Issue #3390693 by mikelutz, smustgrave:...

Issue #3390693 by mikelutz, smustgrave: MigrationPluginManager::ExpandPluginIds can lose derivative plugins under certain circumstances

core/modules/migrate/src/Plugin/MigrationPluginManager.php

@@ -145,7 +145,7 @@ public function expandPluginIds(array $migration_ids) {
     $plugin_ids = [];
     $all_ids = array_keys($this->getDefinitions());
     foreach ($migration_ids as $id) {
-      $plugin_ids += preg_grep('/^' . preg_quote($id, '/') . PluginBase::DERIVATIVE_SEPARATOR . '/', $all_ids);
+      $plugin_ids = array_merge($plugin_ids, preg_grep('/^' . preg_quote($id, '/') . PluginBase::DERIVATIVE_SEPARATOR . '/', $all_ids));
       if ($this->hasDefinition($id)) {
         $plugin_ids[] = $id;
       }

core/modules/migrate/tests/src/Unit/MigrationPluginManagerTest.php

@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\migrate\Unit;
 
+use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\migrate\Plugin\Migration;
 use Drupal\migrate\Plugin\MigrationPluginManager;
 use Drupal\Tests\UnitTestCase;
@@ -65,6 +66,25 @@ public function testDependencyBuilding($migrations_data, $result_ids) {
     }
   }
 
+  /**
+   * Tests that expandPluginIds returns all derivatives.
+   */
+  public function testExpandPluginIds() {
+    $backend = $this->prophesize(CacheBackendInterface::class);
+    $cache = new \stdClass();
+    $cache->data = [
+      'a:a' => ['provider' => 'core'],
+      'a:b' => ['provider' => 'core'],
+      'b' => ['provider' => 'core'],
+    ];
+    $backend->get('migration_plugins')->willReturn($cache);
+    $this->pluginManager->setCacheBackend($backend->reveal(), 'migration_plugins');
+    $plugin_ids = $this->pluginManager->expandPluginIds(['b', 'a']);
+    $this->assertContains('a:a', $plugin_ids);
+    $this->assertContains('a:b', $plugin_ids);
+    $this->assertContains('b', $plugin_ids);
+  }
+
   /**
    * Provide dependency data for testing.
    */