NullReferenceException in Avalonia.X11.X11EventDispatcher.DispatchX11Events

[Whiletru3]

Describe the bug

Hello,

On my linux app I have randomly exceptions in DispatchX11Events

System.NullReferenceException: Object reference not set to an instance of an object.
   at Avalonia.X11.X11EventDispatcher.DispatchX11Events(CancellationToken cancellationToken)
   at Avalonia.X11.X11PlatformThreading.RunLoop(CancellationToken cancellationToken)
   at Avalonia.Threading.DispatcherFrame.Run(IControlledDispatcherImpl impl)
   at Avalonia.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at Avalonia.Threading.Dispatcher.MainLoop(CancellationToken cancellationToken)
   at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.StartCore(String[] args)
   at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(String[] args)
   at Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime(AppBuilder builder, String[] args, Action`1 lifetimeBuilder)

Here is my program.cs (used in prod perfect in win & macos)

class Program
    {
        // Initialization code. Don't use any Avalonia, third-party APIs or any
        // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
        // yet and stuff might break.
        [STAThread]
        public static void Main(string[] args) 
        {
            try
            {
                BuildAvaloniaApp()
                    .StartWithClassicDesktopLifetime(args);
            }
            catch (Exception e)
            {
                App.Logger.Error(e);
                throw;
            }
            
            Environment.Exit(0);
        }
        // Avalonia configuration, don't remove; also used by visual designer.
        public static AppBuilder BuildAvaloniaApp()
            => AppBuilder.Configure<App>()
                .UsePlatformDetect()
                .UseReactiveUI();
    }

To Reproduce

I just use my app then sometimes the exception is triggered

Expected behavior

A stable app :)

Avalonia version

11.3.9

OS

Linux

Additional context

No response

[Whiletru3]

the exception is triggered on this line and appears ramdomly.
In this case, handler is null
else if (_eventHandlers.TryGetValue(xev.AnyEvent.window, out var handler))
handler(ref xev);

public unsafe void DispatchX11Events(CancellationToken cancellationToken)
    {
        while (IsPending)
        {
            if (cancellationToken.IsCancellationRequested)
                return;
                
            XNextEvent(_display, out var xev);
            if(XFilterEvent(ref xev, IntPtr.Zero))
                continue;

            if (xev.type == XEventName.GenericEvent)
                XGetEventData(_display, &xev.GenericEventCookie);
            try
            {
                if (xev.type == XEventName.GenericEvent)
                {
                    if (_platform.XI2 != null && _platform.Info.XInputOpcode ==
                        xev.GenericEventCookie.extension)
                    {
                        _platform.XI2.OnEvent((XIEvent*)xev.GenericEventCookie.data);
                    }
                }
                else if (_eventHandlers.TryGetValue(xev.AnyEvent.window, out var handler))
                    handler(ref xev);
            }
            finally
            {
                if (xev.type == XEventName.GenericEvent && xev.GenericEventCookie.data != null)
                    XFreeEventData(_display, &xev.GenericEventCookie);
            }
        }
        Flush();
    }

[timunie]

I wonder if you may use something async which should be on the UiThread. A reproduction sample would be great.

[Carti-it]

@MrJul What if these two lines

Avalonia/src/Avalonia.X11/Dispatching/X11EventDispatcher.cs

Lines 49 to 50 in 0715676

	else if (_eventHandlers.TryGetValue(xev.AnyEvent.window, out var handler))
	handler(ref xev);

were replaced with:

else if (_eventHandlers.TryGetValue(xev.AnyEvent.window, out var handler))
{
    if (handler is null)
        throw new NullReferenceException("Fatal: Event handler for '{xev?.AnyEvent?.window}' is null!");

    handler(ref xev);
}

Perhaps it would help with debugging of this issue. What do you think?

[timunie]

@Carti-it feel free to open a PR with this changes. It will give you a preview nuget package for you to test locally. If you find a solution from there, the PR can be updated.

[Carti-it]

I opened #20592.

[Whiletru3]

I still have this issue using avalonia 12.1.1, when it hqppen, the XEvent struct is not null but is all set to 0

I also logged this other issue on windows in the same method :
#22148