Django app for custom user model and django_db fixture: during test setup of db migrations: error when run on Linux but Windows is OK.

I am setting up my testing environment and my first pytest-django test works on Windows but errors out on Linux.

I'm using a django custom user model as recommended for new projects:
https://docs.djangoproject.com/en/3.2/topics/auth/customizing/#using-a-custom-user-model-when-starting-a-project

For pytest-django, I specify django_db:
pytestmark = [ pytest.mark.django_db ]
so I can do db testing.

On both Windows and Linux platforms, I've done the django migration for my custom user model.

dj_user_app
 [X] 0001_initial

When I run on Windows:

>pytest -vs  all_tests/django_tests/django_test.py::Test_django_operations::test_auth_a_non_user
===================================================================== test session starts =====================================================================
platform win32 -- Python 3.9.2, pytest-6.2.4, py-1.10.0, pluggy-0.13.1 -- h:\business\projects\...\scripts\python.exe
cachedir: .pytest_cache
django: settings: www_dj_project.mysite.settings (from ini)
rootdir: H:\business\projects\..._1, configfile: pytest.ini
plugins: django-4.4.0, pythonpath-0.7.3
collected 1 item

all_tests/django_tests/django_test.py::Test_django_operations::test_auth_a_non_user Creating test database for alias 'default'...
Creating test database for alias 'reg_test_queen_bee_engine'...
Creating test database for alias 'reg_test_worker_bee_engine'...
PASSEDDestroying test database for alias 'default'...
Destroying test database for alias 'reg_test_queen_bee_engine'...
Destroying test database for alias 'reg_test_worker_bee_engine'...


====================================================================== 1 passed in 4.07s ======================================================================

When I run on Linux:

$ pytest -vs  all_tests/django_tests/django_test.py::Test_django_operations::test_auth_a_non_user
====================================================================== test session starts =======================================================================
platform linux -- Python 3.8.10, pytest-6.2.4, py-1.10.0, pluggy-0.13.1 -- /home/.../bin/python3
cachedir: .pytest_cache
django: settings: www_dj_project.mysite.settings (from ini)
rootdir: /home/..._1, configfile: pytest.ini
plugins: django-4.4.0, pythonpath-0.7.3
collected 1 item

all_tests/django_tests/django_test.py::Test_django_operations::test_auth_a_non_user Creating test database for alias 'default'...
ERROR

============================================================================= ERRORS =============================================================================
_________________________________________________ ERROR at setup of Test_django_operations.test_auth_a_non_user __________________________________________________

self = <django.db.migrations.loader.MigrationLoader object at 0x7f7e715f5ee0>, key = ('dj_user_app', '__first__'), current_app = 'admin'

    def check_key(self, key, current_app):
        if (key[1] != "__first__" and key[1] != "__latest__") or key in self.graph:
            return key
        # Special-case __first__, which means "the first migration" for
        # migrated apps, and is ignored for unmigrated apps. It allows
        # makemigrations to declare dependencies on apps before they even have
        # migrations.
        if key[0] == current_app:
            # Ignore __first__ references to the same app (#22325)
            return
        if key[0] in self.unmigrated_apps:
            # This app isn't migrated, but something depends on it.
            # The models will get auto-added into the state, though
            # so we're fine.
            return
        if key[0] in self.migrated_apps:
            try:
                if key[1] == "__first__":
>                   return self.graph.root_nodes(key[0])[0]
E                   IndexError: list index out of range

../../../../webapps/.../lib/python3.8/site-packages/django/db/migrations/loader.py:174: IndexError

During handling of the above exception, another exception occurred:

request = <SubRequest '_django_db_marker' for <Function test_auth_a_non_user>>

    @pytest.fixture(autouse=True)
    def _django_db_marker(request) -> None:
        """Implement the django_db marker, internal to pytest-django.

        This will dynamically request the ``db``, ``transactional_db`` or
        ``django_db_reset_sequences`` fixtures as required by the django_db marker.
        """
        marker = request.node.get_closest_marker("django_db")
        if marker:
            transaction, reset_sequences, databases = validate_django_db(marker)

            # TODO: Use pytest Store (item.store) once that's stable.
            request.node._pytest_django_databases = databases

            if reset_sequences:
                request.getfixturevalue("django_db_reset_sequences")
            elif transaction:
                request.getfixturevalue("transactional_db")
            else:
>               request.getfixturevalue("db")

../../../../webapps/.../lib/python3.8/site-packages/pytest_django/plugin.py:470:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../../../webapps/.../lib/python3.8/site-packages/pytest_django/fixtures.py:117: in django_db_setup
    db_cfg = setup_databases(
../../../../webapps/.../lib/python3.8/site-packages/django/test/utils.py:179: in setup_databases
    connection.creation.create_test_db(
../../../../webapps/.../lib/python3.8/site-packages/django/db/backends/base/creation.py:74: in create_test_db
    call_command(
../../../../webapps/.../lib/python3.8/site-packages/django/core/management/__init__.py:181: in call_command
    return command.execute(*args, **defaults)
../../../../webapps/.../lib/python3.8/site-packages/django/core/management/base.py:398: in execute
    output = self.handle(*args, **options)
../../../../webapps/.../lib/python3.8/site-packages/django/core/management/base.py:89: in wrapped
    res = handle_func(*args, **kwargs)
../../../../webapps/.../lib/python3.8/site-packages/django/core/management/commands/migrate.py:92: in handle
    executor = MigrationExecutor(connection, self.migration_progress_callback)
../../../../webapps/.../lib/python3.8/site-packages/django/db/migrations/executor.py:18: in __init__
    self.loader = MigrationLoader(self.connection)
../../../../webapps/.../lib/python3.8/site-packages/django/db/migrations/loader.py:53: in __init__
    self.build_graph()
../../../../webapps/.../lib/python3.8/site-packages/django/db/migrations/loader.py:235: in build_graph
    self.add_external_dependencies(key, migration)
../../../../webapps/.../lib/python3.8/site-packages/django/db/migrations/loader.py:199: in add_external_dependencies
    parent = self.check_key(parent, key[0])
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <django.db.migrations.loader.MigrationLoader object at 0x7f7e715f5ee0>, key = ('dj_user_app', '__first__'), current_app = 'admin'

    def check_key(self, key, current_app):
        if (key[1] != "__first__" and key[1] != "__latest__") or key in self.graph:
            return key
        # Special-case __first__, which means "the first migration" for
        # migrated apps, and is ignored for unmigrated apps. It allows
        # makemigrations to declare dependencies on apps before they even have
        # migrations.
        if key[0] == current_app:
            # Ignore __first__ references to the same app (#22325)
            return
        if key[0] in self.unmigrated_apps:
            # This app isn't migrated, but something depends on it.
            # The models will get auto-added into the state, though
            # so we're fine.
            return
        if key[0] in self.migrated_apps:
            try:
                if key[1] == "__first__":
                    return self.graph.root_nodes(key[0])[0]
                else:  # "__latest__"
                    return self.graph.leaf_nodes(key[0])[0]
            except IndexError:
                if self.ignore_no_migrations:
                    return None
                else:
>                   raise ValueError("Dependency on app with no migrations: %s" % key[0])
E                   ValueError: Dependency on app with no migrations: dj_user_app

../../../../webapps/.../lib/python3.8/site-packages/django/db/migrations/loader.py:181: ValueError
==================================================================== short test summary info =====================================================================
ERROR all_tests/django_tests/django_test.py::Test_django_operations::test_auth_a_non_user - ValueError: Dependency on app with no migrations: dj_user_app
======================================================================== 1 error in 0.61s ========================================================================

Since the same code and db state works on Windows but errors out on Linux, I'm thinking it's a bug and maybe relating to django custom user models because the Linux error out is in db migration of the user model in the django admin app.

Start with all_tests/django_tests/django_test.py::Test_django_operations::test_auth_a_non_user and reproduce the Linux setup failure. Trace pytest_django/plugin.py and fixtures.py into Django's django/db/migrations/loader.py, comparing the migration state for dj_user_app on Windows and Linux. Done means the cause of the platform difference is identified and the test database setup succeeds consistently.