Issue #2210521 by mitrpaka, vlad.n, Ema.jar, donquixote, dawehner, klausi, alexpott: Settings::getInstance() should throw an exception if called before being initialized

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

@@ -24,7 +24,7 @@ final class Settings {
    *
    * @var \Drupal\Core\Site\Settings
    */
-  private static $instance;
+  private static $instance = NULL;
 
   /**
    * Constructor.
@@ -44,8 +44,14 @@ final class Settings {
    * available.
    *
    * @return \Drupal\Core\Site\Settings
+   *
+   * @throws \BadMethodCallException
+   *   Thrown when the settings instance has not been initialized yet.
    */
   public static function getInstance() {
+    if (self::$instance === NULL) {
+      throw new \BadMethodCallException('Settings::$instance is not initialized yet. Whatever you are trying to do, it might be too early for that. You could call Settings::initialize(), but it is probably better to wait until it is called in the regular way. Also check for recursions.');
+    }
     return self::$instance;
   }
 

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

@@ -126,4 +126,21 @@ class SettingsTest extends UnitTestCase {
     $this->assertNotEquals($settings::getApcuPrefix('cache_test', '/test/a'), $settings::getApcuPrefix('cache_test', '/test/b'));
   }
 
+  /**
+   * Tests that an exception is thrown when settings are not initialized yet.
+   *
+   * @covers ::getInstance
+   */
+  public function testGetInstanceReflection() {
+    $settings = new Settings(array());
+
+    $class = new \ReflectionClass(Settings::class);
+    $instace_property = $class->getProperty("instance");
+    $instace_property->setAccessible(TRUE);
+    $instace_property->setValue(NULL);
+
+    $this->setExpectedException(\BadMethodCallException::class);
+    $settings->getInstance();
+  }
+
 }