Don't capture async locals in resolver (#2426)

Author: JamesNK

src/Grpc.Net.Client/Balancer/PollingResolver.cs

@@ -135,14 +135,33 @@ public sealed override void Refresh()
 
             if (_resolveTask.IsCompleted)
             {
-                // Run ResolveAsync in a background task.
-                // This is done to prevent synchronous block inside ResolveAsync from blocking future Refresh calls.
-                _resolveTask = Task.Run(() => ResolveNowAsync(_cts.Token), _cts.Token);
-                _resolveTask.ContinueWith(static (t, state) =>
+                // Don't capture the current ExecutionContext and its AsyncLocals onto the connect
+                var restoreFlow = false;
+                try
+                {
+                    if (!ExecutionContext.IsFlowSuppressed())
+                    {
+                        ExecutionContext.SuppressFlow();
+                        restoreFlow = true;
+                    }
+
+                    // Run ResolveAsync in a background task.
+                    // This is done to prevent synchronous block inside ResolveAsync from blocking future Refresh calls.
+                    _resolveTask = Task.Run(() => ResolveNowAsync(_cts.Token), _cts.Token);
+                    _resolveTask.ContinueWith(static (t, state) =>
+                    {
+                        var pollingResolver = (PollingResolver)state!;
+                        Log.ResolveTaskCompleted(pollingResolver._logger, pollingResolver.GetType());
+                    }, this);
+                }
+                finally
                 {
-                    var pollingResolver = (PollingResolver)state!;
-                    Log.ResolveTaskCompleted(pollingResolver._logger, pollingResolver.GetType());
-                }, this);
+                    // Restore the current ExecutionContext
+                    if (restoreFlow)
+                    {
+                        ExecutionContext.RestoreFlow();
+                    }
+                }
             }
             else
             {

src/Grpc.Net.Client/Balancer/Subchannel.cs

@@ -257,7 +257,7 @@ public void RequestConnection()
         }
 
         // Don't capture the current ExecutionContext and its AsyncLocals onto the connect
-        bool restoreFlow = false;
+        var restoreFlow = false;
         if (!ExecutionContext.IsFlowSuppressed())
         {
             ExecutionContext.SuppressFlow();

src/Shared/NonCapturingTimer.cs

@@ -13,7 +13,7 @@ public static Timer Create(TimerCallback callback, object? state, TimeSpan dueTi
         ArgumentNullThrowHelper.ThrowIfNull(callback);
 
         // Don't capture the current ExecutionContext and its AsyncLocals onto the timer
-        bool restoreFlow = false;
+        var restoreFlow = false;
         try
         {
             if (!ExecutionContext.IsFlowSuppressed())

test/Grpc.Net.Client.Tests/Balancer/ResolverTests.cs

@@ -111,6 +111,59 @@ protected override Task ResolveAsync(CancellationToken cancellationToken)
         }
     }
 
+    [Test]
+    public async Task Refresh_AsyncLocal_NotCaptured()
+    {
+        // Arrange
+        var services = new ServiceCollection();
+        services.AddNUnitLogger();
+        var loggerFactory = services.BuildServiceProvider().GetRequiredService<ILoggerFactory>();
+
+        var asyncLocal = new AsyncLocal<object>();
+        asyncLocal.Value = new object();
+
+        var callbackAsyncLocalValues = new List<object>();
+
+        var resolver = new CallbackPollingResolver(loggerFactory, new TestBackoffPolicyFactory(TimeSpan.FromMilliseconds(100)), (listener) =>
+        {
+            callbackAsyncLocalValues.Add(asyncLocal.Value);
+            if (callbackAsyncLocalValues.Count >= 2)
+            {
+                listener(ResolverResult.ForResult(new List<BalancerAddress>()));
+            }
+
+            return Task.CompletedTask;
+        });
+
+        var tcs = new TaskCompletionSource<ResolverResult>(TaskCreationOptions.RunContinuationsAsynchronously);
+        resolver.Start(result => tcs.TrySetResult(result));
+
+        // Act
+        resolver.Refresh();
+
+        // Assert
+        await tcs.Task.DefaultTimeout();
+
+        Assert.AreEqual(2, callbackAsyncLocalValues.Count);
+        Assert.IsNull(callbackAsyncLocalValues[0]);
+        Assert.IsNull(callbackAsyncLocalValues[1]);
+    }
+
+    private class CallbackPollingResolver : PollingResolver
+    {
+        private readonly Func<Action<ResolverResult>, Task> _callback;
+
+        public CallbackPollingResolver(ILoggerFactory loggerFactory, IBackoffPolicyFactory backoffPolicyFactory, Func<Action<ResolverResult>, Task> callback) : base(loggerFactory, backoffPolicyFactory)
+        {
+            _callback = callback;
+        }
+
+        protected override Task ResolveAsync(CancellationToken cancellationToken)
+        {
+            return _callback(Listener);
+        }
+    }
+
     [Test]
     public async Task Resolver_ResolveNameFromServices_Success()
     {