The export storage manager dispatches an event for the export storage.
+ *
+ * @internal
+ */
+class ExportStorageManager implements StorageManagerInterface, EventSubscriberInterface {
+
+  use StorageCopyTrait;
+
+  /**
+   * The state key indicating that the export storage needs to be rebuilt.
+   */
+  const NEEDS_REBUILD_KEY = 'config_export_needs_rebuild';
+
+  /**
+   * The active configuration storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $active;
+
+  /**
+   * The drupal state.
+   *
+   * @var \Drupal\Core\State\StateInterface
+   */
+  protected $state;
+
+  /**
+   * The database storage.
+   *
+   * @var \Drupal\Core\Config\DatabaseStorage
+   */
+  protected $storage;
+
+  /**
+   * The event dispatcher.
+   *
+   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
+   */
+  protected $eventDispatcher;
+
+  /**
+   * ExportStorageManager constructor.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $active
+   *   The active config storage to prime the export storage.
+   * @param \Drupal\Core\State\StateInterface $state
+   *   The drupal state.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   The database connection.
+   * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher.
+   */
+  public function __construct(StorageInterface $active, StateInterface $state, Connection $connection, EventDispatcherInterface $event_dispatcher) {
+    $this->active = $active;
+    $this->state = $state;
+    $this->eventDispatcher = $event_dispatcher;
+    // 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() {
+    if ($this->state->get(self::NEEDS_REBUILD_KEY, TRUE)) {
+      self::replaceStorageContents($this->active, $this->storage);
+      // @todo: Use ConfigEvents::STORAGE_TRANSFORM_EXPORT in #2991683
+      $this->eventDispatcher->dispatch('config.transform.export', new StorageTransformEvent($this->storage));
+      $this->state->set(self::NEEDS_REBUILD_KEY, FALSE);
+    }
+
+    return new ReadOnlyStorage($this->storage);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events[ConfigEvents::SAVE][] = ['onConfigChange', 0];
+    $events[ConfigEvents::DELETE][] = ['onConfigChange', 0];
+    $events[ConfigEvents::RENAME][] = ['onConfigChange', 0];
+    return $events;
+  }
+
+  /**
+   * Set the flag in state that the export storage is out of date.
+   */
+  public function onConfigChange() {
+    if (!$this->state->get(self::NEEDS_REBUILD_KEY, FALSE)) {
+      $this->state->set(self::NEEDS_REBUILD_KEY, TRUE);
+    }
+  }
+
+}
diff --git a/core/modules/config_environment/src/Core/Config/ImportStorageTransformer.php b/core/modules/config_environment/src/Core/Config/ImportStorageTransformer.php
new file mode 100644
index 000000000000..639e982b741b
--- /dev/null
+++ b/core/modules/config_environment/src/Core/Config/ImportStorageTransformer.php
@@ -0,0 +1,82 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Core\Config in #2991683.
+// Use this class with its class alias Drupal\Core\Config\ImportStorageTransformer
+// @codingStandardsIgnoreEnd
+namespace Drupal\config_environment\Core\Config;
+
+use Drupal\Core\Database\Connection;
+use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+use Drupal\Core\Config\DatabaseStorage;
+use Drupal\Core\Config\StorageCopyTrait;
+use Drupal\Core\Config\StorageInterface;
+
+/**
+ * Class ImportStorageTransformer.
+ *
+ * @internal
+ */
+class ImportStorageTransformer {
+
+  use StorageCopyTrait;
+
+  /**
+   * The event dispatcher to get changes to the configuration.
+   *
+   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
+   */
+  protected $eventDispatcher;
+
+  /**
+   * The drupal database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $connection;
+
+  /**
+   * ImportStorageTransformer constructor.
+   *
+   * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   The database connection.
+   */
+  public function __construct(EventDispatcherInterface $event_dispatcher, Connection $connection) {
+    $this->eventDispatcher = $event_dispatcher;
+    $this->connection = $connection;
+  }
+
+  /**
+   * 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.
+   */
+  public function transform(StorageInterface $storage) {
+    // We use a database storage to reduce the memory requirement.
+    $mutable = new DatabaseStorage($this->connection, 'config_import');
+
+    // Copy the sync configuration to the created mutable storage.
+    self::replaceStorageContents($storage, $mutable);
+
+    // Dispatch the event so that event listeners can alter the configuration.
+    // @todo: Use ConfigEvents::STORAGE_TRANSFORM_IMPORT in #2991683
+    $this->eventDispatcher->dispatch('config.transform.import', new StorageTransformEvent($mutable));
+
+    // Return the storage with the altered configuration.
+    return $mutable;
+  }
+
+}
diff --git a/core/modules/config_environment/src/Core/Config/ManagedStorage.php b/core/modules/config_environment/src/Core/Config/ManagedStorage.php
new file mode 100644
index 000000000000..24f7eb52f98f
--- /dev/null
+++ b/core/modules/config_environment/src/Core/Config/ManagedStorage.php
@@ -0,0 +1,157 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Core\Config in #2991683.
+// Use this class with its class alias Drupal\Core\Config\ManagedStorage
+// @codingStandardsIgnoreEnd
+namespace Drupal\config_environment\Core\Config;
+
+use Drupal\Core\Config\StorageInterface;
+
+/**
+ * The managed storage defers all the storage method calls to the manager.
+ *
+ * The reason for deferring all the method calls is that the storage interface
+ * is the API but we potentially need to do an expensive transformation before
+ * the storage can be used so we can't do it in the constructor but we also
+ * don't know which method is called first.
+ *
+ * @internal
+ */
+class ManagedStorage implements StorageInterface {
+
+  /**
+   * The decorated storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $storage;
+
+  /**
+   * The storage manager to get the storage to decorate.
+   *
+   * @var \Drupal\Core\Config\StorageManagerInterface
+   */
+  protected $manager;
+
+  /**
+   * ManagedStorage constructor.
+   *
+   * @param \Drupal\Core\Config\StorageManagerInterface $manager
+   *   The storage manager.
+   */
+  public function __construct(StorageManagerInterface $manager) {
+    $this->manager = $manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function exists($name) {
+    return $this->getStorage()->exists($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function read($name) {
+    return $this->getStorage()->read($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function readMultiple(array $names) {
+    return $this->getStorage()->readMultiple($names);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function write($name, array $data) {
+    return $this->getStorage()->write($name, $data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete($name) {
+    return $this->getStorage()->delete($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function rename($name, $new_name) {
+    return $this->getStorage()->rename($name, $new_name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function encode($data) {
+    return $this->getStorage()->encode($data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function decode($raw) {
+    return $this->getStorage()->decode($raw);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function listAll($prefix = '') {
+    return $this->getStorage()->listAll($prefix);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteAll($prefix = '') {
+    return $this->getStorage()->deleteAll($prefix);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function createCollection($collection) {
+    // We return the collection directly.
+    // This means that the collection will not be an instance of ManagedStorage
+    // But this doesn't matter because the storage is retrieved from the
+    // manager only the first time it is accessed.
+    return $this->getStorage()->createCollection($collection);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getAllCollectionNames() {
+    return $this->getStorage()->getAllCollectionNames();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCollectionName() {
+    return $this->getStorage()->getCollectionName();
+  }
+
+  /**
+   * Get the decorated storage from the manager if necessary.
+   *
+   * @return \Drupal\Core\Config\StorageInterface
+   *   The config storage.
+   */
+  protected function getStorage() {
+    // Get the storage from the manager the first time it is needed.
+    if (!isset($this->storage)) {
+      $this->storage = $this->manager->getStorage();
+    }
+
+    return $this->storage;
+  }
+
+}
diff --git a/core/modules/config_environment/src/Core/Config/StorageManagerInterface.php b/core/modules/config_environment/src/Core/Config/StorageManagerInterface.php
new file mode 100644
index 000000000000..042d29de82cc
--- /dev/null
+++ b/core/modules/config_environment/src/Core/Config/StorageManagerInterface.php
@@ -0,0 +1,24 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Core\Config in #2991683.
+// Use this class with its class alias Drupal\Core\Config\StorageManagerInterface
+// @codingStandardsIgnoreEnd
+namespace Drupal\config_environment\Core\Config;
+
+/**
+ * Interface for a storage manager.
+ *
+ * @internal
+ */
+interface StorageManagerInterface {
+
+  /**
+   * Get the config storage.
+   *
+   * @return \Drupal\Core\Config\StorageInterface
+   *   The config storage.
+   */
+  public function getStorage();
+
+}
diff --git a/core/modules/config_environment/src/Core/Config/StorageTransformEvent.php b/core/modules/config_environment/src/Core/Config/StorageTransformEvent.php
new file mode 100644
index 000000000000..e0bc29cbfa66
--- /dev/null
+++ b/core/modules/config_environment/src/Core/Config/StorageTransformEvent.php
@@ -0,0 +1,52 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Core\Config in #2991683.
+// Use this class with its class alias Drupal\Core\Config\StorageTransformEvent
+// @codingStandardsIgnoreEnd
+namespace Drupal\config_environment\Core\Config;
+
+use Symfony\Component\EventDispatcher\Event;
+// @todo: below removed when namespace is \Drupal\Core\Config in 2991683.
+use Drupal\Core\Config\StorageInterface;
+
+/**
+ * Class StorageTransformEvent.
+ *
+ * This event allows subscribers to alter the configuration of the storage that
+ * is being transformed.
+ */
+class StorageTransformEvent extends Event {
+
+  /**
+   * The configuration storage which is transformed.
+   *
+   * This storage can be interacted with by event subscribers and will be
+   * used instead of the original storage after all event subscribers have been
+   * called.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $storage;
+
+  /**
+   * StorageTransformEvent constructor.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $storage
+   *   The storage with the configuration to transform.
+   */
+  public function __construct(StorageInterface $storage) {
+    $this->storage = $storage;
+  }
+
+  /**
+   * Returns the mutable storage ready to be read from and written to.
+   *
+   * @return \Drupal\Core\Config\StorageInterface
+   *   The config storage.
+   */
+  public function getStorage() {
+    return $this->storage;
+  }
+
+}
diff --git a/core/modules/config_environment/src/Form/ConfigSync.php b/core/modules/config_environment/src/Form/ConfigSync.php
new file mode 100644
index 000000000000..47b04c0ad5cc
--- /dev/null
+++ b/core/modules/config_environment/src/Form/ConfigSync.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\config_environment\Form;
+
+use Drupal\config\Form\ConfigSync as OriginalConfigSync;
+use Drupal\Core\Form\FormStateInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Overrides the ConfigSync form.
+ */
+class ConfigSync extends OriginalConfigSync {
+
+  /**
+   * The import transformer service.
+   *
+   * @var \Drupal\Core\Config\ImportStorageTransformer
+   */
+  protected $importTransformer;
+
+  /**
+   * The sync storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $originalSyncStorage;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    $form = parent::create($container);
+    $form->importTransformer = $container->get('config.import_transformer');
+    $form->originalSyncStorage = $form->syncStorage;
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $this->syncStorage = $this->importTransformer->transform($this->originalSyncStorage);
+
+    return parent::buildForm($form, $form_state);
+  }
+
+}
diff --git a/core/modules/config_environment/tests/config_transformer_test/config_transformer_test.info.yml b/core/modules/config_environment/tests/config_transformer_test/config_transformer_test.info.yml
new file mode 100644
index 000000000000..3fbdd561a0a8
--- /dev/null
+++ b/core/modules/config_environment/tests/config_transformer_test/config_transformer_test.info.yml
@@ -0,0 +1,10 @@
+# @todo: Move this test module under the config module in #2991683.
+name: 'Configuration Storage Transformer Test'
+type: module
+package: Testing
+version: VERSION
+core: 8.x
+dependencies:
+  - drupal:config
+# @todo: remove dependency on config_environment in #2991683.
+  - drupal:config_environment
diff --git a/core/modules/config_environment/tests/config_transformer_test/config_transformer_test.services.yml b/core/modules/config_environment/tests/config_transformer_test/config_transformer_test.services.yml
new file mode 100644
index 000000000000..05b9fccb8a0b
--- /dev/null
+++ b/core/modules/config_environment/tests/config_transformer_test/config_transformer_test.services.yml
@@ -0,0 +1,6 @@
+services:
+  config_transformer_test.event_subscriber:
+    class: Drupal\config_transformer_test\EventSubscriber
+    arguments: ['@config.storage', '@config.storage.sync']
+    tags:
+      - { name: event_subscriber }
diff --git a/core/modules/config_environment/tests/config_transformer_test/src/EventSubscriber.php b/core/modules/config_environment/tests/config_transformer_test/src/EventSubscriber.php
new file mode 100644
index 000000000000..72c8516087de
--- /dev/null
+++ b/core/modules/config_environment/tests/config_transformer_test/src/EventSubscriber.php
@@ -0,0 +1,102 @@
+<?php
+
+namespace Drupal\config_transformer_test;
+
+use Drupal\Core\Config\StorageInterface;
+use Drupal\Core\Config\StorageTransformEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Class EventSubscriber.
+ *
+ * The transformations here are for testing purposes only and do not constitute
+ * a well-behaved config storage transformation.
+ */
+class EventSubscriber implements EventSubscriberInterface {
+
+  /**
+   * The active config storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $active;
+
+  /**
+   * The sync config storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $sync;
+
+  /**
+   * EventSubscriber constructor.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $active
+   *   The active config storage.
+   * @param \Drupal\Core\Config\StorageInterface $sync
+   *   The sync config storage.
+   */
+  public function __construct(StorageInterface $active, StorageInterface $sync) {
+    $this->active = $active;
+    $this->sync = $sync;
+  }
+
+  /**
+   * The storage is transformed for importing.
+   *
+   * In this transformation we ignore the site name from the sync storage and
+   * set it always to the currently active site name with an additional string
+   * so that there will always be something to import.
+   * Do not do this outside of tests.
+   *
+   * @param \Drupal\Core\Config\StorageTransformEvent $event
+   *   The config storage transform event.
+   */
+  public function onImportTransform(StorageTransformEvent $event) {
+    $storage = $event->getStorage();
+    $site = $storage->read('system.site');
+    // Only change something if the sync storage has data.
+    if (!empty($site)) {
+      // Add "Arrr" to the site name. Because pirates!
+      // The site name which is in the sync directory will be ignored.
+      $current = $this->active->read('system.site');
+      $site['name'] = $current['name'] . ' Arrr';
+      $storage->write('system.site', $site);
+    }
+  }
+
+  /**
+   * The storage is transformed for exporting.
+   *
+   * In this transformation we ignore the site slogan from the site if the sync
+   * storage has it. Just export it again with an additional string so that
+   * there will always be something new exported.
+   * Do not do this outside of tests.
+   *
+   * @param \Drupal\Core\Config\StorageTransformEvent $event
+   *   The config storage transform event.
+   */
+  public function onExportTransform(StorageTransformEvent $event) {
+    $sync = $this->sync->read('system.site');
+    // Only change something if the sync storage has data.
+    if (!empty($sync)) {
+      $storage = $event->getStorage();
+      $site = $storage->read('system.site');
+      // Add "Arrr" to the site slogan. Because pirates!
+      // The active slogan will be ignored.
+      $site['slogan'] = $sync['slogan'] . ' Arrr';
+      $storage->write('system.site', $site);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    // @todo: use class constants when they get added in #2991683
+    $events['config.transform.import'][] = ['onImportTransform'];
+    $events['config.transform.export'][] = ['onExportTransform'];
+    return $events;
+  }
+
+}
diff --git a/core/modules/config_environment/tests/src/Functional/TransformedConfigExportImportUITest.php b/core/modules/config_environment/tests/src/Functional/TransformedConfigExportImportUITest.php
new file mode 100644
index 000000000000..6de7b7fbb3d3
--- /dev/null
+++ b/core/modules/config_environment/tests/src/Functional/TransformedConfigExportImportUITest.php
@@ -0,0 +1,100 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\Tests\config\Functional in #2991683.
+// @codingStandardsIgnoreEnd
+namespace Drupal\Tests\config_environment\Functional;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the user interface for importing/exporting transformed configuration.
+ *
+ * @group config
+ */
+class TransformedConfigExportImportUITest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'config',
+    'config_transformer_test',
+    'config_environment',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $permissions = [
+      'export configuration',
+      'import configuration',
+      'synchronize configuration',
+    ];
+    $this->webUser = $this->drupalCreateUser($permissions);
+    $this->drupalLogin($this->webUser);
+
+    // Start off with the sync storage being the same as the active storage.
+    $this->copyConfig($this->container->get('config.storage'), $this->container->get('config.storage.sync'));
+  }
+
+  /**
+   * Tests a simple site export import case.
+   */
+  public function testTransformedExportImport() {
+    // After installation there is no snapshot but a new site name.
+    $this->drupalGet('admin/config/development/configuration');
+    $this->assertNoText('Warning message');
+    $this->assertNoText('There are no configuration changes to import.');
+
+    // Tests changes of system.site.
+    $this->drupalGet('admin/config/development/configuration/sync/diff/system.site');
+    $this->assertText('name: Drupal');
+    $this->assertText(Html::escape("name: 'Drupal Arrr'"));
+
+    // Add a slogan.
+    $originalSlogan = $this->config('system.site')->get('slogan');
+    $this->assertEmpty($originalSlogan);
+    $newSlogan = $this->randomMachineName(16);
+    $this->assertNotEqual($newSlogan, $originalSlogan);
+    $this->config('system.site')
+      ->set('slogan', $newSlogan)
+      ->save();
+    $this->assertEqual($this->config('system.site')->get('slogan'), $newSlogan);
+
+    // Tests changes of system.site.
+    $this->drupalGet('admin/config/development/configuration/sync/diff/system.site');
+    $this->assertText(Html::escape("slogan: ''"));
+    $this->assertText(Html::escape("slogan: $newSlogan"));
+
+    // Export the configuration.
+    $this->drupalPostForm('admin/config/development/configuration/full/export', [], 'Export');
+    $tarball = $this->getSession()->getPage()->getContent();
+
+    // Import the configuration from the tarball.
+    $filename = 'temporary://' . $this->randomMachineName();
+    file_put_contents($filename, $tarball);
+    $this->drupalPostForm('admin/config/development/configuration/full/import', ['files[import_tarball]' => $filename], 'Upload');
+
+    // Assert the new name and slogan.
+    $this->drupalGet('admin/config/development/configuration/sync/diff/system.site');
+    $this->assertText(Html::escape("name: 'Drupal Arrr'"));
+    $this->assertText(Html::escape("slogan: '$originalSlogan Arrr'"));
+    $this->assertEqual($this->config('system.site')->get('name'), 'Drupal');
+    $this->assertEqual($this->config('system.site')->get('slogan'), $newSlogan);
+
+    // Sync the configuration.
+    $this->drupalPostForm('admin/config/development/configuration', [], 'Import all');
+    $this->assertEqual($this->config('system.site')->get('name'), 'Drupal Arrr');
+    $this->assertEqual($this->config('system.site')->get('slogan'), $originalSlogan . " Arrr");
+
+    // Assert that the event was dispatched again on the new config.
+    $this->drupalGet('admin/config/development/configuration/sync/diff/system.site');
+    $this->assertText(Html::escape("name: 'Drupal Arrr Arrr'"));
+  }
+
+}
diff --git a/core/modules/config_environment/tests/src/Kernel/Core/Config/ExportStorageManagerTest.php b/core/modules/config_environment/tests/src/Kernel/Core/Config/ExportStorageManagerTest.php
new file mode 100644
index 000000000000..ab6e30997683
--- /dev/null
+++ b/core/modules/config_environment/tests/src/Kernel/Core/Config/ExportStorageManagerTest.php
@@ -0,0 +1,62 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\KernelTests\Core\Config in #2991683.
+// @codingStandardsIgnoreEnd
+namespace Drupal\Tests\config_environment\Kernel\Core\Config;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests the export storage manager.
+ *
+ * @group config
+ */
+class ExportStorageManagerTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'system',
+    'config_transformer_test',
+    'config_environment',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->installConfig(['system']);
+  }
+
+  /**
+   * Test getting the export storage.
+   */
+  public function testGetStorage() {
+    // Get the raw system.site config and set it in the sync storage.
+    $rawConfig = $this->config('system.site')->getRawData();
+    $this->container->get('config.storage.sync')->write('system.site', $rawConfig);
+
+    $storage = $this->container->get('config.storage.export.manager')->getStorage();
+    $exported = $storage->read('system.site');
+    // The test subscriber adds "Arrr" to the slogan of the sync config.
+    $this->assertEquals($rawConfig['name'], $exported['name']);
+    $this->assertEquals($rawConfig['slogan'] . ' Arrr', $exported['slogan']);
+
+    // Save the config to trigger the rebuild.
+    $this->config('system.site')
+      ->set('name', 'New name')
+      ->set('slogan', 'New slogan')
+      ->save();
+
+    // Get the storage again.
+    $storage = $this->container->get('config.storage.export.manager')->getStorage();
+    $exported = $storage->read('system.site');
+    // The test subscriber adds "Arrr" to the slogan of the sync config.
+    $this->assertEquals('New name', $exported['name']);
+    $this->assertEquals($rawConfig['slogan'] . ' Arrr', $exported['slogan']);
+  }
+
+}
diff --git a/core/modules/config_environment/tests/src/Kernel/Core/Config/ImportStorageTransformerTest.php b/core/modules/config_environment/tests/src/Kernel/Core/Config/ImportStorageTransformerTest.php
new file mode 100644
index 000000000000..14a0f0526040
--- /dev/null
+++ b/core/modules/config_environment/tests/src/Kernel/Core/Config/ImportStorageTransformerTest.php
@@ -0,0 +1,63 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\KernelTests\Core\Config in #2991683.
+// @codingStandardsIgnoreEnd
+namespace Drupal\Tests\config_environment\Kernel\Core\Config;
+
+use Drupal\Core\Config\MemoryStorage;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests the import storage transformer.
+ *
+ * @group config
+ */
+class ImportStorageTransformerTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'system',
+    'config_transformer_test',
+    'config_environment',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->installConfig(['system']);
+  }
+
+  /**
+   * Test the import transformation.
+   */
+  public function testTransform() {
+    // Get the raw system.site config and set it in the sync storage.
+    $rawConfig = $this->config('system.site')->getRawData();
+
+    $storage = new MemoryStorage();
+    $this->copyConfig($this->container->get('config.storage'), $storage);
+
+    $import = $this->container->get('config.import_transformer')->transform($storage);
+    $config = $import->read('system.site');
+    // The test subscriber always adds "Arrr" to the current site name.
+    $this->assertEquals($rawConfig['name'] . ' Arrr', $config['name']);
+    $this->assertEquals($rawConfig['slogan'], $config['slogan']);
+
+    // Update the site config in the storage to test a second transformation.
+    $config['name'] = 'New name';
+    $config['slogan'] = 'New slogan';
+    $storage->write('system.site', $config);
+
+    $import = $this->container->get('config.import_transformer')->transform($storage);
+    $config = $import->read('system.site');
+    // The test subscriber always adds "Arrr" to the current site name.
+    $this->assertEquals($rawConfig['name'] . ' Arrr', $config['name']);
+    $this->assertEquals('New slogan', $config['slogan']);
+  }
+
+}
diff --git a/core/modules/config_environment/tests/src/Kernel/Core/Config/Storage/ManagedStorageTest.php b/core/modules/config_environment/tests/src/Kernel/Core/Config/Storage/ManagedStorageTest.php
new file mode 100644
index 000000000000..1b6ae9a72786
--- /dev/null
+++ b/core/modules/config_environment/tests/src/Kernel/Core/Config/Storage/ManagedStorageTest.php
@@ -0,0 +1,77 @@
+<?php
+
+// @codingStandardsIgnoreStart
+// @todo: Move this back to \Drupal\KernelTests\Core\Config\Storage in #2991683.
+// @codingStandardsIgnoreEnd
+namespace Drupal\Tests\config_environment\Kernel\Core\Config\Storage;
+
+// include class aliases. @todo: remove in #2991683.
+include_once __DIR__ . '/../../../../../../config_environment.module';
+
+use Drupal\Core\Config\StorageManagerInterface;
+use Drupal\Core\Config\ManagedStorage;
+use Drupal\Core\Config\MemoryStorage;
+use Drupal\KernelTests\Core\Config\Storage\ConfigStorageTestBase;
+
+/**
+ * Tests ManagedStorage operations.
+ *
+ * @group config
+ */
+class ManagedStorageTest extends ConfigStorageTestBase implements StorageManagerInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorage() {
+    // We return a new storage every time to make sure the managed storage
+    // only calls this once and retains the configuration by itself.
+    return new MemoryStorage();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->storage = new ManagedStorage($this);
+    // ::listAll() verifications require other configuration data to exist.
+    $this->storage->write('system.performance', []);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function read($name) {
+    return $this->storage->read($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function insert($name, $data) {
+    $this->storage->write($name, $data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function update($name, $data) {
+    $this->storage->write($name, $data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function delete($name) {
+    $this->storage->delete($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testInvalidStorage() {
+    $this->markTestSkipped('ManagedStorage cannot be invalid.');
+  }
+
+}