[2.1.x] Fixed #29827 -- Fixed reuse of test databases with --keepdb on MySQL.

Regression in e1253bc26f.
Backport of 9a88c6dd6a from master.

django/db/backends/mysql/creation.py

@@ -19,16 +19,6 @@ class DatabaseCreation(BaseDatabaseCreation):
 
     def _execute_create_test_db(self, cursor, parameters, keepdb=False):
         try:
-            if keepdb:
-                # If the database should be kept, add "IF NOT EXISTS" to avoid
-                # "database exists" error, also temporarily disable "database
-                # exists" warning.
-                cursor.execute('''
-                    SET @_tmp_sql_notes := @@sql_notes, sql_notes = 0;
-                    CREATE DATABASE IF NOT EXISTS %(dbname)s %(suffix)s;
-                    SET sql_notes = @_tmp_sql_notes;
-                ''' % parameters)
-            else:
             super()._execute_create_test_db(cursor, parameters, keepdb)
         except Exception as e:
             if len(e.args) < 1 or e.args[0] != 1007:
@@ -49,6 +39,9 @@ class DatabaseCreation(BaseDatabaseCreation):
             try:
                 self._execute_create_test_db(cursor, test_db_params, keepdb)
             except Exception:
+                if keepdb:
+                    # If the database should be kept, skip everything else.
+                    return
                 try:
                     if verbosity >= 1:
                         print("Destroying old test database for alias %s..." % (
@@ -59,7 +52,9 @@ class DatabaseCreation(BaseDatabaseCreation):
                 except Exception as e:
                     sys.stderr.write("Got an error recreating the test database: %s\n" % e)
                     sys.exit(2)
+        self._clone_db(source_database_name, target_database_name)
 
+    def _clone_db(self, source_database_name, target_database_name):
         dump_cmd = DatabaseClient.settings_to_cmd_args(self.connection.settings_dict)
         dump_cmd[0] = 'mysqldump'
         dump_cmd[-1] = source_database_name

docs/releases/2.1.3.txt

@@ -14,3 +14,6 @@ Bugfixes
 
 * Fixed a regression in Django 1.11 where ``django-admin shell`` may hang
   on startup (:ticket:`29774`).
+
+* Fixed a regression in Django 2.0 where test databases aren't reused with
+  ``manage.py test --keepdb`` on MySQL (:ticket:`29827`).

tests/backends/mysql/test_creation.py

@@ -43,3 +43,10 @@ class DatabaseCreationTests(SimpleTestCase):
         with self.patch_test_db_creation(self._execute_raise_access_denied):
             with self.assertRaises(SystemExit):
                 creation._create_test_db(verbosity=0, autoclobber=False, keepdb=False)
+
+    def test_clone_test_db_database_exists(self):
+        creation = DatabaseCreation(connection)
+        with self.patch_test_db_creation(self._execute_raise_database_exists):
+            with mock.patch.object(DatabaseCreation, '_clone_db') as _clone_db:
+                creation._clone_test_db('suffix', verbosity=0, keepdb=True)
+                _clone_db.assert_not_called()