Bugfix correct the dispatcher middleware root path and path

The path shouldn't have been adjusted, rather the root_path should
have been changed - this matches WSGI whereby the script_name is
changed. In addition the changes should have been made in a copied
scope to isolate them.

Author: pgjones

src/hypercorn/middleware/dispatcher.py

@@ -20,8 +20,9 @@ async def __call__(self, scope: Scope, receive: Callable, send: Callable) -> Non
         else:
             for path, app in self.mounts.items():
                 if scope["path"].startswith(path):
-                    scope["path"] = scope["path"][len(path) :] or "/"
-                    return await app(scope, receive, send)
+                    local_scope = scope.copy()
+                    local_scope["root_path"] += path
+                    return await app(local_scope, receive, send)
             await send(
                 {
                     "type": "http.response.start",

tests/middleware/test_dispatcher.py

@@ -40,10 +40,10 @@ async def send(message: dict) -> None:
     await app({**http_scope, **{"path": "/api/b"}}, None, send)  # type: ignore
     await app({**http_scope, **{"path": "/"}}, None, send)  # type: ignore
     assert sent_events == [
-        {"type": "http.response.start", "status": 200, "headers": [(b"content-length", b"7")]},
-        {"type": "http.response.body", "body": b"apix-/b"},
-        {"type": "http.response.start", "status": 200, "headers": [(b"content-length", b"6")]},
-        {"type": "http.response.body", "body": b"api-/b"},
+        {"type": "http.response.start", "status": 200, "headers": [(b"content-length", b"13")]},
+        {"type": "http.response.body", "body": b"apix-/api/x/b"},
+        {"type": "http.response.start", "status": 200, "headers": [(b"content-length", b"10")]},
+        {"type": "http.response.body", "body": b"api-/api/b"},
         {"type": "http.response.start", "status": 404, "headers": [(b"content-length", b"0")]},
         {"type": "http.response.body"},
     ]