[3.1.x] Fixed #32299 -- Prevented mutating handlers when processing middlewares marking as unused in an async context.

Thanks Hubert Bielenia for the report.
Backport of 98ad327864 from master

django/core/handlers/base.py

@@ -51,11 +51,11 @@ class BaseHandler:
                 middleware_is_async = middleware_can_async
             try:
                 # Adapt handler, if needed.
-                handler = self.adapt_method_mode(
+                adapted_handler = self.adapt_method_mode(
                     middleware_is_async, handler, handler_is_async,
                     debug=settings.DEBUG, name='middleware %s' % middleware_path,
                 )
-                mw_instance = middleware(handler)
+                mw_instance = middleware(adapted_handler)
             except MiddlewareNotUsed as exc:
                 if settings.DEBUG:
                     if str(exc):
@@ -63,6 +63,8 @@ class BaseHandler:
                     else:
                         logger.debug('MiddlewareNotUsed: %r', middleware_path)
                 continue
+            else:
+                handler = adapted_handler
 
             if mw_instance is None:
                 raise ImproperlyConfigured(

docs/releases/3.1.5.txt

@@ -12,3 +12,7 @@ Bugfixes
 * Fixed ``__isnull=True`` lookup on key transforms for
   :class:`~django.db.models.JSONField` with Oracle and SQLite
   (:ticket:`32252`).
+
+* Fixed a bug in Django 3.1 that caused a crash when processing middlewares in
+  an async context with a middleware that raises a ``MiddlewareNotUsed``
+  exception (:ticket:`32299`).

tests/middleware_exceptions/tests.py

@@ -181,6 +181,25 @@ class MiddlewareNotUsedTests(SimpleTestCase):
             with self.assertLogs('django.request', 'DEBUG'):
                 self.client.get('/middleware_exceptions/view/')
 
+    @override_settings(MIDDLEWARE=[
+        'middleware_exceptions.middleware.SyncAndAsyncMiddleware',
+        'middleware_exceptions.tests.MyMiddleware',
+    ])
+    async def test_async_and_sync_middleware_chain_async_call(self):
+        with self.assertLogs('django.request', 'DEBUG') as cm:
+            response = await self.async_client.get('/middleware_exceptions/view/')
+        self.assertEqual(response.content, b'OK')
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(
+            cm.records[0].getMessage(),
+            'Asynchronous middleware middleware_exceptions.tests.MyMiddleware '
+            'adapted.',
+        )
+        self.assertEqual(
+            cm.records[1].getMessage(),
+            "MiddlewareNotUsed: 'middleware_exceptions.tests.MyMiddleware'",
+        )
+
 
 @override_settings(
     DEBUG=True,