Make sync login() and logout() set request.auser if present

[Jacob Walls]

Analogous to #37017, we should make sync logout() clear request.auser if present. If auser is not present, I doubt login() should set it, but if it is present, it probably should be set as well.

[Sarah Boyce]

Regression tests from the PR discussion if useful

tests/auth_tests/test_middleware.py

@@ +1 @@
+from asgiref.sync import sync_to_async
+
 from django.conf import settings
-from django.contrib.auth import REDIRECT_FIELD_NAME, alogin, alogout
+from django.contrib.auth import REDIRECT_FIELD_NAME, alogin, alogout, login, logout
 from django.contrib.auth.middleware import (
     AuthenticationMiddleware,
     LoginRequiredMiddleware,
@@ -68 +70 @@
         auser_second = await self.request.auser()
         self.assertEqual(auser_second, self.user2)
 
+    async def test_auser_after_login(self):
+        self.middleware(self.request)
+        auser = await self.request.auser()
+        self.assertEqual(auser, self.user)
+        await sync_to_async(login)(self.request, self.user2)
+        auser_second = await self.request.auser()
+        self.assertEqual(auser_second, self.user2)
+
     async def test_auser_after_alogout(self):
         self.middleware(self.request)
         auser = await self.request.auser()
@@ -76 +86 @@
         auser_second = await self.request.auser()
         self.assertTrue(auser_second.is_anonymous)
 
+    async def test_auser_after_logout(self):
+        self.middleware(self.request)
+        auser = await self.request.auser()
+        self.assertEqual(auser, self.user)
+        await sync_to_async(logout)(self.request)
+        auser_second = await self.request.auser()
+        self.assertTrue(auser_second.is_anonymous)
+

I personally think auser may need to be set as using sync_to_async should still be valid and perhaps third-party code may have sync only code assuming things will work using sync_to_async. I understand in your own project you should use alogin instead