Using UI.setPollInterval should not choke the client-server communication

[mstahv]

Description of the bug / feature

If poll interval is set, the client queues server visits if server cannot be reached or there is an ongoing server visit. Server visit should only be queued if previous server poll has finished. This works fine in Vaadin 8 applications, but with V10+ apps the servervisits are queued and finally dumped to server as fast as possible when the clint-server communication is free again. This may cause extra lag when returning from a bad connection and causes obsolete resource usage.

Minimal reproducible example

@Route
public class MainView extends VerticalLayout {

    public MainView() {
        Button button = new Button("Make a long blockking call",
                e -> {
                    try {
                        Thread.sleep(10000);
                    } catch (InterruptedException ex) {
                        Logger.getLogger(MainView.class.getName()).log(Level.SEVERE, null, ex);
                    }

                    Notification.show("Process done");
                });
        add(button);

        UI.getCurrent().addPollListener(p -> {
            System.out.println("POLL " + LocalDateTime.now());
        });

        UI.getCurrent().setPollInterval(1000);

    }

}

Expected behavior

Server visits should be skipped if the communication has stalled.

Actual behavior

Obsolete server visits.

Versions:

- Vaadin / Flow version: 10+
- Java version: any
- OS version: any
- Browser version (if applicable): any
- Application Server (if applicable): any
- IDE (if applicable): any

[Legioth]

I guess that on a low level, what's missing compared to Vaadin 8 is something corresponding to @Delayed(lastOnly = true).

[mstahv]

I was shortly looking into this and tracked the implementation to this class. Had no idea how to try to solve that. https://github.com/vaadin/flow/blob/master/flow-client/src/main/java/com/vaadin/client/communication/Poller.java

[gscaparrotti]

Is there an ETA for a fix?