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.

1 files changed, 32 insertions, 3 deletions

diff --git a/django/test/utils.py b/django/test/utils.py
index b91532e597..c019d77354 100644
--- a/django/test/utils.py
+++ b/django/test/utils.py
@@ -25,6 +25,7 @@ from django.db.models.options import Options
 from django.template import Template
 from django.test.signals import setting_changed, template_rendered
 from django.urls import get_script_prefix, set_script_prefix
+from django.utils.deprecation import RemovedInDjango50Warning
 from django.utils.translation import deactivate
 
 try:
@@ -156,8 +157,18 @@ def teardown_test_environment():
     del mail.outbox
 
 
-def setup_databases(verbosity, interactive, *, time_keeper=None, keepdb=False, debug_sql=False, parallel=0,
-                    aliases=None, **kwargs):
+def setup_databases(
+    verbosity,
+    interactive,
+    *,
+    time_keeper=None,
+    keepdb=False,
+    debug_sql=False,
+    parallel=0,
+    aliases=None,
+    serialized_aliases=None,
+    **kwargs,
+):
     """Create the test databases."""
     if time_keeper is None:
         time_keeper = NullTimeKeeper()
@@ -176,11 +187,29 @@ def setup_databases(verbosity, interactive, *, time_keeper=None, keepdb=False, d
             if first_alias is None:
                 first_alias = alias
                 with time_keeper.timed("  Creating '%s'" % alias):
+                    # RemovedInDjango50Warning: when the deprecation ends,
+                    # replace with:
+                    # serialize_alias = serialized_aliases is None or alias in serialized_aliases
+                    try:
+                        serialize_alias = connection.settings_dict['TEST']['SERIALIZE']
+                    except KeyError:
+                        serialize_alias = (
+                            serialized_aliases is None or
+                            alias in serialized_aliases
+                        )
+                    else:
+                        warnings.warn(
+                            'The SERIALIZE test database setting is '
+                            'deprecated as it can be inferred from the '
+                            'TestCase/TransactionTestCase.databases that '
+                            'enable the serialized_rollback feature.',
+                            category=RemovedInDjango50Warning,
+                        )
                     connection.creation.create_test_db(
                         verbosity=verbosity,
                         autoclobber=not interactive,
                         keepdb=keepdb,
-                        serialize=connection.settings_dict['TEST'].get('SERIALIZE', True),
+                        serialize=serialize_alias,
                     )
                 if parallel > 1:
                     for index in range(parallel):