Make sure TryGetGlyphTypeface does not fail for concurrent access

src/Avalonia.Base/Media/FontManager.cs

@@ -360,25 +360,20 @@ namespace Avalonia.Media
                 source = SystemFontsKey;
             }
 
-            if (!_fontCollections.TryGetValue(source, out fontCollection))
+            fontCollection = _fontCollections.GetOrAdd(source, static (key, platformImpl) =>
             {
-                if (source == SystemFontsKey)
+                if (key == SystemFontsKey)
                 {
-                    fontCollection = new SystemFontCollection(PlatformImpl);
-                }
-                else
-                {
-                    if (source.IsAbsoluteResm() || source.IsAvares())
-                    {
-                        fontCollection = new EmbeddedFontCollection(source, source);
-                    }
+                    return new SystemFontCollection(platformImpl);
                 }
 
-                if (fontCollection != null)
+                if (key.IsAbsoluteResm() || key.IsAvares())
                 {
-                    return _fontCollections.TryAdd(fontCollection.Key, fontCollection);
+                    return new EmbeddedFontCollection(key, key);
                 }
-            }
+
+                return null!;
+            }, PlatformImpl);
 
             return fontCollection != null;
         }

tests/Avalonia.Base.UnitTests/Media/FontManagerTests.cs

@@ -1,5 +1,10 @@
 using System;
+using System.Collections.Concurrent;
+using System.Reflection;
+using System.Threading;
+using System.Threading.Tasks;
 using Avalonia.Media;
+using Avalonia.Media.Fonts;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -86,5 +91,59 @@ namespace Avalonia.Base.UnitTests.Media
                 Assert.Equal("DejaVu", FontManager.Current.DefaultFontFamily.Name);
             }
         }
+
+        [Fact]
+        public async Task TryGetGlyphTypeface_Should_Be_Thread_Safe_For_Embedded_Fonts()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var fontManager = FontManager.Current;
+                var fontCollections = (ConcurrentDictionary<Uri, IFontCollection>)
+                    typeof(FontManager)
+                        .GetField("_fontCollections", BindingFlags.NonPublic | BindingFlags.Instance)!
+                        .GetValue(fontManager)!;
+
+                const string fontUri =
+                    "resm:Avalonia.Base.UnitTests.Assets?assembly=Avalonia.Base.UnitTests#Noto Mono";
+                var collectionKey =
+                    new Uri("resm:Avalonia.Base.UnitTests.Assets?assembly=Avalonia.Base.UnitTests");
+
+                // Warm up to validate the font URI is correct.
+                Assert.True(fontManager.TryGetGlyphTypeface(new Typeface(new FontFamily(fontUri)), out _));
+
+                const int iterations = 50;
+                int failures = 0;
+
+                for (int i = 0; i < iterations; i++)
+                {
+                    // Remove the cached collection so both threads must re-create it.
+                    fontCollections.TryRemove(collectionKey, out _);
+
+                    using var barrier = new Barrier(2);
+                    bool r1 = false, r2 = false;
+
+                    var t1 = Task.Run(() =>
+                    {
+                        barrier.SignalAndWait();
+                        r1 = fontManager.TryGetGlyphTypeface(
+                            new Typeface(new FontFamily(fontUri)), out _);
+                    });
+
+                    var t2 = Task.Run(() =>
+                    {
+                        barrier.SignalAndWait();
+                        r2 = fontManager.TryGetGlyphTypeface(
+                            new Typeface(new FontFamily(fontUri)), out _);
+                    });
+
+                    await Task.WhenAll(t1, t2);
+
+                    if (!r1 || !r2)
+                        Interlocked.Increment(ref failures);
+                }
+
+                Assert.Equal(0, failures);
+            }
+        }
     }
 }