Adding TaskQueue

src/Microsoft.AspNetCore.Sockets.Client/TaskQueue.cs

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+// 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.Tasks;
+
+namespace Microsoft.AspNetCore.Sockets.Client.Internal
+{
+    // Allows serial queuing of Task instances
+    // The tasks are not called on the current synchronization context
+
+    public sealed class TaskQueue
+    {
+        private readonly object _lockObj = new object();
+        private Task _lastQueuedTask;
+        private volatile bool _drained;
+
+        public TaskQueue()
+            : this(Task.CompletedTask)
+        { }
+
+        public TaskQueue(Task initialTask)
+        {
+            _lastQueuedTask = initialTask;
+        }
+
+        public bool IsDrained
+        {
+            get { return _drained; }
+        }
+
+        public Task Enqueue(Func<Task> taskFunc)
+        {
+            return Enqueue(s => taskFunc(), null);
+        }
+
+        public Task Enqueue(Func<object, Task> taskFunc, object state)
+        {
+            lock (_lockObj)
+            {
+                if (_drained)
+                {
+                    return _lastQueuedTask;
+                }
+
+                var newTask = _lastQueuedTask.ContinueWith((t, s1) =>
+                {
+                    if (t.IsFaulted || t.IsCanceled)
+                    {
+                        return t;
+                    }
+                    return taskFunc(s1);
+                },
+                state).Unwrap();
+                _lastQueuedTask = newTask;
+                return newTask;
+            }
+        }
+
+        public Task Drain()
+        {
+            lock (_lockObj)
+            {
+                _drained = true;
+
+                return _lastQueuedTask;
+            }
+        }
+    }
+}

test/Microsoft.AspNetCore.Sockets.Client.Tests/TaskQueueTests.cs

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+// 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.Tasks;
+using Microsoft.AspNetCore.Sockets.Client.Internal;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Client.Tests
+{
+    public class TaskQueueTests
+    {
+        [Fact]
+        public async Task DrainingTaskQueueShutsQueueOff()
+        {
+            var queue = new TaskQueue();
+            await queue.Enqueue(() => Task.CompletedTask);
+            await queue.Drain();
+
+            // This would throw if the task was queued successfully
+            await queue.Enqueue(() => Task.FromException(new Exception()));
+        }
+
+        [Fact]
+        public async Task TaskQueueDoesNotQueueNewTasksIfPreviousTaskFaulted()
+        {
+            var exception = new Exception();
+            var queue = new TaskQueue();
+            var ignore = queue.Enqueue(() => Task.FromException(exception));
+            var task = queue.Enqueue(() => Task.CompletedTask);
+
+            var actual = await Assert.ThrowsAsync<Exception>(async () => await task);
+            Assert.Same(exception, actual);
+        }
+
+        [Fact]
+        public void TaskQueueRunsTasksInSequence()
+        {
+            var queue = new TaskQueue();
+            int n = 0;
+            queue.Enqueue(() =>
+            {
+                n++;
+                return Task.CompletedTask;
+            });
+
+            Task task = queue.Enqueue(() =>
+            {
+                return Task.Delay(100).ContinueWith(t => n++);
+            });
+
+            task.Wait();
+            Assert.Equal(n, 2);
+        }
+    }
+}