Add HeadlessTestRunner with simulated MultiThreaded mode

osu.Framework/Platform/HeadlessGameHost.cs

@@ -9,6 +9,7 @@
 using osu.Framework.Graphics.Rendering.Dummy;
 using osu.Framework.Input.Handlers;
 using osu.Framework.Logging;
+using osu.Framework.Testing;
 using osu.Framework.Threading;
 using osu.Framework.Timing;
 
@@ -106,6 +107,8 @@ protected override void UpdateFrame()
 
         protected override IEnumerable<InputHandler> CreateAvailableInputHandlers() => Array.Empty<InputHandler>();
 
+        protected override ThreadRunner CreateThreadRunner(InputThread mainThread) => new HeadlessThreadRunner(mainThread);
+
         private class FastClock : IClock
         {
             private readonly double increment;

osu.Framework/Testing/HeadlessThreadRunner.cs

@@ -0,0 +1,51 @@
+// Copyright (c) ppy Pty Ltd <[email protected]>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Development;
+using osu.Framework.Platform;
+using osu.Framework.Threading;
+using osu.Framework.Utils;
+
+namespace osu.Framework.Testing
+{
+    public class HeadlessThreadRunner : ThreadRunner
+    {
+        public HeadlessThreadRunner(InputThread mainThread)
+            : base(mainThread)
+        {
+        }
+
+        public override void RunMainLoop()
+        {
+            // Run true single-threaded mode.
+            if (ThreadSafety.ExecutionMode == ExecutionMode.SingleThread)
+            {
+                base.RunMainLoop();
+                return;
+            }
+
+            // Run a simulated multi-threaded mode.
+            // This is because the CI test runners are usually CPU-limited and may be preferring one thread (audio) over another (update).
+
+            ThreadSafety.ExecutionMode = ExecutionMode.SingleThread;
+
+            for (int i = 0; i < Threads.Count; i++)
+            {
+                int from;
+                int to;
+
+                do
+                {
+                    from = RNG.Next(Threads.Count);
+                    to = RNG.Next(Threads.Count);
+                } while (from == to);
+
+                (Threads[from], Threads[to]) = (Threads[to], Threads[from]);
+            }
+
+            base.RunMainLoop();
+
+            ThreadSafety.ExecutionMode = ExecutionMode.MultiThreaded;
+        }
+    }
+}