Version 5.0 breaks show_toolbar callback accessing database

[adamantike]

Description

Since version 5.0, setting SHOW_TOOLBAR_CALLBACK to a function that uses the ORM to access the database raises the following exception:

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

An example function that triggers this issue:

def show_toolbar(request):
    return request.user.is_staff

As request.user is a lazy object, it'll be retrieved during the show_toolbar execution. As some panels are asynchronous now, Django Debug Toolbar will try to access to the database from an asynchronous context.

Converting the callback to asynchronous does not work either, because:

• The DebugToolbarMiddleware.__acall__ method does not await for the show_toolbar(request) call.
• The DebugToolbarMiddleware.__call__ method calls show_toolbar(request), which will always be true, as Python will return a coroutine object.

Possible solution

Allow both sync and async SHOW_TOOLBAR_CALLBACK functions, and:

• In DebugToolbarMiddleware.__call__ method, cast the callback using async_to_sync if the provided function is asynchronous.
• In DebugToolbarMiddleware.__acall__ method, cast the callback using sync_to_async if the provided function is synchronous, and add the await to its execution.

[tim-schilling]

Can you confirm that this only happens when you're using an ASGI request? (ASGI_APPLICATION is specified in your settings?)

[tim-schilling]

I've confirmed it does break for ASGI applications and not WSGI.

[adamantike]

Yes, I can confirm it's not reproducible on my end when running runserver with a WSGI application, but it is reproducible when using Django's get_asgi_application(), even without ASGI_APPLICATION setting explicitly set.

[tim-schilling]

@adamantike do you mind giving #2066 a review when you have time?