id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35148	Missed request cleanup for async views	Hugo Heyman	nobody	"Hello!

''I posted this on django-dev mailing list at first, which was probably not the correct place for this (sorry), so I'm posting it here instead.''

I've been troubleshooting an issue with lingering idle db connections (postgres) since upgrading from django 4.2 to 5.0. The issue manifested by db raising OperationalError FATAL: sorry, too many clients already not long after deploying the new version. We're running asgi with uvicorn + gunicorn.

After extensive troubleshooting I installed django from a local repo and ran a git bisect to pin down the commit where this first occured and ended up on this commit https://github.com/django/django/commit/64cea1e48f285ea2162c669208d95188b32bbc82

Since I've recently built something quite async heavy I decided to dig into how the `ASGIHandler` code works and see if I can understand it and possibly make a fix if there really is some issue there (I have never found a real bug in django so I'm still having doubts :D).

The change introduced in Django 5.0 to allow handling of `asyncio.CancelledError` in views seems to also allow async views with normal `HttpResponse` handling client disconnects. As a consequence though the `request_finished` signal may not run leading to things like db connection not being closed.

What currently seems to be happening in `ASGIHandler` in `django/core/handlers/asgi.py` goes something like this:

1. Concurrent tasks are started for
   - disconnect listener (listen for ""http.disconnect"" from webserver)
   - process_request
2. If disconnect happens first (e.g. browser refresh) `cancel()` is run on `process_request` task and `request_finished` may not have been triggered
3. db connection for the request is not closed :(

Here are the lines of code where all this happens, marked: https://github.com/django/django/blob/9c6d7b4a678b7bbc6a1a14420f686162ba9016f5/django/core/handlers/asgi.py#L191-L232

Possible fix? I'm thinking only views that return a `HTTPStreamingResponse` would need to allow for cancellation cleanup handling via the view. If so `response = await self.run_get_response(request)` could run before and outside of any task so we could check `response.streaming` attribute and only run `listen_for_disconnect` task when there's a streaming response.

I've made a patch to try the above out and it seems to fix the issue: 
https://github.com/HeyHugo/django/commit/e25a1525654e00dcd5b483689ef16e0dc74d32d1

Here's a minimal setup to reproduce the issue via print debugging: 
https://github.com/HeyHugo/django_async_issue"	Bug	closed	HTTP handling	5.0	Normal	duplicate			Unreviewed	0	0	0	0	0	0