Fixed #32446 -- Deprecated SERIALIZE test database setting.

Whether or not the state of a test database should be serialized can be
inferred from the set of databases allowed to be access from discovered
TestCase/TransactionTestCase enabling the serialized_rollback feature
which makes this setting unnecessary.

This should make a significant test suite bootstraping time difference
on large projects that didn't explicitly disable test database
serialization.

2 files changed, 25 insertions, 10 deletions

diff --git a/tests/test_runner/test_discover_runner.py b/tests/test_runner/test_discover_runner.py
index ee95f9da72..1b5177ccf7 100644
--- a/tests/test_runner/test_discover_runner.py
+++ b/tests/test_runner/test_discover_runner.py
@@ -354,7 +354,7 @@ class DiscoverRunnerGetDatabasesTests(SimpleTestCase):
     def assertSkippedDatabases(self, test_labels, expected_databases):
         databases, output = self.get_databases(test_labels)
         self.assertEqual(databases, expected_databases)
-        skipped_databases = set(connections) - expected_databases
+        skipped_databases = set(connections) - set(expected_databases)
         if skipped_databases:
             self.assertIn(self.skip_msg + ', '.join(sorted(skipped_databases)), output)
         else:
@@ -362,31 +362,37 @@ class DiscoverRunnerGetDatabasesTests(SimpleTestCase):
 
     def test_mixed(self):
         databases, output = self.get_databases(['test_runner_apps.databases.tests'])
-        self.assertEqual(databases, set(connections))
+        self.assertEqual(databases, {'default': True, 'other': False})
         self.assertNotIn(self.skip_msg, output)
 
     def test_all(self):
         databases, output = self.get_databases(['test_runner_apps.databases.tests.AllDatabasesTests'])
-        self.assertEqual(databases, set(connections))
+        self.assertEqual(databases, {alias: False for alias in connections})
         self.assertNotIn(self.skip_msg, output)
 
     def test_default_and_other(self):
         self.assertSkippedDatabases([
             'test_runner_apps.databases.tests.DefaultDatabaseTests',
             'test_runner_apps.databases.tests.OtherDatabaseTests',
-        ], {'default', 'other'})
+        ], {'default': False, 'other': False})
 
     def test_default_only(self):
         self.assertSkippedDatabases([
             'test_runner_apps.databases.tests.DefaultDatabaseTests',
-        ], {'default'})
+        ], {'default': False})
 
     def test_other_only(self):
         self.assertSkippedDatabases([
             'test_runner_apps.databases.tests.OtherDatabaseTests'
-        ], {'other'})
+        ], {'other': False})
 
     def test_no_databases_required(self):
         self.assertSkippedDatabases([
             'test_runner_apps.databases.tests.NoDatabaseTests'
-        ], set())
+        ], {})
+
+    def test_serialize(self):
+        databases, _ = self.get_databases([
+            'test_runner_apps.databases.tests.DefaultDatabaseSerializedTests'
+        ])
+        self.assertEqual(databases, {'default': True})
diff --git a/tests/test_runner/tests.py b/tests/test_runner/tests.py
index 87d432a2ab..5dc3126581 100644
--- a/tests/test_runner/tests.py
+++ b/tests/test_runner/tests.py
@@ -11,12 +11,15 @@ from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
 from django.core.management import call_command
 from django.core.management.base import SystemCheckError
-from django.test import TransactionTestCase, skipUnlessDBFeature
+from django.test import (
+    SimpleTestCase, TransactionTestCase, skipUnlessDBFeature,
+)
 from django.test.runner import DiscoverRunner
 from django.test.testcases import connections_support_transactions
 from django.test.utils import (
     captured_stderr, dependency_ordered, get_unique_databases_and_mirrors,
 )
+from django.utils.deprecation import RemovedInDjango50Warning
 
 from .models import B, Person, Through
 
@@ -315,7 +318,7 @@ class AliasedDefaultTestSetupTest(unittest.TestCase):
             runner_instance.teardown_databases(old_config)
 
 
-class SetupDatabasesTests(unittest.TestCase):
+class SetupDatabasesTests(SimpleTestCase):
 
     def setUp(self):
         self.runner_instance = DiscoverRunner(verbosity=0)
@@ -398,9 +401,15 @@ class SetupDatabasesTests(unittest.TestCase):
                 'TEST': {'SERIALIZE': False},
             },
         })
+        msg = (
+            'The SERIALIZE test database setting is deprecated as it can be '
+            'inferred from the TestCase/TransactionTestCase.databases that '
+            'enable the serialized_rollback feature.'
+        )
         with mock.patch('django.db.backends.dummy.base.DatabaseWrapper.creation_class') as mocked_db_creation:
             with mock.patch('django.test.utils.connections', new=tested_connections):
-                self.runner_instance.setup_databases()
+                with self.assertWarnsMessage(RemovedInDjango50Warning, msg):
+                    self.runner_instance.setup_databases()
         mocked_db_creation.return_value.create_test_db.assert_called_once_with(
             verbosity=0, autoclobber=False, serialize=False, keepdb=False
         )