Any way to check for disconnection without side effects (like discarding request body)

[msdrigg]

For some background, I got here looking for a way to cancel costly actions if the client disconnects. I submitted this same discussion to starlette encode/starlette#2215, but I thought that this might be a better form:

I see that starlette offers an async is_disconnected method on their Request:

async def is_disconnected(self) -> bool:
    if not self._is_disconnected:
        message: Message = {}

        # If message isn't immediately available, move on
        with anyio.CancelScope() as cs:
            cs.cancel()
            message = await self._receive()

        if message.get("type") == "http.disconnect":
            self._is_disconnected = True

    return self._is_disconnected

https://github.com/encode/starlette/blob/844220819f1cfcf2726d1628dd4ea597182d6f3e/starlette/requests.py#L293

Here we see that is_disconnected calls self._receive() (which is the asgi receive function). I believe that this could possibly consume and discard a http.request message containing the request body.

Is there any way to listen for disconnection (ideally create an Event that signals disconnection) without discarding any messages from receive? I read the spec, but unless I buffer arbitrarily many messages from receive, I don't know how I would do this.

I am relatively new to asgi, so I may be missing something (or not asking in the right place), but I wanted to reach out to see if anyone here could point me in the right direction

[hari01584]

Can you see if fastapi/fastapi#11360 can work for you?

[msdrigg]

So your solution is really good actually. All my attempts just lose the body, while yours keeps it and returns it after buffering. I still see this as limiting though, because it will buffer the entire body, so any kind of streamed request will get buffered entirely in memory and this will remove any kind of tcp window control/backpressure support (https://sumofbytes.com/blog/understanding-tcp-protocol-and-backpressure).

Most ASGI servers (uvicorn for example: https://www.uvicorn.org/server-behavior/#flow-control) support tcp flow control based off when the client calls thereceive() method.

[msdrigg]

I'd support the PR though and realistically I think it's the best that can be done without an extension to the ASGI protocol with a .receiveDisconnect() method or something