Error "Trying to start a new request while another is active" in browser while setting the theme, pushing, and adding a grid in the same request.

[eriklumme]

Steps to reproduce

1. Enable @Push
2. In a click listener, do the following
   2.1. Set the theme to an existing theme other than the current using UI#setTheme
   2.2. Manually push using UI#push
   2.3. Add a Grid

Actual outcome

An error Trying to start a new request while another is active is displayed in the browser console.

I believe this is the root cause of sometimes infinite resynchronizations of the page in a real application, but I have not been able to reproduce it completely. The real app does not use manual pushing, but it does have push enabled (long polling) and it accesses the UI from background threads.

Expected outcome

No errors in the console.

Example code

After clicking the first button, the theme must be reset before the error occurs again.

@Theme("mytheme")
@Push
public class MyUI extends UI {

    @Override
    protected void init(VaadinRequest vaadinRequest) {
        VerticalLayout layout = new VerticalLayout();
        UI ui = UI.getCurrent();

        Button button = new Button("Explosion button");
        button.addClickListener(e -> {
            // Must be a different theme, and its style files must exist
            ui.setTheme("my-theme");
            ui.push();
            layout.addComponent(new Grid<>());
        });

        Button reset = new Button("Reset theme");
        reset.addClickListener(e -> ui.setTheme("mytheme"));
        
        layout.addComponents(button, reset);
        setContent(layout);
    }
    
    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
    public static class MyUIServlet extends VaadinServlet {
    }
}

[eriklumme]

I initially thought it was reproducible with a ComboBox too, but it seems that was just due to my session being serialized across server restarts.

[ShawnRG]

What version of vaadin are you using?

[ShawnRG]

If you're using background threads to update your UI you should always provide them with the UI instance as since vaadin version 8 the UI.getCurrent() is a weak reference to avoid memory leaks.
So you you could cache your UI in either of the following ways:

// This should be called from the main UI thread.
final UI ui = UI.getCurrent();

or preferably when calling from within a Component

// From within the component
final UI ui = getUI();

Then what you should do is wrap your code used in the ClickListener in a UI.access() runnable block, like so:

final UI ui = UI.getCurrent();
ui.access(() -> {
    ui.setTheme("mytheme2");
    ui.push();
    layout.addComponent(new Grid<>());
})

Hope this helps

[eriklumme]

Hi @ShawnRG . Thanks for your reply, but as you can see from the MWE it's not an issue with background threads. It should be reproducible with any Vaadin 8.X version.

It seems to be an issue with Vaadin's MessageSender not taking active requests into account when issuing a resynchronization due to theme changes.

I have made a PoC for a solution here
8.8...eriklumme:8.8