pathsend support

[apawlucz-cisco]

Hello!

I'm having a hard time setting up static file server using:

• starlette 0.50.0
• granian 2.6.0
• FastAPI 0.122.0 (if this matters)

My requirement is the static file requests need to go through the ASGI app (starlette) for the authorization purposes, i.e. some files cannot be seen by unauthorized users. So I can't just use the bare static file serving mechanism from granian.

The error I'm getting now is:

  File "venvs/3.11/core/lib/python3.11/site-packages/starlette/_utils.py", line 85, in collapse_excgroups
    raise exc
  File "venvs/3.11/core/lib/python3.11/site-packages/starlette/responses.py", line 273, in wrap
    await func()
  File "venvs/3.11/core/lib/python3.11/site-packages/starlette/responses.py", line 255, in stream_response
    chunk = chunk.encode(self.charset)
            ^^^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'encode'

In my case the chunk looks like this:

{
    "type": "http.response.pathsend",
    "path": "<PATH_TO_STATIC_FILE>"
}

Corresponding part of the code in starlette:

• https://github.com/Kludex/starlette/blob/0.50.0/starlette/responses.py#L253-L256

I am aware about the discussions taking place in the past in both starlette and granian:

• pathsend causing issues with BaseHTTPMiddleware #2613
• fix: pathsend handling in middlewares #2616
• Strange error with starlette / fastapi emmett-framework/granian#320

And the chronological history on the starlette side:

• https://github.com/Kludex/starlette/pulls?q=is%3Apr+pathsend+is%3Aclosed

I do understand pathsend support was once added, then reverted, then added back so my understanding is it should work in the latest version (0.50.0 at the time of writing this post).

We do use BaseHTTPMiddleware in our app if this matters.

After digging through I realized that here:

• https://github.com/Kludex/starlette/pull/2671/files

The pathsend support was added only to the internal starlette.middleware.base._StreamingResponse, but not the public starlette.responses.StreamingResponse. The case is we have one middleware which uses this public StreamingResponse which causes the issue at hand. When I switch it to use internal _StreamingResponse - then all works.

Our middleware is pure ASGI middleware (so doesn't use BaseHTTPMiddleware).

This is part of this middleware:

            async def body_stream() -> AsyncGenerator[bytes]:
                async for message in recv_stream:
                    if message["type"] == "http.response.pathsend":
                        yield message
                        break
                    assert message["type"] == "http.response.body"
                    yield message.get("body", b"")

            # Peviously:
            # response = StreamingResponse(
            response = _StreamingResponse(
                status_code=message["status"], content=body_stream()
            )

What would be a long-term solution in case like this? Using internal _StreamingResponse doesn't seem to be right thing to do.
I believe StreamingResponse needs to be fixed in a similar fashion in starlette.