#32299: MiddlewareNotUsed leaves undesired side effects when loading middleware
in
ASGI context
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Reporter: Hubert Bielenia | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: 3.1
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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I experienced strange issues when working with
[https://docs.djangoproject.com/en/3.1/howto/deployment/asgi/ ASGI] ,
[https://github.com/jazzband/django-debug-toolbar django-debug-toolbar]
and my own small middleware. It was hard problem to debug, I uploaded an
example project here: [https://github.com/hbielenia/asgi-djangotoolbar-
bug] (the name is misleading - I initially thought it's a bug with django-
debug-toolbar).
The {{{SESSION_FILE_PATH}}} setting is intentionally broken to cause a 500
error. When starting the application and accessing {{{/admin}}} (any
location really, but I wanted to leave it at a minimum and didn't add any
views) it gives {{{TypeError: object HttpResponse can't be used in 'await'
expression}}}. Commenting out
{{{asgi_djangotoolbar_bug.middleware.DummyMiddleware}}} fixes the issue
(in that I receive a 500 {{{ImproperlyConfigured}}} exception). I'm not
sure about the overall role of django-debug-toolbar here - removing it
causes Daphne to return a 500 error page but without debug information and
there's no traceback in console either. I decided to leave it since it
helped me approximate the causes of issue.
I notice that in
[https://github.com/django/django/blob/3.1.4/django/core/handlers/base.py#L58]
while {{{MiddlewareNotUsed}}} causes the loop to skip futher processing
and go to next middleware, it does leave {{{handler}}} variable
overwritten with output of {{{self.adapt_method_mode()}}}. On next pass,
this {{{handler}}} is passed to next middleware instance, disregarding all
the previous checks for (lack of) async support. This likely causes the
middleware chain to be "poisoned" from this point onwards, resulting in
last middleware in response cycle to return an HttpResponse as a
synchronous middleware would, instead of coroutine that is expected.
This is probably avoided by adding async support to my middleware, but
unless I'm missing something
[https://docs.djangoproject.com/en/3.1/topics/http/middleware/ docs]
indicate it should work as it is. It is my intention that it's applied
only on synchronous requests, so I didn't make it async compatible on
purpose. If it's intentional in Django that every middleware needs to
support async if the application is run as ASGI app, the documentation
should probably state that clearly. Though it kinda defeats the purpose of
having {{{async_capable = False}}} flag in the first place.
--
Ticket URL: <https://code.djangoproject.com/ticket/32299>
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