Fix TaskChain performing the action in-line, add test

osu.Game.Tests/NonVisual/TaskChainTest.cs

@@ -0,0 +1,83 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Threading;
+using System.Threading.Tasks;
+using NUnit.Framework;
+using osu.Game.Utils;
+
+namespace osu.Game.Tests.NonVisual
+{
+    [TestFixture]
+    public class TaskChainTest
+    {
+        private TaskChain taskChain;
+        private int currentTask;
+
+        [SetUp]
+        public void Setup()
+        {
+            taskChain = new TaskChain();
+            currentTask = 0;
+        }
+
+        [Test]
+        public async Task TestChainedTasksRunSequentially()
+        {
+            var task1 = addTask();
+            var task2 = addTask();
+            var task3 = addTask();
+
+            task3.mutex.Set();
+            task2.mutex.Set();
+            task1.mutex.Set();
+
+            await Task.WhenAll(task1.task, task2.task, task3.task);
+
+            Assert.That(task1.task.Result, Is.EqualTo(1));
+            Assert.That(task2.task.Result, Is.EqualTo(2));
+            Assert.That(task3.task.Result, Is.EqualTo(3));
+        }
+
+        [Test]
+        public async Task TestChainedTaskWithIntermediateCancelRunsInSequence()
+        {
+            var task1 = addTask();
+            var task2 = addTask();
+            var task3 = addTask();
+
+            // Cancel task2, allow task3 to complete.
+            task2.cancellation.Cancel();
+            task2.mutex.Set();
+            task3.mutex.Set();
+
+            // Allow task3 to potentially complete.
+            Thread.Sleep(1000);
+
+            // Allow task1 to complete.
+            task1.mutex.Set();
+
+            // Wait on both tasks.
+            await Task.WhenAll(task1.task, task3.task);
+
+            Assert.That(task1.task.Result, Is.EqualTo(1));
+            Assert.That(task2.task.IsCompleted, Is.False);
+            Assert.That(task3.task.Result, Is.EqualTo(2));
+        }
+
+        private (Task<int> task, ManualResetEventSlim mutex, CancellationTokenSource cancellation) addTask()
+        {
+            var mutex = new ManualResetEventSlim(false);
+            var cancellationSource = new CancellationTokenSource();
+            var completionSource = new TaskCompletionSource<int>();
+
+            taskChain.Add(() =>
+            {
+                mutex.Wait(CancellationToken.None);
+                completionSource.SetResult(Interlocked.Increment(ref currentTask));
+            }, cancellationSource.Token);
+
+            return (completionSource.Task, mutex, cancellationSource);
+        }
+    }
+}