Add support for ASGI pathsend extension in BaseHTTPMiddleware

docs/middleware.md

@@ -264,7 +264,6 @@ around explicitly, rather than mutating the middleware instance.
 Currently, the `BaseHTTPMiddleware` has some known limitations:
 
 - Using `BaseHTTPMiddleware` will prevent changes to [`contextlib.ContextVar`](https://docs.python.org/3/library/contextvars.html#contextvars.ContextVar)s from propagating upwards. That is, if you set a value for a `ContextVar` in your endpoint and try to read it from a middleware you will find that the value is not the same value you set in your endpoint (see [this test](https://github.com/encode/starlette/blob/621abc747a6604825190b93467918a0ec6456a24/tests/middleware/test_base.py#L192-L223) for an example of this behavior).
-- Using `BaseHTTPMiddleware` will prevent [ASGI pathsend extension](https://asgi.readthedocs.io/en/latest/extensions.html#path-send) to work properly. Thus, if you run your Starlette application with a server implementing this extension, routes returning [FileResponse](responses.md#fileresponse) should avoid the usage of this middleware.
 
 To overcome these limitations, use [pure ASGI middleware](#pure-asgi-middleware), as shown below.
 

starlette/middleware/base.py

@@ -12,6 +12,7 @@
 
 RequestResponseEndpoint = typing.Callable[[Request], typing.Awaitable[Response]]
 DispatchFunction = typing.Callable[[Request, RequestResponseEndpoint], typing.Awaitable[Response]]
+BodyStreamGenerator = typing.AsyncGenerator[typing.Union[bytes, typing.MutableMapping[str, typing.Any]], None]
 T = typing.TypeVar("T")
 
 
@@ -165,9 +166,12 @@ async def coro() -> None:
 
             assert message["type"] == "http.response.start"
 
-            async def body_stream() -> typing.AsyncGenerator[bytes, None]:
+            async def body_stream() -> BodyStreamGenerator:
                 async with recv_stream:
                     async for message in recv_stream:
+                        if message["type"] == "http.response.pathsend":
+                            yield message
+                            break
                         assert message["type"] == "http.response.body"
                         body = message.get("body", b"")
                         if body:

tests/middleware/test_base.py

@@ -2,6 +2,7 @@
 
 import contextvars
 from contextlib import AsyncExitStack
+from pathlib import Path
 from typing import (
     Any,
     AsyncGenerator,
@@ -18,7 +19,7 @@
 from starlette.middleware import Middleware, _MiddlewareClass
 from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
 from starlette.requests import ClientDisconnect, Request
-from starlette.responses import PlainTextResponse, Response, StreamingResponse
+from starlette.responses import FileResponse, PlainTextResponse, Response, StreamingResponse
 from starlette.routing import Route, WebSocketRoute
 from starlette.testclient import TestClient
 from starlette.types import ASGIApp, Message, Receive, Scope, Send
@@ -1132,3 +1133,54 @@ async def send(message: Message) -> None:
         {"type": "http.response.body", "body": b"good!", "more_body": True},
         {"type": "http.response.body", "body": b"", "more_body": False},
     ]
+
+
+@pytest.mark.anyio
+async def test_asgi_pathsend_events(tmpdir: Path) -> None:
+    path = tmpdir / "example.txt"
+    with path.open("w") as file:
+        file.write("<file content>")
+
+    request_body_sent = False
+    response_complete = anyio.Event()
+    events: list[Message] = []
+
+    async def endpoint_with_pathsend(_: Request) -> FileResponse:
+        return FileResponse(path)
+
+    async def passthrough(
+        request: Request, call_next: RequestResponseEndpoint
+    ) -> Response:
+        return await call_next(request)
+
+    app = Starlette(
+        middleware=[Middleware(BaseHTTPMiddleware, dispatch=passthrough)],
+        routes=[Route("/", endpoint_with_pathsend)],
+    )
+
+    scope = {
+        "type": "http",
+        "version": "3",
+        "method": "GET",
+        "path": "/",
+        "extensions": {"http.response.pathsend": {}},
+    }
+
+    async def receive() -> Message:
+        nonlocal request_body_sent
+        if not request_body_sent:
+            request_body_sent = True
+            return {"type": "http.request", "body": b"", "more_body": False}
+        await response_complete.wait()
+        return {"type": "http.disconnect"}
+
+    async def send(message: Message) -> None:
+        events.append(message)
+        if message["type"] == "http.response.pathsend":
+            response_complete.set()
+
+    await app(scope, receive, send)
+
+    assert len(events) == 2
+    assert events[0]["type"] == "http.response.start"
+    assert events[1]["type"] == "http.response.pathsend"