[django-channels] Testing events from post_save signal

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Hello,

I've just migrated my project to django-channels 2.x. Thanks to everyone involved!

I'm trying to write a test for a consumer.

I have a post_save signal receiver, which sends a message to a group. As I understand,

I need to wrap `group_send` with `async_to_sync` because django signals can't be

async functions:

def notify_on_model_changes(model):
    from django.contrib.contenttypes.models import ContentType
    ct = ContentType.objects.get_for_model(model)
    model_label = '.'.join([ct.app_label, ct.model])

    channel_layer = get_channel_layer()

    group_send = async_to_sync(channel_layer.group_send)

    def receiver(sender, instance, **kwargs):
        payload = {
            'type': 'model.changed',
            'pk': instance.pk,
            'model': model_label
        }     
        group_send(f'django.{model_label}', payload)

    post_save.connect(receiver, sender=model, weak=False,
                      dispatch_uid=f'django.{model_label}')

# in AppConfig.ready:

# notify_on_model_changes(Conversation)

My test suite, however, is async function:

@pytest.fixture
async def communicator():
    communicator = WebsocketCommunicator(GraphqlSubcriptionConsumer, "/")
    await communicator.connect()
    yield communicator
    await communicator.disconnect()
    
async def test_subscription_start(communicator):
    def make_conversation():
        return Conversation.objects.create()

    # function body truncated

    # subscribe for changes in Conversation model
    await communicator.send_json_to(data)

    conversation = await sync_to_async(make_conversation)()
    response = await communicator.receive_json_from()
    assert response['type'] == 'data'

I can't use `Conversation.objects.create()` directly, because it uses `async_to_sync`.

First, I need to convert it to async and await the result. I kinda feel this is hackish

jumping from async to sync and back to async, but so far everything works as expected

and test works.

Here comes the punchline:

The tests fail to teardown cleanly, because apparently there's hanging DB connection

and after a while pytest just fails with `There is 1 other session using the database.`.

Breadcrumbs:

1. If I comment out last three lines of test (make_conversations and waiting for result),

the test exits cleanly - seems like there's no problem with passing `sync_to_async` function

to `post_save.connect`.

2. If I create `async_make = sync_to_async(make_conversation)`, but don't call it at all,

the test exists cleanly - I thought that there might be problem with calling `async_to_sync`

inside code wrapped with `sync_to_async`.

I suspect there's a hanging db connection which isn't cleaned and/or garbage collected.

I would also appreciate any comments about structure of such tests - is there cleaner way

test django signals inside async test cases?

Cheers,

   Tom