[tokio] Detect connection loss

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It's not a question about timeouts: When a client has connected to the server it requests to subscribe to a stream of messages. While the server is waiting for a message to arrive [from the internal mpsc channel] to forward to the client, it needs to detect if the client has disconnected.

I'll add some comments to the original code to better illustrate the problem

async fn handle_client_sub_conn(chrx: Receiver<Msg>, frmio: Framed<...>) {
  loop {
    // This blocks.  If the client disconnects while we're waiting for
    // for a message to arrive over the channel, then we won't notice
    // it here.
    let element = chrx.recv().await.unwrap();

    let buf = serialize(element);

    // It's not until we reach this point where we notice that the
    // client has disconnected
    frmio.send(buf).await.unwrap();
  }
}

The question is simply: How can one detect that the client connection has been lost while waiting for a message to arrive over the mpsc channel. Does one need to attempt a fake read, or is there some other mechanism?