Wrap access to GrContext in locks

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Threading;
 using Avalonia.Media;
 using Avalonia.Platform;
 using Avalonia.Rendering;
@@ -20,7 +21,7 @@ namespace Avalonia.Skia
     /// </summary>
     public class DrawingContextImpl : IDrawingContextImpl
     {
-        private readonly IDisposable[] _disposables;
+        private IDisposable[] _disposables;
         private readonly Vector _dpi;
         private readonly Stack<PaintWrapper> _maskStack = new Stack<PaintWrapper>();
         private readonly Stack<double> _opacityStack = new Stack<double>();
@@ -30,7 +31,7 @@ namespace Avalonia.Skia
         private readonly bool _canTextUseLcdRendering;
         private Matrix _currentTransform;
         private GRContext _grContext;
-
+        private bool _disposed;
         /// <summary>
         /// Context create info.
         /// </summary>
@@ -74,6 +75,8 @@ namespace Avalonia.Skia
             _disposables = disposables;
             _canTextUseLcdRendering = !createInfo.DisableTextLcdRendering;
             _grContext = createInfo.GrContext;
+            if (_grContext != null)
+                Monitor.Enter(_grContext);
             
             Canvas = createInfo.Canvas;
 
@@ -257,14 +260,26 @@ namespace Avalonia.Skia
         /// <inheritdoc />
         public virtual void Dispose()
         {
-            if (_disposables == null)
-            {
+            if(_disposed)
                 return;
-            }
+            try
+            {
+                if (_grContext != null)
+                {
+                    Monitor.Exit(_grContext);
+                    _grContext = null;
+                }
 
-            foreach (var disposable in _disposables)
+                if (_disposables != null)
+                {
+                    foreach (var disposable in _disposables)
+                        disposable?.Dispose();
+                    _disposables = null;
+                }
+            }
+            finally
             {
-                disposable?.Dispose();
+                _disposed = true;
             }
         }
 

src/Skia/Avalonia.Skia/GlRenderTarget.cs

@@ -35,31 +35,36 @@ namespace Avalonia.Skia
             gl.ClearStencil(0);
             gl.ClearColor(0, 0, 0, 0);
             gl.Clear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
-            _grContext.ResetContext();
-
-            GRBackendRenderTarget renderTarget =
-                new GRBackendRenderTarget(size.Width, size.Height, disp.SampleCount, disp.StencilSize,
-                    new GRGlFramebufferInfo((uint)fb, GRPixelConfig.Rgba8888.ToGlSizedFormat()));
-            var surface = SKSurface.Create(_grContext, renderTarget,
-                GRSurfaceOrigin.BottomLeft,
-                GRPixelConfig.Rgba8888.ToColorType());
-            
-            var nfo = new DrawingContextImpl.CreateInfo
+            lock (_grContext)
             {
-                GrContext = _grContext,
-                Canvas = surface.Canvas,
-                Dpi = SkiaPlatform.DefaultDpi * scaling,
-                VisualBrushRenderer = visualBrushRenderer,
-                DisableTextLcdRendering = true
-            };
+                _grContext.ResetContext();
 
-            return new DrawingContextImpl(nfo, Disposable.Create(() =>
-            {
-                surface.Canvas.Flush();
-                surface.Dispose();
-                renderTarget.Dispose();
-                session.Dispose();
-            }));
+                GRBackendRenderTarget renderTarget =
+                    new GRBackendRenderTarget(size.Width, size.Height, disp.SampleCount, disp.StencilSize,
+                        new GRGlFramebufferInfo((uint)fb, GRPixelConfig.Rgba8888.ToGlSizedFormat()));
+                var surface = SKSurface.Create(_grContext, renderTarget,
+                    GRSurfaceOrigin.BottomLeft,
+                    GRPixelConfig.Rgba8888.ToColorType());
+
+                var nfo = new DrawingContextImpl.CreateInfo
+                {
+                    GrContext = _grContext,
+                    Canvas = surface.Canvas,
+                    Dpi = SkiaPlatform.DefaultDpi * scaling,
+                    VisualBrushRenderer = visualBrushRenderer,
+                    DisableTextLcdRendering = true
+                };
+
+                return new DrawingContextImpl(nfo, Disposable.Create(() =>
+                {
+                    
+                    surface.Canvas.Flush();
+                    surface.Dispose();
+                    renderTarget.Dispose();
+                    _grContext.Flush();
+                    session.Dispose();
+                }));
+            }
         }
     }
 }