Fix thread-safety issue causing infinite loops in TypeHelper cache (#20113)

* Initial plan

* Fix thread-safety issue by replacing Dictionary with ConcurrentDictionary

Co-authored-by: grokys <1775141+grokys@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: grokys <1775141+grokys@users.noreply.github.com>

src/Avalonia.Remote.Protocol/MetsysBson.cs

@@ -30,6 +30,7 @@ Code imported from https://github.com/elaberge/Metsys.Bson without any changes
 
 using System;
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
@@ -695,7 +696,7 @@ namespace Metsys.Bson
     [UnconditionalSuppressMessage("Trimming", "IL3050", Justification = "Bson uses reflection")]
     internal class TypeHelper
     {
-        private static readonly IDictionary<Type, TypeHelper> _cachedTypeLookup = new Dictionary<Type, TypeHelper>();
+        private static readonly ConcurrentDictionary<Type, TypeHelper> _cachedTypeLookup = new ConcurrentDictionary<Type, TypeHelper>();
         private static readonly BsonConfiguration _configuration = BsonConfiguration.Instance;
 
         private readonly IDictionary<string, MagicProperty> _properties;
@@ -725,13 +726,7 @@ namespace Metsys.Bson
 
         public static TypeHelper GetHelperForType(Type type)
         {
-            TypeHelper helper;
-            if (!_cachedTypeLookup.TryGetValue(type, out helper))
-            {
-                helper = new TypeHelper(type);
-                _cachedTypeLookup[type] = helper;
-            }
-            return helper;
+            return _cachedTypeLookup.GetOrAdd(type, t => new TypeHelper(t));
         }
 
         public static string FindProperty(LambdaExpression lambdaExpression)

tests/Avalonia.DesignerSupport.Tests/RemoteProtocolTests.cs

@@ -162,6 +162,48 @@ namespace Avalonia.DesignerSupport.Tests
 
         }
 
+        [Fact]
+        [SuppressMessage("Usage", "xUnit1031:Do not use blocking task operations in test method", Justification = "Sync context is explicitly disabled")]
+        void BsonSerializationIsThreadSafe()
+        {
+            Init();
+            // This test verifies that concurrent serialization doesn't cause infinite loops
+            // or corruption in the TypeHelper cache
+            var messages = Enumerable.Range(0, 100).Select(i => new MeasureViewportMessage
+            {
+                Width = i,
+                Height = i * 2
+            }).ToArray();
+
+            var tasks = new List<Task>();
+            var exceptions = new ConcurrentBag<Exception>();
+
+            // Spawn multiple threads that all try to serialize messages concurrently
+            for (int i = 0; i < 10; i++)
+            {
+                var task = Task.Run(() =>
+                {
+                    try
+                    {
+                        foreach (var message in messages)
+                        {
+                            _client.Send(message).Wait(TimeoutInMs);
+                        }
+                    }
+                    catch (Exception ex)
+                    {
+                        exceptions.Add(ex);
+                    }
+                });
+                tasks.Add(task);
+            }
+
+            Task.WaitAll(tasks.ToArray(), TimeoutInMs * messages.Length * 10);
+            
+            // Verify no exceptions occurred
+            Assert.Empty(exceptions);
+        }
+
         public void Dispose()
         {
             _disposables.ForEach(d => d.Dispose());