Added TransactionTestCase.available_apps.

This can be used to make Django's test suite significantly faster by
reducing the number of models for which content types and permissions
must be created and tables must be flushed in each non-transactional
test.

It's documented for Django contributors and committers but it's branded
as a private API to preserve our freedom to change it in the future.

Most of the credit goes to Anssi. He got the idea and did the research.

Fixed #20483.

django/contrib/auth/management/__init__.py

@@ -11,7 +11,7 @@ from django.contrib.auth import models as auth_app, get_user_model
 from django.core import exceptions
 from django.core.management.base import CommandError
 from django.db import DEFAULT_DB_ALIAS, router
-from django.db.models import get_models, signals
+from django.db.models import get_model, get_models, signals, UnavailableApp
 from django.utils.encoding import DEFAULT_LOCALE_ENCODING
 from django.utils import six
 from django.utils.six.moves import input
@@ -60,6 +60,11 @@ def _check_permission_clashing(custom, builtin, ctype):
         pool.add(codename)
 
 def create_permissions(app, created_models, verbosity, db=DEFAULT_DB_ALIAS, **kwargs):
+    try:
+        get_model('auth', 'Permission')
+    except UnavailableApp:
+        return
+
     if not router.allow_syncdb(db, auth_app.Permission):
         return
 
@@ -101,9 +106,13 @@ def create_permissions(app, created_models, verbosity, db=DEFAULT_DB_ALIAS, **kw
 
 
 def create_superuser(app, created_models, verbosity, db, **kwargs):
-    from django.core.management import call_command
+    try:
+        get_model('auth', 'Permission')
+        UserModel = get_user_model()
+    except UnavailableApp:
+        return
 
-    UserModel = get_user_model()
+    from django.core.management import call_command
 
     if UserModel in created_models and kwargs.get('interactive', True):
         msg = ("\nYou just installed Django's auth system, which means you "

django/contrib/contenttypes/management.py

@@ -1,6 +1,6 @@
 from django.contrib.contenttypes.models import ContentType
 from django.db import DEFAULT_DB_ALIAS, router
-from django.db.models import get_apps, get_models, signals
+from django.db.models import get_apps, get_model, get_models, signals, UnavailableApp
 from django.utils.encoding import smart_text
 from django.utils import six
 from django.utils.six.moves import input
@@ -11,6 +11,11 @@ def update_contenttypes(app, created_models, verbosity=2, db=DEFAULT_DB_ALIAS, *
     Creates content types for models in the given app, removing any model
     entries that no longer have a matching model class.
     """
+    try:
+        get_model('contenttypes', 'ContentType')
+    except UnavailableApp:
+        return
+
     if not router.allow_syncdb(db, ContentType):
         return