Fix coordinator timer disposal race

src/MSBuild.Coordinator.UnitTests/CoordinatorServer_Tests.cs

@@ -6,6 +6,7 @@
 using Microsoft.Build.BackEnd;
 using Microsoft.Build.Framework;
 using Microsoft.Build.Framework.Coordinator;
+using Microsoft.Build.UnitTests;
 using Shouldly;
 using Xunit;
 
@@ -417,6 +418,56 @@ public async Task Heartbeat_DoesNotCauseError()
         await serverTask;
     }
 
+    [Fact]
+    public async Task Dispose_DefersSharedStateDisposalUntilInFlightHeartbeatCompletes()
+    {
+        using TestEnvironment env = TestEnvironment.Create(testOutput);
+        using ManualResetEventSlim heartbeatStarted = new();
+        using ManualResetEventSlim continueHeartbeat = new();
+        CoordinatorServer server = CreateServer(
+            DefaultSettings with
+            {
+                TotalNodeBudget = 16,
+                HeartbeatIntervalMs = CoordinatorSettings.MaxHeartbeatIntervalMs,
+            });
+        CoordinatorServer.TestAccessor accessor = server.GetTestAccessor();
+        accessor.SetHeartbeatCallbackStarted(
+            () =>
+            {
+                heartbeatStarted.Set();
+                continueHeartbeat.Wait();
+            });
+
+        Task serverTask = server.RunAsync(_cts.Token);
+
+        try
+        {
+            accessor.TriggerHeartbeatCheck();
+            heartbeatStarted.Wait(TimeSpan.FromSeconds(5)).ShouldBeTrue();
+
+            Task disposeTask = Task.Run(
+                () =>
+                {
+                    Parallel.Invoke(server.Dispose, server.Dispose, server.Dispose);
+                });
+            (await Task.WhenAny(disposeTask, Task.Delay(5000))).ShouldBe(disposeTask);
+            await disposeTask;
+            accessor.IsDisposing.ShouldBeTrue();
+
+            continueHeartbeat.Set();
+
+            (await Task.WhenAny(serverTask, Task.Delay(5000))).ShouldBe(serverTask);
+            await serverTask;
+
+            server.Dispose();
+        }
+        finally
+        {
+            continueHeartbeat.Set();
+            server.Dispose();
+        }
+    }
+
     [Fact]
     public async Task ClientDisconnect_ReleasesGrant()
     {

src/MSBuild.Coordinator/CoordinatorServer.TestAccessor.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.Build.Coordinator;
+
+internal sealed partial class CoordinatorServer
+{
+    internal TestAccessor GetTestAccessor() => new(this);
+
+    internal readonly struct TestAccessor(CoordinatorServer server)
+    {
+        public bool IsDisposing => Volatile.Read(ref server._disposeState) == Disposing;
+
+        public void SetHeartbeatCallbackStarted(Action callback)
+            => server._heartbeatCallbackStartedForTests = callback;
+
+        public void TriggerHeartbeatCheck()
+        {
+            lock (server._lifecycleLock)
+            {
+                server._heartbeatMonitor!.Change(dueTime: 0, period: Timeout.Infinite);
+            }
+        }
+    }
+}