Add AsyncLazy<T>.SuppressRecursiveFactoryDetection property

This allows folks to opt-out of the recursive detection altogether, such that `AsyncLazy<T>.SuppressRelevance()` isn't required (though that call would still carry out the JTF suppression function).

src/Microsoft.VisualStudio.Threading/AsyncLazy`1.cs

@@ -38,7 +38,7 @@ public class AsyncLazy<T>
     /// <summary>
     /// The unique instance identifier.
     /// </summary>
-    private readonly AsyncLocal<object> recursiveFactoryCheck = new AsyncLocal<object>();
+    private AsyncLocal<object>? recursiveFactoryCheck;
 
     /// <summary>
     /// The function to invoke to produce the task.
@@ -72,6 +72,30 @@ public AsyncLazy(Func<Task<T>> valueFactory, JoinableTaskFactory? joinableTaskFa
         this.jobFactory = joinableTaskFactory;
     }
 
+    /// <summary>
+    /// Gets a value indicating whether to suppress detection of a value factory depending on itself.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// A value factory that truly depends on itself (e.g. by calling <see cref="GetValueAsync()"/> on the same instance)
+    /// would deadlock, and by default this class will throw an exception if it detects such a condition.
+    /// However this detection relies on the .NET ExecutionContext, which can flow to "spin off" contexts that are not awaited
+    /// by the factory, and thus could legally await the result of the value factory without deadlocking.
+    /// </para>
+    /// <para>
+    /// When this flows improperly, it can cause <see cref="InvalidOperationException"/> to be thrown, but only when the value factory
+    /// has not already been completed, leading to a difficult to reproduce race condition.
+    /// Such a case can be resolved by calling <see cref="SuppressRelevance"/> around the non-awaited fork in <see cref="ExecutionContext" />,
+    /// or the entire instance can be configured to suppress this check by setting this property to <see langword="true"/>.
+    /// </para>
+    /// <para>
+    /// When this property is set to <see langword="true" />, the recursive factory check will not be performed,
+    /// but <see cref="SuppressRelevance"/> will still call into <see cref="JoinableTaskContext.SuppressRelevance"/>
+    /// if a <see cref="JoinableTaskFactory"/> was provided to the constructor.
+    /// </para>
+    /// </remarks>
+    public bool SuppressRecursiveFactoryDetection { get; init; }
+
     /// <summary>
     /// Gets a value indicating whether the value factory has been invoked.
     /// </summary>
@@ -137,7 +161,7 @@ public bool IsValueFactoryCompleted
     /// <exception cref="ObjectDisposedException">Thrown after <see cref="DisposeValue"/> is called.</exception>
     public Task<T> GetValueAsync(CancellationToken cancellationToken)
     {
-        if (!((this.value is object && this.value.IsCompleted) || this.recursiveFactoryCheck.Value is null))
+        if (this.value is not { IsCompleted: true } && this.recursiveFactoryCheck is { Value: not null })
         {
             // PERF: we check the condition and *then* retrieve the string resource only on failure
             // because the string retrieval has shown up as significant on ETL traces.
@@ -183,7 +207,12 @@ public Task<T> GetValueAsync(CancellationToken cancellationToken)
                         }
                     };
 
-                    this.recursiveFactoryCheck.Value = RecursiveCheckSentinel;
+                    if (!this.SuppressRecursiveFactoryDetection)
+                    {
+                        Assumes.Null(this.recursiveFactoryCheck);
+                        this.recursiveFactoryCheck = new AsyncLocal<object>() { Value = RecursiveCheckSentinel };
+                    }
+
                     try
                     {
                         if (this.jobFactory is object)
@@ -201,7 +230,10 @@ public Task<T> GetValueAsync(CancellationToken cancellationToken)
                     }
                     finally
                     {
-                        this.recursiveFactoryCheck.Value = null;
+                        if (this.recursiveFactoryCheck is not null)
+                        {
+                            this.recursiveFactoryCheck.Value = null;
+                        }
                     }
                 }
             }
@@ -451,7 +483,11 @@ internal RevertRelevance(AsyncLazy<T> owner)
             Requires.NotNull(owner, nameof(owner));
             this.owner = owner;
 
-            (this.oldCheckValue, owner.recursiveFactoryCheck.Value) = (owner.recursiveFactoryCheck.Value, null);
+            if (owner.recursiveFactoryCheck is not null)
+            {
+                (this.oldCheckValue, owner.recursiveFactoryCheck.Value) = (owner.recursiveFactoryCheck.Value, null);
+            }
+
             this.joinableRelevance = owner.jobFactory?.Context.SuppressRelevance();
         }
 
@@ -460,9 +496,9 @@ internal RevertRelevance(AsyncLazy<T> owner)
         /// </summary>
         public void Dispose()
         {
-            if (this.owner is object)
+            if (this.owner?.recursiveFactoryCheck is { } check)
             {
-                this.owner.recursiveFactoryCheck.Value = this.oldCheckValue;
+                check.Value = this.oldCheckValue;
             }
 
             this.joinableRelevance?.Dispose();

src/Microsoft.VisualStudio.Threading/net472/PublicAPI.Unshipped.txt

@@ -13,4 +13,6 @@ Microsoft.VisualStudio.Threading.AsyncLazy<T>.DisposeValueAsync() -> System.Thre
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.IsValueDisposed.get -> bool
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance.Dispose() -> void
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.get -> bool
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.init -> void
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRelevance() -> Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance

src/Microsoft.VisualStudio.Threading/net6.0-windows/PublicAPI.Unshipped.txt

@@ -13,4 +13,6 @@ Microsoft.VisualStudio.Threading.AsyncLazy<T>.DisposeValueAsync() -> System.Thre
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.IsValueDisposed.get -> bool
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance.Dispose() -> void
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.get -> bool
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.init -> void
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRelevance() -> Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance

src/Microsoft.VisualStudio.Threading/net6.0/PublicAPI.Unshipped.txt

@@ -13,4 +13,6 @@ Microsoft.VisualStudio.Threading.AsyncLazy<T>.DisposeValueAsync() -> System.Thre
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.IsValueDisposed.get -> bool
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance.Dispose() -> void
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.get -> bool
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.init -> void
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRelevance() -> Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance

src/Microsoft.VisualStudio.Threading/netstandard2.0/PublicAPI.Unshipped.txt

@@ -13,4 +13,6 @@ Microsoft.VisualStudio.Threading.AsyncLazy<T>.DisposeValueAsync() -> System.Thre
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.IsValueDisposed.get -> bool
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance.Dispose() -> void
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.get -> bool
+Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRecursiveFactoryDetection.init -> void
 Microsoft.VisualStudio.Threading.AsyncLazy<T>.SuppressRelevance() -> Microsoft.VisualStudio.Threading.AsyncLazy<T>.RevertRelevance

test/Microsoft.VisualStudio.Threading.Tests/AsyncLazyTests.cs

@@ -6,10 +6,13 @@
 using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
+
 using Microsoft;
 using Microsoft.VisualStudio.Threading;
+
 using Xunit;
 using Xunit.Abstractions;
+
 using NamedSyncContext = AwaitExtensionsTests.NamedSyncContext;
 
 public class AsyncLazyTests : TestBase
@@ -748,6 +751,105 @@ async Task<int> FireAndForgetCodeAsync()
         }
     }
 
+    [Fact]
+    public async Task SuppressRecursiveFactoryDetection_WithoutJTF()
+    {
+        AsyncManualResetEvent allowValueFactoryToFinish = new();
+        Task<int>? fireAndForgetTask = null;
+        AsyncLazy<int> asyncLazy = null!;
+        asyncLazy = new AsyncLazy<int>(
+            async delegate
+            {
+                fireAndForgetTask = FireAndForgetCodeAsync();
+                await allowValueFactoryToFinish;
+                return 1;
+            },
+            null)
+        {
+            SuppressRecursiveFactoryDetection = true,
+        };
+
+        bool fireAndForgetCodeAsyncEntered = false;
+        Task<int> lazyValue = asyncLazy.GetValueAsync();
+        Assert.True(fireAndForgetCodeAsyncEntered);
+        allowValueFactoryToFinish.Set();
+
+        // Assert that the value factory was allowed to finish.
+        Assert.Equal(1, await lazyValue.WithCancellation(this.TimeoutToken));
+
+        // Assert that the fire-and-forget task was allowed to finish and did so without throwing.
+        Assert.Equal(1, await fireAndForgetTask!.WithCancellation(this.TimeoutToken));
+
+        async Task<int> FireAndForgetCodeAsync()
+        {
+            fireAndForgetCodeAsyncEntered = true;
+            return await asyncLazy.GetValueAsync();
+        }
+    }
+
+    [Theory, PairwiseData]
+    public async Task SuppressRecursiveFactoryDetection_WithJTF(bool suppressWithJTF)
+    {
+        JoinableTaskContext? context = this.InitializeJTCAndSC();
+        SingleThreadedTestSynchronizationContext.IFrame frame = SingleThreadedTestSynchronizationContext.NewFrame();
+
+        JoinableTaskFactory? jtf = context.Factory;
+        AsyncManualResetEvent allowValueFactoryToFinish = new();
+        Task<int>? fireAndForgetTask = null;
+        AsyncLazy<int> asyncLazy = null!;
+        asyncLazy = new AsyncLazy<int>(
+            async delegate
+            {
+                using (suppressWithJTF ? jtf.Context.SuppressRelevance() : default)
+                using (suppressWithJTF ? default : asyncLazy.SuppressRelevance())
+                {
+                    fireAndForgetTask = FireAndForgetCodeAsync();
+                }
+
+                await allowValueFactoryToFinish;
+                return 1;
+            },
+            jtf)
+        {
+            SuppressRecursiveFactoryDetection = true,
+        };
+
+        bool fireAndForgetCodeAsyncEntered = false;
+        bool fireAndForgetCodeAsyncReachedUIThread = false;
+        jtf.Run(async delegate
+        {
+            Task<int> lazyValue = asyncLazy.GetValueAsync();
+            Assert.True(fireAndForgetCodeAsyncEntered);
+            await Task.Delay(AsyncDelay);
+            Assert.False(fireAndForgetCodeAsyncReachedUIThread);
+            allowValueFactoryToFinish.Set();
+
+            // Assert that the value factory was allowed to finish.
+            Assert.Equal(1, await lazyValue.WithCancellation(this.TimeoutToken));
+        });
+
+        // Run a main thread pump so the fire-and-forget task can finish.
+        SingleThreadedTestSynchronizationContext.PushFrame(SynchronizationContext.Current!, frame);
+
+        // Assert that the fire-and-forget task was allowed to finish and did so without throwing.
+        Assert.Equal(1, await fireAndForgetTask!.WithCancellation(this.TimeoutToken));
+
+        async Task<int> FireAndForgetCodeAsync()
+        {
+            fireAndForgetCodeAsyncEntered = true;
+
+            // Yield the caller's thread.
+            // Resuming will require the main thread, since the caller was on the main thread.
+            await Task.Yield();
+
+            fireAndForgetCodeAsyncReachedUIThread = true;
+
+            int result = await asyncLazy.GetValueAsync();
+            frame.Continue = false;
+            return result;
+        }
+    }
+
     [Fact]
     public async Task Dispose_ValueType_Completed()
     {