Add a convenience API for using timer in the right way

Author: rynowak

NuGetPackageVerifier.json

@@ -9,6 +9,7 @@
       "Microsoft.Extensions.CommandLineUtils.Sources": {},
       "Microsoft.Extensions.CopyOnWriteDictionary.Sources": {},
       "Microsoft.Extensions.HashCodeCombiner.Sources": {},
+      "Microsoft.Extensions.NonCapturingTimer.Sources": {},
       "Microsoft.Extensions.ObjectMethodExecutor.Sources": {},
       "Microsoft.Extensions.Process.Sources": {},
       "Microsoft.Extensions.PropertyActivator.Sources": {},

shared/Microsoft.Extensions.NonCapturingTimer.Sources/NonCapturingTimer.cs

@@ -0,0 +1,43 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading;
+
+namespace Microsoft.Extensions.Internal
+{
+    // A convenience API for interacting with System.Threading.Timer in a way
+    // that doesn't capture the ExecutionContext. We should be using this (or equivalent)
+    // everywhere we use timers to avoid rooting any values stored in asynclocals.
+    internal static class NonCapturingTimer
+    {
+        public static Timer Create(TimerCallback callback, object state, TimeSpan dueTime, TimeSpan period)
+        {
+            if (callback == null)
+            {
+                throw new ArgumentNullException(nameof(callback));
+            }
+
+            // Don't capture the current ExecutionContext and its AsyncLocals onto the timer
+            bool restoreFlow = false;
+            try
+            {
+                if (!ExecutionContext.IsFlowSuppressed())
+                {
+                    ExecutionContext.SuppressFlow();
+                    restoreFlow = true;
+                }
+
+                return new Timer(callback, state, dueTime, period);
+            }
+            finally
+            {
+                // Restore the current ExecutionContext
+                if (restoreFlow)
+                {
+                    ExecutionContext.RestoreFlow();
+                }
+            }
+        }
+    }
+}

test/Microsoft.Extensions.Internal.Test/Microsoft.Extensions.Internal.Test.csproj

@@ -13,6 +13,7 @@
     <Compile Include="..\..\shared\Microsoft.Extensions.CopyOnWriteDictionary.Sources\**\*.cs" />
     <Compile Include="..\..\shared\Microsoft.Extensions.DotnetToolDispatcher.Sources\**\*.cs" />
     <Compile Include="..\..\shared\Microsoft.Extensions.HashCodeCombiner.Sources\**\*.cs" />
+    <Compile Include="..\..\shared\Microsoft.Extensions.NonCapturingTimer.Sources\**\*.cs" />
     <Compile Include="..\..\shared\Microsoft.Extensions.ObjectMethodExecutor.Sources\**\*.cs" />
     <Compile Include="..\..\shared\Microsoft.Extensions.ParameterDefaultValue.Sources\**\*.cs" />
     <Compile Include="..\..\shared\Microsoft.Extensions.Process.Sources\**\*.cs" />

test/Microsoft.Extensions.Internal.Test/NonCapturingTimerTest.cs

@@ -0,0 +1,40 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.Extensions.Internal
+{
+    public class NonCapturingTimerTest
+    {
+        [Fact]
+        public async Task NonCapturingTimer_DoesntCaptureExecutionContext()
+        {
+            // Arrange
+            var message = new AsyncLocal<string>();
+            message.Value = "Hey, this is a value stored in the execuion context";
+
+            var tcs = new TaskCompletionSource<string>();
+
+            // Act
+            var timer = NonCapturingTimer.Create((_) =>
+            {
+                // Observe the value based on the current execution context
+                tcs.SetResult(message.Value);
+            }, state: null, dueTime: TimeSpan.FromMilliseconds(1), Timeout.InfiniteTimeSpan);
+
+            // Assert
+            var messageFromTimer = await tcs.Task;
+            timer.Dispose();
+
+            // ExecutionContext didn't flow to timer callback
+            Assert.Null(messageFromTimer);
+
+            // ExecutionContext was restored
+            Assert.NotNull(await Task.Run(() => message.Value));
+        }
+    }
+}