Add listener for disconnect to FileResponse

[idotobi]

This fixes #1301.

This makes FileResponse behave the same way as StreamResponse,
namely aborting sending data once a http.disconnect is received from the client.

Note that this behaviour is currently not tested in StreamResponse.
I found one way to test this behaviour. However, it would require to add httpx as a testing requirement hence it is not part of the PR yet.
FileResponse is even more tricky to test without having a way to mark a FileResponse as finished (would require a new property).

[idotobi]

Note: The code for __call__ and listen_for_disconnect are implemented exactly as in StreamResponse. It might be worthwhile to make FileResponse inherit from StreamResponse in the future as they are quite similar in nature.

[Kludex]

Interesting. We don't have a specific test for StramingResponse as well, for the same case.

[Kludex]

I found one way to test this behaviour. However, it would require to add httpx as a testing requirement hence it is not part of the PR yet.

@idotobi How?

[tomchristie]

Hiya @idotobi - thank you for the neatly tackled bit of work here.

I'd like to close this off with the justification as outlined on #1301

There's always a complexity trade-off in these kinds of cases, and my feeling is that Starlette ought to lean very strongly towards as-simple-as-possible-wherever-possible.

(Something we need to get better at as a team is making sure we're fielding incoming requests like this, so that you're not sending time on neatly done pull requests like this one, and then having them closed off. We're working on that.)

[idotobi]

@Kludex : It was a rather messy prototype, where I had to remove some asserts within the httpx testclient, so I wouldn't recommend it for production usage.

The idea was basically to exchange the receive callback in the app definition

    async def app(scope, receive, send):
        async def receive() -> dict:
            return {"type": "http.disconnect"}

This way the server would abort the connection itself, if it listened to http.disconnect.
...As I said. Not pretty but sadly there is to Testclient I found that was able to send disconnect while waiting on the response out-of-the-box.

[idotobi]

@tomchristie : No worries. I learned a bit on the way, so it's no time wasted.