tests.file_storage.tests can fail when run in isolation

[Adam Zapletal]

The full test suite run with multiple threads passes for me, but when I run the tests.file_storage.tests file in isolation (./runtests.py file_storage.tests), I get a failure. It passes in isolation if I run the tests with only one thread (./runtests.py --parallel 1 file_storage.tests). This happened to others when I asked about it in the Django Discord server, but not everyone. For what it's worth, I'm on the latest macOS running Python v3.12.

When a failure happens, I get the following error:

django.core.exceptions.ImproperlyConfigured: AUTH_USER_MODEL refers to model 'auth.User' that has not been installed

I have narrowed the issue down to the fact that this test file contains a class that inherits from LiveServerTestCase. It seems like any test file that mixes LiveServerTestCase and something more normal like unittest.TestCase will fail for me when run in isolation with more than one thread.

Here's a minimal example if anyone is interested:

from unittest import TestCase

from django.test import LiveServerTestCase


class TestCaseTests(TestCase):
    def test(self):
        self.assertEqual(2, 2)


class LiveServerTestCaseTests(LiveServerTestCase):
    available_apps = []

    def test(self):
        self.client.get(self.live_server_url + '/')

I have a fix, but I'm not sure if it's a good one. Setting available_apps = ['django.contrib.auth'] on the LiveServerTestCase-based class fixes it as one would expect from the error message, but I wonder if this is hiding something like a race condition. I wonder why it passes the full test suite when run with multiple threads, but it can't pass in isolation when run with multiple threads.

Should I open a pull request to fix ​the test in the way I mentioned above, or is something deeper going on here? It could be that I'm misunderstanding something about how LiveServerTestCase is isolated during test runs.