Database connection differs between Middleware and views running under ASGI.

[Hugo Osvaldo Barrera]

(Was: Middleware behaviour inconsistent between runserver and daphne)

All my views expect a particular middleware to have run (it detects the current tenant based on the request's host).

I'm having an issue which only occurs on production, but I could not reproduce in development mode (even with DEBUG=False) nor via unit tests.

After just inspecting stack traces, it seems that when handler404 is run, behaviour varies between runserver and daphne.

Looking at the backtrace of my handler404 function:

• When running django-admin runserver 0.0.0.0:8000, all my middlewares are in my backtrace, meaning they've gotten executed and the function is called in their context.
• When running daphne myapp.asgi:application --bind 0.0.0.0 --port 8000 --http-timeout 90, I put a tracepoint in my handler404, and the middleware are not in the backtrace. It seems that the view is run outside the context of these middlewares.

I haven't been able to find anything in the docs clarifying whether error handlers should expect middleware to have run successfully, but whatever the intended behaviour is, it should be consistent across the two.

[Carlton Gibson]

Hi Hugo. Can you provide a minimal project that reproduces this please?

[Hugo Osvaldo Barrera]

This reproduces the issue. Hope it's clear enough.

​https://github.com/WhyNotHugo/django-32049

[Carlton Gibson]

This is an interesting one. The issue is the way the ASGIHandler runs your code in separate threads. (The bottom-line will be that what you're trying to do isn't supported: you're going to have to find a thread-safe way of doing this if you want to use ASGI here, now.)

Your middleware sets connection.tenant in Thread 1. Via the DefaultConnectionProxy this fetches and sets the property on the "default" connection for Thread 1.
Your view is then run on Thread 2. Attempting to get connection.tenant the DefaultConnectionProxy fetches the "default" connection for Thread 2.
Since DB connections are not shared between thread, this is not the same connection, so the tenant attribute you set doesn't exist.

I'll cc Andrew since it's an interesting case, but it's expected behaviour. ASGI/async implies/requires much more thought about what thread you're going to be running on here, and hence how you pass this kind of info around.

[Hugo Osvaldo Barrera]

You mention this is expected behaviour, but which one? runserver does one thing, but daphne does another.
If one of them is behaving as expected, then the other is misbehaving, right? I don't think the intent is for them to run different, is it?

Side note:
I find it curious that for non-404 views, the same db connection is used for both the middleware and the view. The exception seems to _only_ how the 404 view is called.

you're going to have to find a thread-safe way of doing this if you want to use ASGI here, now

Any hints on something that's safe?
What I'm doing right now is setting a tenant based on the domain name, and this is used in various places (e.g.: by my custom staticfiles' Storage).

[Carlton Gibson]

runserver doesn't yet support ASGI, so it's running as a WSGI app in a single thread.

[Hugo Osvaldo Barrera]

Ah, right, makes sense. I'll be sure to do testing of this kind of functionality with daphne too then.

Thanks a lot for your feedback/insight here.

It should be noted that, apparently, since both unittests and runserver run synchronously, these bugs are very hard to pick up -- for me they _only_ occur on the 404 page, and on production.

Do you have any hints on how to safely do what I'm trying to do? E.g.: keep a pseudo-global state isolated to the context of a single request?

[Simon Charette]

Do you have any hints on how to safely do what I'm trying to do? E.g.: keep a pseudo-global state isolated to the context of a single request?

I think you'll want to use asgiref.local.Local for that.

[Andrew Godwin]

It's worth noting that I still consider this a bug since it's a type of failure, but it's already been fixed in asgiref: ​https://github.com/django/asgiref/commit/7becc9daca2628c46af1cb7e46b4c47c1ea27adf

As of the next release, that will enforce that all synchronous calls run inside the same, single thread, slightly hurting performance but crucially meaning that this issue would not have happened.

[Carlton Gibson]

Hey Andrew. I hadn't seen that go in. It's a good move I think but, do you think it would be worth a post on the Django blog highlighting the change in behaviour for people?

[Andrew Godwin]

Replying to Carlton Gibson:

Hey Andrew. I hadn't seen that go in. It's a good move I think but, do you think it would be worth a post on the Django blog highlighting the change in behaviour for people?

It might be worth it, yes, or at least have a personal blog from me about it - I also called it out in my DjangoCon EU talk. It's not released in asgiref yet, but hopefully I'll get that out this weekend.