Function finish 

async fn finish<S, E, T>(
    writer: JoinHandle<Option<T>>,
    written: &Activity,
    messages: &mut S,
)
where
    S: Stream<Item = Result<Message, E>> + Unpin,

End a WebSocket: wait for the writer, which sends the close of the server, see join_or_abort, then for the answer of the client, see await_close_answer. The writer returns its sink once its close went out, a client gets no time to answer a close it did not get.