[5.2.x] Fixed #36390 -- Deprecated RemoteUserMiddleware subclasses missing aprocess_request().

Regression in 50f89ae850f6b4e35819fe725a08c7e579bfd099.
Thank you to shamoon for the report and Natalia Bidart for the review.

Backport of 1704c49a9b149b66b6a0e67abc8c95293bc35649 from main.

5 files changed, 90 insertions, 9 deletions

diff --git a/django/contrib/auth/middleware.py b/django/contrib/auth/middleware.py
index 880563bc5b..6be3552cad 100644
--- a/django/contrib/auth/middleware.py
+++ b/django/contrib/auth/middleware.py
@@ -1,7 +1,8 @@
+import warnings
 from functools import partial
 from urllib.parse import urlsplit
 
-from asgiref.sync import iscoroutinefunction, markcoroutinefunction
+from asgiref.sync import iscoroutinefunction, markcoroutinefunction, sync_to_async
 
 from django.conf import settings
 from django.contrib import auth
@@ -10,7 +11,7 @@ from django.contrib.auth.backends import RemoteUserBackend
 from django.contrib.auth.views import redirect_to_login
 from django.core.exceptions import ImproperlyConfigured
 from django.shortcuts import resolve_url
-from django.utils.deprecation import MiddlewareMixin
+from django.utils.deprecation import MiddlewareMixin, RemovedInDjango61Warning
 from django.utils.functional import SimpleLazyObject
 
 
@@ -128,6 +129,10 @@ class RemoteUserMiddleware:
     def __call__(self, request):
         if self.is_async:
             return self.__acall__(request)
+        self.process_request(request)
+        return self.get_response(request)
+
+    def process_request(self, request):
         # AuthenticationMiddleware is required so that request.user exists.
         if not hasattr(request, "user"):
             raise ImproperlyConfigured(
@@ -145,13 +150,13 @@ class RemoteUserMiddleware:
             # AnonymousUser by the AuthenticationMiddleware).
             if self.force_logout_if_no_header and request.user.is_authenticated:
                 self._remove_invalid_user(request)
-            return self.get_response(request)
+            return
         # If the user is already authenticated and that user is the user we are
         # getting passed in the headers, then the correct user is already
         # persisted in the session and we don't need to continue.
         if request.user.is_authenticated:
             if request.user.get_username() == self.clean_username(username, request):
-                return self.get_response(request)
+                return
             else:
                 # An authenticated user is associated with the request, but
                 # it does not match the authorized user in the header.
@@ -165,9 +170,26 @@ class RemoteUserMiddleware:
             # by logging the user in.
             request.user = user
             auth.login(request, user)
-        return self.get_response(request)
 
     async def __acall__(self, request):
+        # RemovedInDjango61Warning.
+        if (
+            self.__class__.process_request is not RemoteUserMiddleware.process_request
+            and self.__class__.aprocess_request is RemoteUserMiddleware.aprocess_request
+        ):
+            warnings.warn(
+                "Support for subclasses of RemoteUserMiddleware that override "
+                "process_request() without overriding aprocess_request() is "
+                "deprecated.",
+                category=RemovedInDjango61Warning,
+                stacklevel=2,
+            )
+            await sync_to_async(self.process_request, thread_sensitive=True)(request)
+            return await self.get_response(request)
+        await self.aprocess_request(request)
+        return await self.get_response(request)
+
+    async def aprocess_request(self, request):
         # AuthenticationMiddleware is required so that request.user exists.
         if not hasattr(request, "user"):
             raise ImproperlyConfigured(
@@ -187,14 +209,14 @@ class RemoteUserMiddleware:
                 user = await request.auser()
                 if user.is_authenticated:
                     await self._aremove_invalid_user(request)
-            return await self.get_response(request)
+            return
         user = await request.auser()
         # If the user is already authenticated and that user is the user we are
         # getting passed in the headers, then the correct user is already
         # persisted in the session and we don't need to continue.
         if user.is_authenticated:
             if user.get_username() == self.clean_username(username, request):
-                return await self.get_response(request)
+                return
             else:
                 # An authenticated user is associated with the request, but
                 # it does not match the authorized user in the header.
@@ -209,8 +231,6 @@ class RemoteUserMiddleware:
             request.user = user
             await auth.alogin(request, user)
 
-        return await self.get_response(request)
-
     def clean_username(self, username, request):
         """
         Allow the backend to clean the username, if the backend defines a
diff --git a/docs/internals/deprecation.txt b/docs/internals/deprecation.txt
index 71e4e19cd8..067686e0e2 100644
--- a/docs/internals/deprecation.txt
+++ b/docs/internals/deprecation.txt
@@ -27,6 +27,10 @@ details on these changes.
   ``django.contrib.postgres.aggregates.JSONBAgg``, and
   ``django.contrib.postgres.aggregates.StringAgg`` will be removed.
 
+* Support for subclasses of ``RemoteUserMiddleware`` that override
+  ``process_request()`` without overriding ``aprocess_request()`` will be
+  removed.
+
 .. _deprecation-removed-in-6.0:
 
 6.0
diff --git a/docs/releases/5.2.2.txt b/docs/releases/5.2.2.txt
index f6787bff08..2cf5c750ff 100644
--- a/docs/releases/5.2.2.txt
+++ b/docs/releases/5.2.2.txt
@@ -18,3 +18,7 @@ Bugfixes
 
 * Fixed a regression in Django 5.2 that caused a crash when no arguments were
   passed into ``QuerySet.union()`` (:ticket:`36388`).
+
+* Fixed a regression in Django 5.2 where subclasses of ``RemoteUserMiddleware``
+  that had overridden ``process_request()`` were no longer supported
+  (:ticket:`36390`).
diff --git a/docs/releases/5.2.txt b/docs/releases/5.2.txt
index 32a0b35951..feaeb9436a 100644
--- a/docs/releases/5.2.txt
+++ b/docs/releases/5.2.txt
@@ -488,3 +488,7 @@ Miscellaneous
   ``django.contrib.postgres.aggregates.JSONBAgg``, and
   ``django.contrib.postgres.aggregates.StringAgg`` is deprecated in favor
   of the ``order_by`` argument.
+
+* Support for subclasses of ``RemoteUserMiddleware`` that override
+  ``process_request()`` without overriding ``aprocess_request()`` is
+  deprecated.
diff --git a/tests/auth_tests/test_remote_user.py b/tests/auth_tests/test_remote_user.py
index 85de931c1a..e84c1b2e4c 100644
--- a/tests/auth_tests/test_remote_user.py
+++ b/tests/auth_tests/test_remote_user.py
@@ -13,6 +13,7 @@ from django.test import (
     modify_settings,
     override_settings,
 )
+from django.utils.deprecation import RemovedInDjango61Warning
 
 
 @override_settings(ROOT_URLCONF="auth_tests.urls")
@@ -487,3 +488,51 @@ class PersistentRemoteUserTest(RemoteUserTest):
         response = await self.async_client.get("/remote_user/")
         self.assertFalse(response.context["user"].is_anonymous)
         self.assertEqual(response.context["user"].username, "knownuser")
+
+
+# RemovedInDjango61Warning.
+class CustomProcessRequestMiddlewareSyncOnly(RemoteUserMiddleware):
+    def process_request(self, request):
+        raise NotImplementedError("process_request has not been implemented.")
+
+
+# RemovedInDjango61Warning.
+class CustomProcessRequestMiddleware(RemoteUserMiddleware):
+    def process_request(self, request):
+        raise NotImplementedError("process_request has not been implemented.")
+
+    async def aprocess_request(self, request):
+        raise NotImplementedError("aprocess_request has not been implemented.")
+
+
+# RemovedInDjango61Warning.
+@override_settings(ROOT_URLCONF="auth_tests.urls")
+class CustomProcessRequestMiddlewareTest(TestCase):
+    @modify_settings(
+        MIDDLEWARE={
+            "append": "auth_tests.test_remote_user."
+            "CustomProcessRequestMiddlewareSyncOnly"
+        }
+    )
+    async def test_async_warns_sync_only_middleware(self):
+        deprecation_msg = (
+            "Support for subclasses of RemoteUserMiddleware that override "
+            "process_request() without overriding aprocess_request() is "
+            "deprecated."
+        )
+        error_msg = "process_request has not been implemented."
+        with (
+            self.assertWarnsMessage(RemovedInDjango61Warning, deprecation_msg),
+            self.assertRaisesMessage(NotImplementedError, error_msg),
+        ):
+            await self.async_client.get("/remote_user/")
+
+    @modify_settings(
+        MIDDLEWARE={
+            "append": "auth_tests.test_remote_user.CustomProcessRequestMiddleware"
+        }
+    )
+    async def test_async_no_warning_sync_and_async_middleware(self):
+        error_msg = "aprocess_request has not been implemented."
+        with self.assertRaisesMessage(NotImplementedError, error_msg):
+            await self.async_client.get("/remote_user/")