[Django] #36819: Async login does not reuse authenticated user instance

[Django]

#36819: Async login does not reuse authenticated user instance
-------------------------------------+-------------------------------------
Reporter: Mykhailo Havelia | Type:
| Cleanup/optimization
Status: new | Component:
| contrib.auth
Version: 6.0 | Severity: Normal
Keywords: async | 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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**Problem**

When using alogin, calling `request.auser` does not reuse the already
authenticated user instance, resulting in redundant authentication
requests.

**Details**

**Middleware Setup**

{{{
class AuthenticationMiddleware(MiddlewareMixin):
def process_request(self, request):
...
request.user = SimpleLazyObject(lambda: get_user(request))
request.auser = partial(auser, request)
}}}

**Sync Login**

{{{
def login(request, user, backend=None):
...
request.user = user
}}}

**Async Login**

{{{
def alogin(request, user, backend=None):
...
request.user = user
}}}

**Problematic Behavior**

After calling `alogin(request, user)`, `request.user` is set to the
authenticated user instance. However, `request.auser()` does not use this
instance and instead performs authentication again, relying on its own
`_acached_user` cache.

**Expected Behavior**

After `alogin`, both `request.user` and `request.auser()` should reference
the same user instance, avoiding duplicate authentication.

**Suggested Solution**

Set a shared cache. `request._cached_user = user`, in `alogin`, so both
sync and async accessors use the same instance.

{{{
async def alogin(request, user, backend=None):
...
request.user = user
request._cached_user = user # <-- suggested addition

async def auser(request):
if not hasattr(request, "_cached_user"): # <-- suggested change
(_acached_user -> _cached_user)
request._cached_user = await auth.aget_user(request)
return request._cached_user
}}}

See related discussion:
https://github.com/django/django/pull/16552#discussion_r1122929933
--
Ticket URL: <https://code.djangoproject.com/ticket/36819>
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