Issue #2451363 by alexpott, Berdir, pjcdawkins: Ensure install_profile is exists in settings.php after installation

core/includes/install.core.inc

@@ -730,6 +730,8 @@ function install_tasks($install_state) {
       'run' => $install_state['settings_verified'] ? INSTALL_TASK_SKIP : INSTALL_TASK_RUN_IF_NOT_COMPLETED,
       'function' => 'Drupal\Core\Installer\Form\SiteSettingsForm',
     ),
+    'install_write_profile' => array(
+    ),
     'install_verify_database_ready' => array(
       'run' => $install_state['database_ready'] ? INSTALL_TASK_SKIP : INSTALL_TASK_RUN_IF_NOT_COMPLETED,
     ),
@@ -2263,3 +2265,20 @@ function install_display_requirements($install_state, $requirements) {
     }
   }
 }
+
+/**
+ * Installation task; ensures install profile is written to settings.php.
+ *
+ * @param array $install_state
+ *   An array of information about the current installation state.
+ */
+function install_write_profile($install_state) {
+  if (Settings::get('install_profile') !== $install_state['parameters']['profile']) {
+    // Remember the profile which was used.
+    $settings['settings']['install_profile'] = (object) array(
+      'value' => $install_state['parameters']['profile'],
+      'required' => TRUE,
+    );
+    drupal_rewrite_settings($settings);
+  }
+}

core/modules/system/src/Tests/Installer/InstallerExistingSettingsNoProfileTest.php

@@ -0,0 +1,79 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Installer\InstallerExistingSettingsNoProfileTest.
+ */
+
+namespace Drupal\system\Tests\Installer;
+
+use Drupal\Core\Site\Settings;
+use Drupal\simpletest\InstallerTestBase;
+use Drupal\Core\Database\Database;
+
+/**
+ * Tests the installer with an existing settings file but no install profile.
+ *
+ * @group Installer
+ */
+class InstallerExistingSettingsNoProfileTest extends InstallerTestBase {
+
+  /**
+   * {@inheritdoc}
+   *
+   * Configures a preexisting settings.php file without an install_profile
+   * setting before invoking the interactive installer.
+   */
+  protected function setUp() {
+    // Pre-configure hash salt.
+    // Any string is valid, so simply use the class name of this test.
+    $this->settings['settings']['hash_salt'] = (object) array(
+      'value' => __CLASS__,
+      'required' => TRUE,
+    );
+
+    // Pre-configure database credentials.
+    $connection_info = Database::getConnectionInfo();
+    unset($connection_info['default']['pdo']);
+    unset($connection_info['default']['init_commands']);
+
+    $this->settings['databases']['default'] = (object) array(
+      'value' => $connection_info,
+      'required' => TRUE,
+    );
+
+    // Pre-configure config directories.
+    $this->settings['config_directories'] = array(
+      CONFIG_ACTIVE_DIRECTORY => (object) array(
+        'value' => conf_path() . '/files/config_active',
+        'required' => TRUE,
+      ),
+      CONFIG_STAGING_DIRECTORY => (object) array(
+        'value' => conf_path() . '/files/config_staging',
+        'required' => TRUE,
+      ),
+    );
+    mkdir($this->settings['config_directories'][CONFIG_ACTIVE_DIRECTORY]->value, 0777, TRUE);
+    mkdir($this->settings['config_directories'][CONFIG_STAGING_DIRECTORY]->value, 0777, TRUE);
+
+    parent::setUp();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUpSettings() {
+    // This step should not appear, since settings.php is fully configured
+    // already.
+  }
+
+  /**
+   * Verifies that installation succeeded.
+   */
+  public function testInstaller() {
+    $this->assertUrl('user/1');
+    $this->assertResponse(200);
+    $this->assertEqual('testing', Settings::get('install_profile'));
+  }
+
+}

core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php

@@ -31,12 +31,10 @@ class InstallerExistingSettingsTest extends InstallerTestBase {
       'required' => TRUE,
     );
 
-    // Actually the install profile should be skipped to because it is written
-    // to settings.php.
-    // @todo https://www.drupal.org/node/2451369 Fix install_profile so that it
-    //   is written to an existing settings.php if possible or if set used.
+    // During interactive install we'll change this to a different profile and
+    // this test will ensure that the new value is written to settings.php.
     $this->settings['settings']['install_profile'] = (object) array(
-      'value' => 'testing',
+      'value' => 'minimal',
       'required' => TRUE,
     );
 
@@ -81,6 +79,7 @@ class InstallerExistingSettingsTest extends InstallerTestBase {
   public function testInstaller() {
     $this->assertUrl('user/1');
     $this->assertResponse(200);
+    $this->assertEqual('testing', drupal_get_profile(), 'Profile was changed from minimal to testing during interactive install.');
   }
 
 }

sites/default/default.settings.php

@@ -255,6 +255,18 @@ $config_directories = array();
  * @see \Drupal\Core\Site\Settings::get()
  */
 
+/**
+ * The active installation profile.
+ *
+ * Changing this after installation is not recommended as it changes which
+ * directories are scanned during extension discovery. If this is set prior to
+ * installation this value will be rewritten according to the profile selected
+ * by the user.
+ *
+ * @see install_select_profile()
+ */
+# $settings['install_profile'] = '';
+
 /**
  * Salt for one-time login links, cancel links, form tokens, etc.
  *