Issue #2999586 by interX, andypost, mondrake: Properly deprecate db_add_index

core/includes/database.inc

@@ -972,7 +972,8 @@ function db_drop_unique_key($table, $name) {
  * @see \Drupal\Core\Database\Schema::addIndex()
  */
 function db_add_index($table, $name, $fields, array $spec) {
-  return Database::getConnection()->schema()->addIndex($table, $name, $fields, $spec);
+  @trigger_error('db_add_index() is deprecated in Drupal 8.0.x and will be removed in Drupal 9.0.0. Instead, get a database connection injected into your service from the container, get its schema driver, and call addIndex() on it. For example, $injected_database->schema()->addIndex($table, $name, $fields, $spec). See https://www.drupal.org/node/2993033', E_USER_DEPRECATED);
+  Database::getConnection()->schema()->addIndex($table, $name, $fields, $spec);
 }
 
 /**

core/modules/system/tests/modules/update_test_schema/update_test_schema.install

@@ -5,6 +5,8 @@
  * Update hooks and schema definition for the update_test_schema module.
  */
 
+use Drupal\Core\Database\Database;
+
 /**
  * Implements hook_schema().
  *
@@ -44,7 +46,7 @@ function update_test_schema_update_8001() {
     ];
 
     // Add a column.
-    db_add_index('update_test_schema_table', 'test', ['a'], $table);
+    Database::getConnection()->schema()->addIndex('update_test_schema_table', 'test', ['a'], $table);
   }
 
 }

core/tests/Drupal/KernelTests/Core/Database/DatabaseLegacyTest.php

@@ -426,4 +426,24 @@ public function testDbQueryRange() {
     $this->assertSame(3, $count);
   }
 
+  /**
+   * Tests deprecation of the db_add_index() function.
+   *
+   * @expectedDeprecation db_add_index() is deprecated in Drupal 8.0.x and will be removed in Drupal 9.0.0. Instead, get a database connection injected into your service from the container, get its schema driver, and call addIndex() on it. For example, $injected_database->schema()->addIndex($table, $name, $fields, $spec). See https://www.drupal.org/node/2993033
+   */
+  public function testDbAddIndex() {
+    $table_specification = [
+      'fields' => [
+        'age' => [
+          'description' => "The person's age",
+          'type' => 'int',
+          'unsigned' => TRUE,
+          'not null' => TRUE,
+          'default' => 0,
+        ],
+      ],
+    ];
+    $this->assertNull(db_add_index('test', 'test', ['age'], $table_specification));
+  }
+
 }