swev-id: django__django-17029 - Clear get_swappable_settings_name cache in Apps.clear_cache() [skip ci]

[casey-brooks]

Resolves #535

Summary

• clear Apps.get_swappable_settings_name when purging registry caches
• add a regression test ensuring the swappable settings cache is purged when INSTALLED_APPS changes

Reproduction

Run the minimal script from the issue on the base branch:

PYTHONPATH=/workspace/django source .venv/bin/activate
PYTHONPATH=/workspace/django python - <<'PY'
import django
from django.conf import settings
from django.apps import apps
from django.test.utils import override_settings

settings.configure(
    INSTALLED_APPS=['django.contrib.contenttypes', 'django.contrib.auth'],
    SECRET_KEY='test',
)
django.setup()

first = apps.get_swappable_settings_name('auth.User')
assert first == 'AUTH_USER_MODEL'

with override_settings(INSTALLED_APPS=['django.contrib.contenttypes']):
    second = apps.get_swappable_settings_name('auth.User')
    assert second is None
PY

Observed failure before the fix:

first: AUTH_USER_MODEL
second: AUTH_USER_MODEL
Traceback (most recent call last):
  File "<stdin>", line 19, in <module>
AssertionError

Testing

• PYTHONPATH=/workspace/django DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py apps.tests.AppsTests.test_clear_cache_clears_swappable_settings_cache --parallel=1
• PYTHONPATH=/workspace/django DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py apps --parallel=1

Local tests pass after the fix.

[casey-brooks]

Test & Lint Summary

• PYTHONPATH=/workspace/django DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py apps.tests.AppsTests.test_clear_cache_clears_swappable_settings_cache --parallel=1 (pass)
• PYTHONPATH=/workspace/django DJANGO_SETTINGS_MODULE=tests.test_sqlite python tests/runtests.py apps --parallel=1 (pass)
• flake8 django/apps/registry.py tests/apps/tests.py (pass)

[noa-lucent]

Looks good. Clearing the swappable settings cache keeps Apps.clear_cache() consistent with its contract, and the regression test covers the reported scenario.

[noa-lucent]

The fix looks good to me. Clearing get_swappable_settings_name alongside get_models keeps Apps.clear_cache() consistent with its contract, and the regression test fails before the change and passes after. Thanks!

[rowan-stein]

Looks good. The cache clear for get_swappable_settings_name in Apps.clear_cache() is correct, and the regression test passes locally.