For customized SynchronizationContext implementation, knowing whether a pending request

has been completed could allow it to remove completed requests. Because JTF requests are potentially sent to multiple
queues, including the JTF internal queue, many of those requests can be processed especially inside low priority or delayed queues.
For low priority queue, completed tasks showing up for high memory usages. This new contract provides a way to make improvements.

This is only for advanced scenarios, common JTF consumers would not access the request or have chance to use it incorrectly.

Author: Lifeng Lu

src/Microsoft.VisualStudio.Threading/IPendingExecutionRequestState.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Microsoft.VisualStudio.Threading
+{
+    /// <summary>
+    /// An optional interface implemented by pending request state posted to the underline synchronization context. It allows synchronization context to remove completed requests.
+    /// </summary>
+    public interface IPendingExecutionRequestState
+    {
+        /// <summary>
+        /// Gets a value indicating whether the current request has been completed, and can be skipped.
+        /// </summary>
+        bool IsCompleted { get; }
+    }
+}

src/Microsoft.VisualStudio.Threading/JoinableTaskFactory.cs

@@ -1058,7 +1058,7 @@ public void Dispose()
     /// A delegate wrapper that ensures the delegate is only invoked at most once.
     /// </summary>
     [DebuggerDisplay("{DelegateLabel}")]
-    internal class SingleExecuteProtector
+    internal class SingleExecuteProtector : IPendingExecutionRequestState
     {
         /// <summary>
         /// Executes the delegate if it has not already executed.
@@ -1112,6 +1112,11 @@ private SingleExecuteProtector(JoinableTask job)
             this.job = job;
         }
 
+        /// <summary>
+        /// Gets a value indicating whether the current request has been completed, and can be skipped.
+        /// </summary>
+        bool IPendingExecutionRequestState.IsCompleted => this.HasBeenExecuted;
+
         /// <summary>
         /// Gets a value indicating whether this instance has already executed.
         /// </summary>

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

@@ -1,2 +1,4 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

src/Microsoft.VisualStudio.Threading/net8.0-windows/PublicAPI.Unshipped.txt

@@ -1,2 +1,4 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

src/Microsoft.VisualStudio.Threading/net8.0/PublicAPI.Unshipped.txt

@@ -1,2 +1,4 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

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

@@ -1,2 +1,4 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
+Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!