Fixed #31905 -- Made MiddlewareMixin call process_request()/process_response() with thread sensitive.

Co-authored-by: Carlton Gibson <carlton.gibson@noumenal.es>

tests/deprecation/test_middleware_mixin.py

@@ -1,11 +1,18 @@
+import threading
+
+from asgiref.sync import async_to_sync
+
 from django.contrib.sessions.middleware import SessionMiddleware
+from django.db import connection
+from django.http.request import HttpRequest
+from django.http.response import HttpResponse
 from django.middleware.cache import (
     CacheMiddleware, FetchFromCacheMiddleware, UpdateCacheMiddleware,
 )
 from django.middleware.common import CommonMiddleware
 from django.middleware.security import SecurityMiddleware
 from django.test import SimpleTestCase
-from django.utils.deprecation import RemovedInDjango40Warning
+from django.utils.deprecation import MiddlewareMixin, RemovedInDjango40Warning
 
 
 class MiddlewareMixinTests(SimpleTestCase):
@@ -37,3 +44,37 @@ class MiddlewareMixinTests(SimpleTestCase):
             with self.subTest(middleware=middleware):
                 with self.assertRaisesMessage(RemovedInDjango40Warning, self.msg):
                     middleware()
+
+    def test_sync_to_async_uses_base_thread_and_connection(self):
+        """
+        The process_request() and process_response() hooks must be called with
+        the sync_to_async thread_sensitive flag enabled, so that database
+        operations use the correct thread and connection.
+        """
+        def request_lifecycle():
+            """Fake request_started/request_finished."""
+            return (threading.get_ident(), id(connection))
+
+        async def get_response(self):
+            return HttpResponse()
+
+        class SimpleMiddleWare(MiddlewareMixin):
+            def process_request(self, request):
+                request.thread_and_connection = request_lifecycle()
+
+            def process_response(self, request, response):
+                response.thread_and_connection = request_lifecycle()
+                return response
+
+        threads_and_connections = []
+        threads_and_connections.append(request_lifecycle())
+
+        request = HttpRequest()
+        response = async_to_sync(SimpleMiddleWare(get_response))(request)
+        threads_and_connections.append(request.thread_and_connection)
+        threads_and_connections.append(response.thread_and_connection)
+
+        threads_and_connections.append(request_lifecycle())
+
+        self.assertEqual(len(threads_and_connections), 4)
+        self.assertEqual(len(set(threads_and_connections)), 1)