Implemented GLib-based dispatcher (#17281)

* Implemented GLib-based dispatcher

This should allow running Avalonia on the same thread as another UI toolkit that supports running on top of GRunLoop (e. g. GTK)

* Force-drain the X11 event queue, since g_runloop_quit won't exit the loop otherwise
#Conflicts:
#	src/Avalonia.X11/X11Platform.cs

Author: kekekeks

samples/ControlCatalog.NetCore/ControlCatalog.NetCore.csproj

@@ -9,6 +9,7 @@
 
   <ItemGroup>
     <Compile Include="..\..\src\Avalonia.X11\NativeDialogs\Gtk.cs" Link="NativeControls\Gtk\Gtk.cs" />
+    <Compile Include="..\..\src\Avalonia.X11\Interop\Glib.cs" Link="NativeControls\Gtk\Glib.cs" />
     <Compile Include="..\..\src\Avalonia.Base\Platform\Interop\Utf8Buffer.cs" Link="NativeControls\Utf8Buffer.cs" />
   </ItemGroup>
 

samples/ControlCatalog.NetCore/NativeControls/Gtk/GtkHelper.cs

@@ -5,7 +5,7 @@
 using Avalonia.X11.Interop;
 using Avalonia.X11.NativeDialogs;
 using static Avalonia.X11.NativeDialogs.Gtk;
-using static Avalonia.X11.NativeDialogs.Glib;
+using static Avalonia.X11.Interop.Glib;
 
 namespace ControlCatalog.NetCore;
 

src/Avalonia.X11/Dispatching/GLibDispatcherImpl.cs

@@ -0,0 +1,313 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Runtime.ExceptionServices;
+using System.Threading;
+using Avalonia.Logging;
+using Avalonia.Threading;
+using static Avalonia.X11.Interop.Glib;
+namespace Avalonia.X11.Dispatching;
+
+internal class GlibDispatcherImpl : 
+    IDispatcherImplWithExplicitBackgroundProcessing,
+    IControlledDispatcherImpl
+{
+    /*
+        GLib priorities and Avalonia priorities are a bit different. Avalonia follows the WPF model when there
+            are "background" and "foreground" priority groups. Foreground jobs are executed before any user input processing,
+            background jobs are executed strictly after user input processing.
+        
+        GLib has numeric priorities that are used in the following way:
+        -100    G_PRIORITY_HIGH - "high" priority sources, not really used by GLib/GTK
+        0       G_PRIORITY_DEFAULT - polling X11 events (GTK) and default value for g_timeout_add
+        100     G_PRIORITY_HIGH_IDLE without a clear definition, used as an anchor value of sorts
+        110     Resize/layout operations (GTK)
+        120     Render operations (GTK)
+        200     G_PRIORITY_DEFAULT_IDLE - "idle" priority sources
+        
+        So, unlike Avalonia, GTK puts way higher priority on input processing, then does resize/layout/render
+        
+        So, to map our model to GLib we do the following:
+        - foreground jobs (including grouped user events) are executed with (-1) priority (_before_ any normal GLib jobs)
+        - X11 socket is polled with G_PRIORITY_DEFAULT, all X11 events are read until socket is empty,
+            we also group input events at that stage (this matches our epoll-based dispatcher)
+        - background jobs are executed with G_PRIORITY_DEFAULT_IDLE, so they would have lower priority than GTK
+            foreground jobs
+        
+        Unfortunately we can't detect if there are pending _non-idle_ GLib jobs using g_main_context_pending, since
+        - g_main_context_pending doesn't accept max_priority argument
+        - even if it did, that would still involve a syscall to the kernel to poll for fds anyway
+        
+        So we just report that we don't support pending input query and let the dispatcher to 
+        call RequestBackgroundProcessing every time, which results in g_idle_add call for every background job.
+        Background jobs are expected to be relatively expensive to execute since on Windows 
+        MsgWaitForMultipleObjectsEx results isn't really free too.
+
+        For signaling (aka waking up dispatcher for processing _high_ priority jobs we are using
+        g_idle_add_full with (-1) priority. While the naming suggests that it would enqueue an idle job,
+        it actually adds an always-triggered source that would be called before other sources with lower priority.
+
+        For timers we are using a simple g_timeout_add_full and discard the previous one when dispatcher requests
+        an update
+        
+        Since GLib dispatches event sources in batches, we force-check for "signaled" flag to run high-prio jobs
+        whenever we get control back from GLib. We can still occasionally get GTK code to run before high-prio
+        Avalonia-jobs, but that should be fine since the point is to keep Avalonia-based jobs ordered properly
+        and to not have our low-priority jobs to prevent GLib-based code from running its own "foreground" jobs 
+
+        Another implementation note here is that GLib (just as any other C library) is NOT aware of C# exceptions,
+        so we are NOT allowed to have exceptions to escape native->managed call boundary. So we have exception handlers
+        that try to propagate those to the nearest run loop frame that was initiated by Avalonia.
+        
+        If there is no such frame, we have no choice but to log/swallow those
+     */
+    
+    private readonly AvaloniaX11Platform _platform;
+    
+    // Note that we can't use g_main_context_is_owner outside a run loop, since context doesn't really have an
+    // inherent owner when run loop is not running and the context isn't explicitly "locked", so we just assume that
+    // the app author is initializing Avalonia on the intended UI thread and won't migrate the default run loop
+    // to a different thread
+    private readonly Thread _mainThread = Thread.CurrentThread;
+    
+    private readonly X11EventDispatcher _x11Events;
+    private bool _signaled;
+    private bool _signaledSourceAdded;
+    private readonly object _signalLock = new();
+    private readonly Stack<ManagedLoopFrame> _runLoopStack = new();
+    
+    private readonly Stopwatch _stopwatch = Stopwatch.StartNew();
+    private uint? _glibTimerSourceTag;
+
+    public GlibDispatcherImpl(AvaloniaX11Platform platform)
+    {
+        _platform = platform;
+        _x11Events = new X11EventDispatcher(platform);
+        var unixFdId = g_unix_fd_add_full(G_PRIORITY_DEFAULT, _x11Events.Fd, GIOCondition.G_IO_IN,
+            X11SourceCallback);
+        // We can trigger a nested event loop when handling X11 events, so we need to mark the source as recursive
+        var unixFdSource = g_main_context_find_source_by_id(IntPtr.Zero, unixFdId);
+        g_source_set_can_recurse(unixFdSource, 1);
+    }
+
+    public bool CurrentThreadIsLoopThread => _mainThread == Thread.CurrentThread;
+    
+    public event Action? Signaled;
+    public void Signal()
+    {
+        lock (_signalLock)
+        {
+            if(_signaled)
+                return;
+            _signaled = true;
+            if(_signaledSourceAdded)
+                return;
+            _signaledSourceAdded = true;
+        }
+        g_idle_add_full(G_PRIORITY_DEFAULT - 1, SignalSourceCallback);
+    }
+
+    private void CheckSignaled()
+    {
+        lock (_signalLock)
+        {
+            if (!_signaled)
+                return;
+            _signaled = false;
+        }
+
+        try
+        {
+            Signaled?.Invoke();
+        }
+        catch (Exception e)
+        {
+            HandleException(e);
+        }
+        _x11Events.Flush();
+    }
+    
+    private bool SignalSourceCallback()
+    {
+        lock (_signalLock)
+        {
+            _signaledSourceAdded = false;
+        }
+        CheckSignaled();
+        return false;
+    }
+    
+    public event Action? Timer;
+    public long Now => _stopwatch.ElapsedMilliseconds;
+    
+    public void UpdateTimer(long? dueTimeInMs)
+    {
+        if (_glibTimerSourceTag.HasValue)
+        {
+            g_source_remove(_glibTimerSourceTag.Value);
+            _glibTimerSourceTag = null;
+        }
+
+        if (dueTimeInMs == null)
+            return;
+
+        var interval = (uint)Math.Max(0, (int)Math.Min(int.MaxValue, dueTimeInMs.Value - Now));
+        _glibTimerSourceTag = g_timeout_add_once(interval, TimerCallback);
+    }
+    
+    private void TimerCallback()
+    {
+        try
+        {
+            Timer?.Invoke();
+        }
+        catch (Exception e)
+        {
+            HandleException(e);
+        }
+        _x11Events.Flush();
+    }
+    
+    public event Action? ReadyForBackgroundProcessing;
+
+    public void RequestBackgroundProcessing() =>
+        g_idle_add_once(() => ReadyForBackgroundProcessing?.Invoke());
+    
+    public bool CanQueryPendingInput => false;
+    public bool HasPendingInput => _platform.EventGrouperDispatchQueue.HasJobs || _x11Events.IsPending;
+    
+    private bool X11SourceCallback(int i, GIOCondition gioCondition)
+    {
+        CheckSignaled();
+        var token = _runLoopStack.Count > 0 ? _runLoopStack.Peek().Cancelled : CancellationToken.None;
+        try
+        {
+            // Completely drain X11 socket while we are at it
+            while (_x11Events.IsPending)
+            {
+                // If we don't actually drain our X11 socket, GLib _will_ call us again even if
+                // we request the run loop to quit
+                _x11Events.DispatchX11Events(CancellationToken.None);
+                if (!token.IsCancellationRequested)
+                {
+                    while (_platform.EventGrouperDispatchQueue.HasJobs)
+                    {
+                        CheckSignaled();
+                        _platform.EventGrouperDispatchQueue.DispatchNext();
+                    }
+
+                    _x11Events.Flush();
+                }
+            }
+        }
+        catch (Exception e)
+        {
+            HandleException(e);
+        }
+
+        return true;
+    }
+    
+    public void RunLoop(CancellationToken token)
+    {
+        if(token.IsCancellationRequested)
+            return;
+
+        using var loop = new ManagedLoopFrame(token);
+        _runLoopStack.Push(loop);
+        loop.Run();
+        _runLoopStack.Pop();
+        
+        // Propagate any managed exceptions that we've captured from this frame
+        if(loop.Exceptions.Count == 1)
+            loop.Exceptions[0].Throw();
+        else if (loop.Exceptions.Count > 1)
+            throw new AggregateException(loop.Exceptions.Select(x => x.SourceException));
+    }
+
+    void HandleException(Exception e)
+    {
+        if (_runLoopStack.Count > 0)
+        {
+            var frame = _runLoopStack.Peek();
+            frame.Exceptions.Add(ExceptionDispatchInfo.Capture(e));
+            frame.Stop();
+        }
+        else
+        {
+            var externalLogger = _platform.Options.ExterinalGLibMainLoopExceptionLogger;
+            if (externalLogger != null)
+                externalLogger.Invoke(e);
+            else
+                Logger.TryGet(LogEventLevel.Error, LogArea.Control)
+                    ?.Log("Dispatcher", "Unhandled exception: {exception}", e);
+        }
+    }
+    
+    private class ManagedLoopFrame : IDisposable
+    {
+        private readonly CancellationToken _externalToken;
+        private CancellationTokenSource? _internalTokenSource;
+        public CancellationToken Cancelled { get; private set; }
+        
+        private readonly IntPtr _loop = g_main_loop_new(IntPtr.Zero, 1);
+        public List<ExceptionDispatchInfo> Exceptions { get; } = new();
+        private readonly object _destroyLock = new();
+        private bool _disposed;
+
+        public ManagedLoopFrame(CancellationToken token)
+        {
+            _externalToken = token;
+        }
+
+        public void Stop()
+        {
+            try
+            {
+                _internalTokenSource?.Cancel();
+            }
+            catch
+            {
+                // Ignore
+            }
+        }
+
+        public void Run()
+        {
+            if (_externalToken.IsCancellationRequested)
+                return;
+            using (_internalTokenSource = new())
+            using (var composite =
+                   CancellationTokenSource.CreateLinkedTokenSource(_externalToken, _internalTokenSource.Token))
+            {
+                Cancelled = composite.Token;
+                using (Cancelled.Register(() =>
+                       {
+                           lock (_destroyLock)
+                           {
+                               if (_disposed)
+                                   return;
+                               g_main_loop_quit(_loop);
+                           }
+                       }))
+                {
+                    g_main_loop_run(_loop);
+                }
+            }
+        }
+
+        public void Dispose()
+        {
+            lock (_destroyLock)
+            {
+                if(_disposed)
+                    return;
+                _disposed = true;
+                g_main_loop_unref(_loop);
+            }
+        }
+    }
+    
+}

src/Avalonia.X11/Dispatching/X11EventDispatcher.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using static Avalonia.X11.XLib;
+namespace Avalonia.X11;
+
+internal class X11EventDispatcher
+{
+    private readonly AvaloniaX11Platform _platform;
+    private readonly IntPtr _display;
+
+    public delegate void EventHandler(ref XEvent xev);
+    public int Fd { get; }
+    private readonly Dictionary<IntPtr, EventHandler> _eventHandlers;
+
+    public X11EventDispatcher(AvaloniaX11Platform platform)
+    {
+        _platform = platform;
+        _display = platform.Display;
+        _eventHandlers = platform.Windows;
+        Fd = XLib.XConnectionNumber(_display);
+    }
+
+    public bool IsPending => XPending(_display) != 0;
+    
+    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();
+    }
+
+    public void Flush() => XFlush(_display);
+}