Issue #2848821 by voleger, harsha012, vidhatanand, Sharique, alexpott, mondrake: Replace all calls to db_close, which is deprecated

core/includes/database.inc

@@ -456,6 +456,7 @@ function db_driver() {
  * @see \Drupal\Core\Database\Database::closeConnection()
  */
 function db_close(array $options = []) {
+  @trigger_error('db_close() is deprecated in Drupal 8.0.x and will be removed before Drupal 9.0.0. Use \Drupal\Core\Database\Database::closeConnection() instead. See https://www.drupal.org/node/2993033.', E_USER_DEPRECATED);
   if (empty($options['target'])) {
     $options['target'] = NULL;
   }

core/lib/Drupal/Core/Database/Driver/pgsql/Install/Tasks.php

@@ -156,7 +156,7 @@ class Tasks extends InstallTasks {
 
         // Close the database connection so that the configuration parameter
         // is applied to the current connection.
-        db_close();
+        Database::closeConnection();
 
         // Recheck, if it fails, finally just rely on the end user to do the
         // right thing.

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

@@ -4,6 +4,8 @@ namespace Drupal\KernelTests\Core\Database;
 
 use Drupal\Core\Database\Transaction;
 
+use Drupal\Core\Database\Database;
+
 /**
  * Deprecation tests cases for the database layer.
  *
@@ -65,4 +67,15 @@ class DatabaseLegacyTest extends DatabaseTestBase {
     $this->assertInstanceOf(Transaction::class, db_transaction());
   }
 
+  /**
+   * Tests the db_close() function.
+   *
+   * @expectedDeprecation db_close() is deprecated in Drupal 8.0.x and will be removed before Drupal 9.0.0. Use \Drupal\Core\Database\Database::closeConnection() instead. See https://www.drupal.org/node/2993033.
+   */
+  public function testDbClose() {
+    $this->assertTrue(Database::isActiveConnection(), 'Database connection is active');
+    db_close();
+    $this->assertFalse(Database::isActiveConnection(), 'Database connection is not active');
+  }
+
 }