Middleware fix up is unexpectedly expensive

[bdraco]

aiohttp/aiohttp/web_middlewares.py

Line 116 in f95bcaf

def _fix_request_current_app(app: "Application") -> Middleware:

it’s likely the contextmanager decorator but need to dig in more

[bdraco]

If I take out the context manager and write it out as a simple try/finally

diff --git a/aiohttp/web_middlewares.py b/aiohttp/web_middlewares.py
index 5da1533c0..e18bd4018 100644
--- a/aiohttp/web_middlewares.py
+++ b/aiohttp/web_middlewares.py
@@ -110,7 +110,12 @@ def normalize_path_middleware(
 def _fix_request_current_app(app: "Application") -> Middleware:
     @middleware
     async def impl(request: Request, handler: Handler) -> StreamResponse:
-        with request.match_info.set_current_app(app):
+        match_info = request.match_info
+        prev = match_info._current_app
+        match_info._current_app = app
+        try:
             return await handler(request)
+        finally:
+            match_info._current_app = prev
 
     return impl

before

Running 10s test @ http://localhost:8123
  10 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   394.67us  279.34us  13.12ms   99.20%
    Req/Sec     2.62k   172.86     2.81k    96.14%
  263493 requests in 10.10s, 1.38GB read
Requests/sec:  26088.61
Transfer/sec:    140.37MB

after

Running 10s test @ http://localhost:8123
  10 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   359.41us  200.94us  11.40ms   99.65%
    Req/Sec     2.83k    91.37     3.00k    92.28%
  284573 requests in 10.10s, 1.50GB read
Requests/sec:  28176.88
Transfer/sec:    151.61MB

[bdraco]

The context manager is 8% of the run time... thats very unexpected