Issue #2530936 by webflo: Ensure that the classloader prefix is really unique

core/lib/Drupal/Core/Site/Settings.php

@@ -170,7 +170,7 @@ public static function getHashSalt() {
    */
   public static function getApcuPrefix($identifier, $root, $site_path = '') {
     if (static::get('apcu_ensure_unique_prefix', TRUE)) {
-      return 'drupal.' . $identifier . '.' . hash_hmac('sha256', $identifier, static::get('hash_salt', $root . '/' . $site_path));
+      return 'drupal.' . $identifier . '.' . hash_hmac('sha256', $identifier, static::get('hash_salt') . '.' . $root . '/' . $site_path);
     }
     return 'drupal.' . $identifier . '.' . Crypt::hashBase64($root . '/' . $site_path);
   }

core/tests/Drupal/Tests/Core/Site/SettingsTest.php

@@ -118,4 +118,17 @@ public function testSerialize() {
     serialize(new Settings([]));
   }
 
+  /**
+   * Tests Settings::getApcuPrefix().
+   *
+   * @covers ::getApcuPrefix
+   */
+  public function testGetApcuPrefix() {
+    $settings = new Settings(array('hash_salt' => 123));
+    $this->assertNotEquals($settings::getApcuPrefix('cache_test', '/test/a'), $settings::getApcuPrefix('cache_test', '/test/b'));
+
+    $settings = new Settings(array('hash_salt' => 123, 'apcu_ensure_unique_prefix' => FALSE));
+    $this->assertNotEquals($settings::getApcuPrefix('cache_test', '/test/a'), $settings::getApcuPrefix('cache_test', '/test/b'));
+  }
+
 }