Make handling of 'root_path' in dispatcher middleware adhere to ASGI …

…spec

Before this change, the dispatcher middleware didn't do anything with
'root_path'. It did, however, modify 'path'.

The problem with this behavior, is that the child ASGI app has no way to
determine what its prefix it. And if it can't determine its prefix, it
doesn't know how to construct URLs.

The ASGI spec isn't super clear on the expected behavior. However, some
resources to review are:

* https://asgi.readthedocs.io/en/latest/specs/www.html#wsgi-compatibility
* encode/starlette#2400
* django/asgiref#229

Based on the above, I believe that the correct behavior is that
"root_path" should be updated by the dispatcher middleware but that
"path" should not be modified.

In addition to the above change, I also updated the tests. And I also
added a new test case where the dispatcher middleware is nested inside
of itself.

src/hypercorn/middleware/dispatcher.py

@@ -18,9 +18,10 @@ async def __call__(self, scope: Scope, receive: Callable, send: Callable) -> Non
         if scope["type"] == "lifespan":
             await self._handle_lifespan(scope, receive, send)
         else:
+            relative_path = scope["path"][len(scope["root_path"]):]
             for path, app in self.mounts.items():
-                if scope["path"].startswith(path):
-                    scope["path"] = scope["path"][len(path) :] or "/"
+                if relative_path.startswith(path):
+                    scope["root_path"] += path
                     return await app(scope, receive, send)
             await send(
                 {

tests/middleware/test_dispatcher.py

@@ -16,7 +16,7 @@ def __init__(self, name: str) -> None:
 
         async def __call__(self, scope: Scope, receive: Callable, send: Callable) -> None:
             scope = cast(HTTPScope, scope)
-            response = f"{self.name}-{scope['path']}"
+            response = f"{self.name}-{scope['path']}-{scope['root_path']}"
             await send(
                 {
                     "type": "http.response.start",
@@ -27,7 +27,15 @@ async def __call__(self, scope: Scope, receive: Callable, send: Callable) -> Non
             await send({"type": "http.response.body", "body": response.encode()})
 
     app = AsyncioDispatcherMiddleware(
-        {"/api/x": EchoFramework("apix"), "/api": EchoFramework("api")}
+        {
+            "/api/x": EchoFramework("apix"),
+            "/api/nested": AsyncioDispatcherMiddleware(
+                {
+                    "/path": EchoFramework("nested"),
+                }
+            ),
+            "/api": EchoFramework("api"),
+        }
     )
 
     sent_events = []
@@ -38,12 +46,15 @@ async def send(message: dict) -> None:
 
     await app({**http_scope, **{"path": "/api/x/b"}}, None, send)  # type: ignore
     await app({**http_scope, **{"path": "/api/b"}}, None, send)  # type: ignore
+    await app({**http_scope, **{"path": "/api/nested/path/x"}}, 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"20")]},
+        {"type": "http.response.body", "body": b"apix-/api/x/b-/api/x"},
+        {"type": "http.response.start", "status": 200, "headers": [(b"content-length", b"15")]},
+        {"type": "http.response.body", "body": b"api-/api/b-/api"},
+        {"type": "http.response.start", "status": 200, "headers": [(b"content-length", b"42")]},
+        {"type": "http.response.body", "body": b"nested-/api/nested/path/x-/api/nested/path"},
         {"type": "http.response.start", "status": 404, "headers": [(b"content-length", b"0")]},
         {"type": "http.response.body"},
     ]