[1.5.x] Fixed #20681 -- Prevented teardown_databases from attempting to tear down aliases

Thanks simonpercivall.

Backport of d9c580306c from master

django/test/simple.py

@@ -310,12 +310,14 @@ class DjangoTestSuiteRunner(object):
 
             for alias in aliases:
                 connection = connections[alias]
-                old_names.append((connection, db_name, True))
                 if test_db_name is None:
                     test_db_name = connection.creation.create_test_db(
                             self.verbosity, autoclobber=not self.interactive)
+                    destroy = True
                 else:
                     connection.settings_dict['NAME'] = test_db_name
+                    destroy = False
+                old_names.append((connection, db_name, destroy))
 
         for alias, mirror_alias in mirrored_aliases.items():
             mirrors.append((alias, connections[alias].settings_dict['NAME']))

tests/regressiontests/test_runner/tests.py

@@ -310,6 +310,40 @@ class AliasedDefaultTestSetupTest(unittest.TestCase):
             db.connections = old_db_connections
 
 
+class AliasedDatabaseTest(unittest.TestCase):
+    def test_setup_aliased_databases(self):
+        from django.db.backends.dummy.base import DatabaseCreation
+
+        runner = DjangoTestSuiteRunner(verbosity=0)
+        old_db_connections = db.connections
+        old_destroy_test_db = DatabaseCreation.destroy_test_db
+        old_create_test_db = DatabaseCreation.create_test_db
+        try:
+            destroyed_names = []
+            DatabaseCreation.destroy_test_db = lambda self, old_database_name, verbosity=1: destroyed_names.append(old_database_name)
+            DatabaseCreation.create_test_db = lambda self, verbosity=1, autoclobber=False: self._get_test_db_name()
+
+            db.connections = db.ConnectionHandler({
+                'default': {
+                    'ENGINE': 'django.db.backends.dummy',
+                    'NAME': 'dbname',
+                },
+                'other': {
+                    'ENGINE': 'django.db.backends.dummy',
+                    'NAME': 'dbname',
+                }
+            })
+
+            old_config = runner.setup_databases()
+            runner.teardown_databases(old_config)
+
+            self.assertEquals(destroyed_names.count('dbname'), 1)
+        finally:
+            DatabaseCreation.create_test_db = old_create_test_db
+            DatabaseCreation.destroy_test_db = old_destroy_test_db
+            db.connections = old_db_connections
+
+
 class DeprecationDisplayTest(AdminScriptTestCase):
     # tests for 19546
     def setUp(self):