ChannelsLiveServerTestCase keeps old ContentType cache between tests

[johanneswilm]

When running multiple ChannelsLiveServerTestCase tests, the id values of the contenttypes change between test runs, yet the cache is not emptied. So ContentType.get_for_model(X) will get the instance from a previous test.

The only work-around I found was to add the following within the django view itself:

if 'test' in sys.argv:
            ContentType.objects.clear_cache()

It is not enough to add this in the setUp or tearDown methods of the test, as the test runs in a separate thread and therefore has a separate cache. If someone can tell me how to, I'd very much like to move this to to the test code.

Tested with Django 5.2.9 and Django Channels 4.3.2.

[carltongibson]

Django's tests add this clean up in the test cases themselves:

class ContentTypesTests(TestCase):
    def setUp(self):
        ContentType.objects.clear_cache()
        self.addCleanup(ContentType.objects.clear_cache)

Likely, that'd be a better place for this kind of thing. 🤔

[carltongibson]

I've suggested a small tweak to the Django docs here:

django/django#20388

[johanneswilm]

Thanks @carltongibson . I've added a comment on that PR.

[johanneswilm]

Likely, that'd be a better place for this kind of thing.

@carltongibson I agree that this is a better location. Unfortunately, it doesn't work for me with a ChannelsLiveServerTestCase. The cache clearing doesn't seem to happen in the thread of the daphne server where the views are called.

[carltongibson]

Ah, OK. That's a different issue then. Are you able to give me a minimal reproduction?

[johanneswilm]

Yeah, I guess I'll have to provide that. My code is all open source and publicly available [1], but it's big and bulky.

Is there some kind of minimal example channels test project that can easily be forked to create minimal reproduction examples?

[1] https://github.com/fiduswriter/fiduswriter/blob/main/fiduswriter/testing/selenium_helper.py#L171-L175

[carltongibson]

Not really. But there is a live server set of tests in the test suite that maybe you can leverage

[johanneswilm]

@carltongibson Ok, I have created the minimal reproduction example here:

https://github.com/johanneswilm/django-channels-contenttype-test

Just run run_tests.sh which should install everything and run the tests. There are two identical tests, and the second one will fail as the ContentType cache has not been cleared between tests.

[johanneswilm]

The problem seems to be in Channels 4.3.0. Downgrade to Django 5.1.0 and Channels 4.2.2 and the tests don't fail. Then upgrade to Channels 4.3.0 and they fail again.

[carltongibson]

@johanneswilm Good work. Thanks. Let me play with it.

[johanneswilm]

@carltongibson I have looked at different possible solutions. AFAICT it is not possible to make self.addCleanup(X) work as expected as they are two separate threads. However, it is possible to define a dictionary with possible commands at the time the class is instantiated and then call these commands later on by means of a shared queue. See #2209 and johanneswilm/django-channels-contenttype-test#1

Whether this works well with your overall ideas about how this app should be designed - I don't know.

[carltongibson]

Does clearing the cache in tearDown() directly work? (addCleanup code runs after this)