ORM query made in `request_finished` signal callback fails in ASGI

[Aditya N]

When running Django with an ASGI server, any ORM query that's made inside a callback connected to the request_finished signal fails with an exception saying:

SynchronousOnlyOperation: You cannot call this from an async context - use a thread or sync_to_async

After inspecting code, I figured out that this was because the django.core.handlers.asgi.ASGIHandler class provides a send_response method which is natively async. This method subsequently fires the request_finished signal and since this happens from within an async context, Django complains and rightly so.

So the issue here is that this behaviour about the request_finished signal hasn't been documented and so it might come as a surprise to people when they try to run their code using an ASGI server. Also, any such ORM queries made from a callback hooked to the request_started signal works just fine without any modifications.

The solution to this problem could be as follows:

1. The synchronous blocking calls made from within the callback hooked to request_finished could be made async using the sync_to_async adapter and this has to be documented appropriately.

(OR)

2. The send_response method could be made natively synchronous and converted into an async callable using the sync_to_async adapter wherein it would run in a background thread without blocking the event loop but this could have performance implications.

If we can finalise on one of the above two or an even better one, I could send a patch for the same.

[Mariusz Felisiak]

Thanks for this ticket, however Django 3.0 is in extended support so it doesn't receive bugfixes anymore (except security fixes and data loss bugs). This issue was fixed in fc0fa72ff4cdbf5861a366e31cb8bbacd44da22d. I would strongly recommend to use Django 3.1+ which supports a fully asynchronous request path.