`EventLoop` cannot gracefully exit because it holds a `request_tx`

[bitcapybara]

Hello everyone! I noticed that the ConnectionError::RequestsDone error in rumqttc can be used to implement graceful program shutdown, but during actual runtime, after AsyncClient exits, the EventLoop still holds request_tx, preventing this error from being caught. Moreover, the request_tx held by EventLoop doesn't seem to serve any purpose within the EventLoop itself. Could it be removed to ensure that only AsyncClient holds request_tx?

In the EventLoop::next_request method, when all senders are dropped, it returns a ConnectionError::RequestsDone error

async fn next_request(
    pending: &mut VecDeque<Request>,
    rx: &Receiver<Request>,
    pending_throttle: Duration,
) -> Result<Request, ConnectionError> {
    if !pending.is_empty() {
        time::sleep(pending_throttle).await;
        // We must call .next() AFTER sleep() otherwise .next() would
        // advance the iterator but the future might be canceled before return
        Ok(pending.pop_front().unwrap())
    } else {
        match rx.recv_async().await {
            Ok(r) => Ok(r),
            Err(_) => Err(ConnectionError::RequestsDone),
        }
    }
}

If this issue is accepted, I'd be happy to submit a PR on this issue. 😊

[bitcapybara]

I submitted a code fix for this issue. Now after all clients exit, theEventLoop can listen for the RequestsDone error to graceful shutdown.

EventLoop code

let (client, mut event_loop) = rumqttc::v5::AsyncClient::new(opts, 1024);

tokio::spawn(async move {
    client.publish(&data.topic, qos, false, data.payload).await.ok();  // here send payload and drop client
});

// eventloop
loop {
    match event_loop.poll().await {
        Ok(_) => {}
        Err(ConnectionError::RequestsDone) => break, // <--- HERE wait for message send and exit loop---->
        Err(e) => {
            println!("polling error: {e}");
            tokio::time::sleep(Duration::from_millis(100)).await;
        }
    }
}

[giridher-art]

@bitcapybara changes look good and mention changes in the CHANGELOG . make sure to run the tests

[bitcapybara]

Hello! I have modified the CHANGELOG in the PR #977 . Please review it, thanks!