Issue #2302617 by dawehner, chx | Crell: Define a standard mechaism for backend-aware service overrides.

core/lib/Drupal/Core/CoreServiceProvider.php

@@ -9,6 +9,7 @@ namespace Drupal\Core;
 
 use Drupal\Core\Cache\CacheContextsPass;
 use Drupal\Core\Cache\ListCacheBinsPass;
+use Drupal\Core\DependencyInjection\Compiler\BackendCompilerPass;
 use Drupal\Core\DependencyInjection\ServiceProviderInterface;
 use Drupal\Core\DependencyInjection\ContainerBuilder;
 use Drupal\Core\DependencyInjection\Compiler\ModifyServiceDefinitionsPass;
@@ -49,6 +50,8 @@ class CoreServiceProvider implements ServiceProviderInterface  {
     // list-building passes are operating on the post-alter services list.
     $container->addCompilerPass(new ModifyServiceDefinitionsPass());
 
+    $container->addCompilerPass(new BackendCompilerPass());
+
     // Collect tagged handler services as method calls on consumer services.
     $container->addCompilerPass(new TaggedHandlersPass());
 

core/lib/Drupal/Core/DependencyInjection/Compiler/BackendCompilerPass.php

@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\DependencyInjection\Compiler\BackendCompilerPass.
+ */
+
+namespace Drupal\Core\DependencyInjection\Compiler;
+
+use Symfony\Component\DependencyInjection\Alias;
+use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+
+/**
+ * Defines a compiler pass to allow automatic override per backend.
+ *
+ * A module developer has to tag his backend service with "backend_overridable":
+ * @code
+ * custom_service:
+ *   class: ...
+ *   tags:
+ *     - { name: backend_overridable }
+ * @endcode
+ *
+ * As a site admin you set the 'default_backend' in your services.yml file:
+ * @code
+ * parameters:
+ *   default_backend: sqlite
+ * @endcode
+ *
+ * As a developer for alternative storage engines you register a service with
+ * $yourbackend.$original_service:
+ *
+ * @code
+ * sqlite.custom_service:
+ *   class: ...
+ * @endcode
+ */
+class BackendCompilerPass implements CompilerPassInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function process(ContainerBuilder $container) {
+    $default_backend = $container->hasParameter('default_backend') ? $container->getParameter('default_backend') : NULL;
+    // No default backend was configured, so continue as normal.
+    if (!isset($default_backend)) {
+      return;
+    }
+
+    foreach ($container->findTaggedServiceIds('backend_overridable') as $id => $attributes) {
+      // If the service is already an alias it is not the original backend, so
+      // we don't want to fallback to other storages any longer.
+      if ($container->hasAlias($id)) {
+        continue;
+      }
+      if ($container->hasDefinition("$default_backend.$id") || $container->hasAlias("$default_backend.$id")) {
+        $container->setAlias($id, new Alias("$default_backend.$id"));
+      }
+    }
+  }
+
+}

core/tests/Drupal/Tests/Core/DependencyInjection/Compiler/BackendCompilerPassTest.php

@@ -0,0 +1,93 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\DependencyInjection\Compiler\BackendCompilerPassTest.
+ */
+
+namespace Drupal\Tests\Core\DependencyInjection\Compiler;
+
+use Drupal\Core\DependencyInjection\Compiler\BackendCompilerPass;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\DependencyInjection\Alias;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Definition;
+
+/**
+ * @coversDefaultClass \Drupal\Core\DependencyInjection\Compiler\BackendCompilerPass
+ * @group DependencyInjection
+ */
+class BackendCompilerPassTest extends UnitTestCase {
+
+  /**
+   * The tested backend compiler pass.
+   *
+   * @var \Drupal\Core\DependencyInjection\Compiler\BackendCompilerPass
+   */
+  protected $backendPass;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    $this->backendPass = new BackendCompilerPass();
+  }
+
+  /**
+   * Tests the process method.
+   *
+   * @param string $expected_class
+   *   The expected used class.
+   * @param \Symfony\Component\DependencyInjection\ContainerBuilder $container
+   *   The container.
+   *
+   * @dataProvider providerTestProcess
+   *
+   * @covers ::process
+   */
+  public function testProcess($expected_class, ContainerBuilder $container) {
+    $this->backendPass->process($container);
+
+    $this->assertInstanceOf($expected_class, $container->get('service'));
+  }
+
+  /**
+   * Provides test data for testProcess().
+   *
+   * @return array
+   */
+  public function providerTestProcess() {
+    $data = array();
+    // Add a container with no set default_backend.
+    $container = new ContainerBuilder();
+    $prefix = '\\' . __NAMESPACE__ . '\\';
+    $container->setDefinition('service', (new Definition($prefix . 'ServiceClassDefault'))->addTag('backend_overridable'));
+    $container->setDefinition('mysql.service', new Definition($prefix . 'ServiceClassMysql'));
+
+    $data[] = array($prefix . 'ServiceClassDefault', $container);
+
+    // Set the default_backend so the mysql service should be used.
+    $container = clone $container;
+    $container->setParameter('default_backend', 'mysql');
+    $data[] = array($prefix . 'ServiceClassMysql', $container);
+
+    // Configure a manual alias for the service, so ensure that it is not
+    // overridden by the default backend.
+    $container = clone $container;
+    $container->setDefinition('mariadb.service', new Definition($prefix . 'ServiceClassMariaDb'));
+    $container->setAlias('service', new Alias('mariadb.service'));
+    $data[] = array($prefix . 'ServiceClassMariaDb', $container);
+
+    return $data;
+  }
+
+}
+
+class ServiceClassDefault {
+}
+
+class ServiceClassMysql extends ServiceClassDefault {
+}
+
+class ServiceClassMariaDb extends ServiceClassMysql {
+}