Make AsyncState thread safe

src/Libraries/Microsoft.Extensions.AsyncState/AsyncState.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections.Generic;
 using System.Threading;
 using Microsoft.Extensions.ObjectPool;
 using Microsoft.Shared.Diagnostics;
@@ -13,7 +12,7 @@ namespace Microsoft.Extensions.AsyncState;
 internal sealed class AsyncState : IAsyncState
 {
     private static readonly AsyncLocal<AsyncStateHolder> _asyncContextCurrent = new();
-    private static readonly ObjectPool<List<object?>> _featuresPool = PoolFactory.CreatePool(new FeaturesPooledPolicy());
+    private static readonly ObjectPool<Features> _featuresPool = PoolFactory.CreatePool(new FeaturesPooledPolicy());
     private int _contextCount;
 
     public void Initialize()
@@ -22,12 +21,12 @@ public void Initialize()
 
         // Use an object indirection to hold the AsyncContext in the AsyncLocal,
         // so it can be cleared in all ExecutionContexts when its cleared.
-        var features = new AsyncStateHolder
+        var asyncStateHolder = new AsyncStateHolder
         {
             Features = _featuresPool.Get()
         };
 
-        _asyncContextCurrent.Value = features;
+        _asyncContextCurrent.Value = asyncStateHolder;
     }
 
     public void Reset()
@@ -60,9 +59,7 @@ public bool TryGet(AsyncStateToken token, out object? value)
             return false;
         }
 
-        EnsureCount(_asyncContextCurrent.Value.Features, token.Index + 1);
-
-        value = _asyncContextCurrent.Value.Features[token.Index];
+        value = _asyncContextCurrent.Value.Features.Get(token.Index);
         return true;
     }
 
@@ -86,28 +83,14 @@ public void Set(AsyncStateToken token, object? value)
             Throw.InvalidOperationException("Context is not initialized");
         }
 
-        EnsureCount(_asyncContextCurrent.Value.Features, token.Index + 1);
-
-        _asyncContextCurrent.Value.Features[token.Index] = value;
-    }
-
-    internal static void EnsureCount(List<object?> features, int count)
-    {
-#if NET6_0_OR_GREATER
-        features.EnsureCapacity(count);
-#endif
-        var difference = count - features.Count;
-
-        for (int i = 0; i < difference; i++)
-        {
-            features.Add(null);
-        }
+        _asyncContextCurrent.Value.Features.Set(token.Index, value);
     }
 
     internal int ContextCount => Volatile.Read(ref _contextCount);
 
     private sealed class AsyncStateHolder
     {
-        public List<object?>? Features { get; set; }
+        public Features? Features { get; set; }
     }
+
 }

src/Libraries/Microsoft.Extensions.AsyncState/Features.cs

@@ -0,0 +1,45 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+
+namespace Microsoft.Extensions.AsyncState;
+
+internal sealed class Features
+{
+    private readonly List<object?> _items = [];
+
+    public object? Get(int index)
+    {
+        return _items.Count <= index ? null : _items[index];
+    }
+
+    public void Set(int index, object? value)
+    {
+        if (_items.Count <= index)
+        {
+            lock (_items)
+            {
+                var count = index + 1;
+
+#if NET6_0_OR_GREATER
+                _items.EnsureCapacity(count);
+#endif
+
+                var difference = count - _items.Count;
+
+                for (int i = 0; i < difference; i++)
+                {
+                    _items.Add(null);
+                }
+            }
+        }
+
+        _items[index] = value;
+    }
+
+    public void Clear()
+    {
+        _items.Clear();
+    }
+}

src/Libraries/Microsoft.Extensions.AsyncState/FeaturesPooledPolicy.cs

@@ -1,27 +1,22 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Collections.Generic;
 using Microsoft.Extensions.ObjectPool;
 
 namespace Microsoft.Extensions.AsyncState;
 
-internal sealed class FeaturesPooledPolicy : IPooledObjectPolicy<List<object?>>
+internal sealed class FeaturesPooledPolicy : IPooledObjectPolicy<Features>
 {
     /// <inheritdoc/>
-    public List<object?> Create()
+    public Features Create()
     {
-        return [];
+        return new Features();
     }
 
     /// <inheritdoc/>
-    public bool Return(List<object?> obj)
+    public bool Return(Features obj)
     {
-        for (int i = 0; i < obj.Count; i++)
-        {
-            obj[i] = null;
-        }
-
+        obj.Clear();
         return true;
     }
 }

test/Libraries/Microsoft.Extensions.AsyncState.Tests/AsyncStateTests.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Threading;
 using System.Threading.Tasks;
@@ -205,28 +204,4 @@ public void RegisterContextCorrectly()
 
         Assert.Equal(3, asyncState.ContextCount);
     }
-
-    [Fact]
-    public void EnsureCount_IncreasesCountCorrectly()
-    {
-        var l = new List<object?>();
-        AsyncState.EnsureCount(l, 5);
-        Assert.Equal(5, l.Count);
-    }
-
-    [Fact]
-    public void EnsureCount_WhenCountLessThanExpected()
-    {
-        var l = new List<object?>(new object?[5]);
-        AsyncState.EnsureCount(l, 2);
-        Assert.Equal(5, l.Count);
-    }
-
-    [Fact]
-    public void EnsureCount_WhenCountEqualWithExpected()
-    {
-        var l = new List<object?>(new object?[5]);
-        AsyncState.EnsureCount(l, 5);
-        Assert.Equal(5, l.Count);
-    }
 }

test/Libraries/Microsoft.Extensions.AsyncState.Tests/FeaturesPooledPolicyTests.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using System.Collections.Generic;
 using Xunit;
 
 namespace Microsoft.Extensions.AsyncState.Test;
@@ -13,20 +12,22 @@ public void Return_ShouldBeTrue()
     {
         var policy = new FeaturesPooledPolicy();
 
-        Assert.True(policy.Return([]));
+        Assert.True(policy.Return(new Features()));
     }
 
     [Fact]
     public void Return_ShouldNullList()
     {
         var policy = new FeaturesPooledPolicy();
 
-        var list = policy.Create();
-        list.Add(string.Empty);
-        list.Add(Array.Empty<int>());
-        list.Add(new object());
+        var features = policy.Create();
+        features.Set(0, string.Empty);
+        features.Set(1, Array.Empty<int>());
+        features.Set(2, new object());
 
-        Assert.True(policy.Return(list));
-        Assert.All(list, el => Assert.Null(el));
+        Assert.True(policy.Return(features));
+        Assert.Null(features.Get(0));
+        Assert.Null(features.Get(1));
+        Assert.Null(features.Get(2));
     }
 }