Preserve view handler function attributes across middlewares

CHANGES/4174.bugfix

@@ -0,0 +1 @@
+Preserve view handler function attributes across middlewares

aiohttp/web_app.py

@@ -1,7 +1,7 @@
 import asyncio
 import logging
 import warnings
-from functools import partial
+from functools import partial, update_wrapper
 from typing import (  # noqa
     TYPE_CHECKING,
     Any,
@@ -325,7 +325,9 @@ async def _handle(self, request: Request) -> StreamResponse:
                 for app in match_info.apps[::-1]:
                     assert app.pre_frozen, "middleware handlers are not ready"
                     for m in app._middlewares_handlers:  # noqa
-                        handler = partial(m, handler=handler)
+                        handler = update_wrapper(
+                            partial(m, handler=handler), handler
+                        )
 
             resp = await handler(request)
 

tests/test_web_middleware.py

@@ -49,8 +49,18 @@ async def test_middleware_chain(loop, aiohttp_client) -> None:
     async def handler(request):
         return web.Response(text='OK')
 
+    handler.annotation = "annotation_value"
+
+    async def handler2(request):
+        return web.Response(text='OK')
+
+    middleware_annotation_seen_values = []
+
     def make_middleware(num):
         async def middleware(request, handler):
+            middleware_annotation_seen_values.append(
+                getattr(handler, "annotation", None)
+            )
             resp = await handler(request)
             resp.text = resp.text + '[{}]'.format(num)
             return resp
@@ -60,11 +70,22 @@ async def middleware(request, handler):
     app.middlewares.append(make_middleware(1))
     app.middlewares.append(make_middleware(2))
     app.router.add_route('GET', '/', handler)
+    app.router.add_route('GET', '/r2', handler2)
     client = await aiohttp_client(app)
     resp = await client.get('/')
     assert 200 == resp.status
     txt = await resp.text()
     assert 'OK[2][1]' == txt
+    assert middleware_annotation_seen_values == [
+        'annotation_value', 'annotation_value'
+    ]
+
+    # check that attributes from handler are not applied to handler2
+    resp = await client.get('/r2')
+    assert 200 == resp.status
+    assert middleware_annotation_seen_values == [
+        'annotation_value', 'annotation_value', None, None
+    ]
 
 
 @pytest.fixture