Issue #1925658 by gielfeldt, mitron: Fixed Use of slaves breaks site.

core/modules/system/lib/Drupal/system/Tests/System/SystemInitTest.php

+<?php
+
+/**
+ * Definition of Drupal\system\Tests\System\SystemInitTest.
+ */
+
+namespace Drupal\system\Tests\System;
+
+use Drupal\Core\Database\Database;
+use \Drupal\simpletest\UnitTestBase;
+use \Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Tests system_init().
+ */
+class SystemInitTest extends UnitTestBase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'System Init',
+      'description' => 'Tests the system_init function in system.module.',
+      'group' => 'System',
+    );
+  }
+
+  /**
+   * Tests that system_init properly ignores slaves when requested.
+   */
+  function testSystemInitIgnoresSlaves() {
+    // Clone the master credentials to a slave connection.
+    // Note this will result in two independent connection objects that happen
+    // to point to the same place.
+    $connection_info = Database::getConnectionInfo('default');
+    Database::addConnectionInfo('default', 'slave', $connection_info['default']);
+
+    db_ignore_slave();
+    system_init();
+
+    $db1 = Database::getConnection('default', 'default');
+    $db2 = Database::getConnection('slave', 'default');
+
+    $this->assertIdentical($db1, $db2, 'System Init ignores slaves when requested.');
+  }
+}

core/modules/system/system.module

@@ -5,6 +5,7 @@
  * Configuration system that lets administrators modify the workings of the site.
  */
 
+use Drupal\Core\Database\Database;
 use Drupal\Core\Utility\ModuleInfo;
 use Drupal\Core\TypedData\Primitive;
 use Drupal\system\Plugin\block\block\SystemMenuBlock;