No way to detect "EOF" on transport

[nikomatsakis]

Problem: there is no "event" to indicate that the input event stream has closed. So a naive "proxy' pattern like:

let (tx, rx) = channel();

Builder::new()
    .on_receive_dispatch(... /* send incoming event to proxy */ ...)
    .connect_with(async |cx| {
        /* receive outgoing event to proxy */
        while let Some(m) = rx.recv().await { cx.send(msg); }
    })
    .await;

will hang forever, because the loop in connect_with never completes. I often create loops like this where the tx endpoint is given to some central dispatcher.

I'm debating the best fix here and I'm not entirely sure.

[nikomatsakis]

Some things that would help

• independent of everything else, we should have a on_close that executes when the incoming transport closes
• independent of everything else, we should make this kind of "message proxying" code easier to write I think. For example, we could have send_from_stream(rx) in the builder and just have it close when there are no more incoming messages

A more heavy-handed option would be to cancel connect_with when the other half drops. That's appealing too, but I wonder if it's too much? It comes back to async cancellation I guess and the lack of a clear ecosystem standard on the best way to think about and recover from that.

[nikomatsakis]

The final option is what #222 does, but I don't think I like the way it does it.