Make WindowsServiceLifetime gracefully stop

WindowsServiceLifetime was not waiting for ServiceBase to stop the service.  As a result
we would sometimes end the process before notifying service control manager that the service
had stopped -- resulting in an error in the eventlog and sometimes a service restart.

We also were permitting multiple calls to Stop to occur - through SCM callbacks, and through
public API.  We must not call SetServiceStatus again once the service is marked as stopped.

src/libraries/Microsoft.Extensions.Hosting.WindowsServices/src/WindowsServiceLifetime.cs

@@ -18,6 +18,7 @@ namespace Microsoft.Extensions.Hosting.WindowsServices
     public class WindowsServiceLifetime : ServiceBase, IHostLifetime
     {
         private readonly TaskCompletionSource<object?> _delayStart = new TaskCompletionSource<object?>(TaskCreationOptions.RunContinuationsAsynchronously);
+        private readonly TaskCompletionSource<object?> _serviceStopped = new TaskCompletionSource<object?>(TaskCreationOptions.RunContinuationsAsynchronously);
         private readonly ManualResetEventSlim _delayStop = new ManualResetEventSlim();
         private readonly HostOptions _hostOptions;
 
@@ -92,14 +93,14 @@ private void Run()
             {
                 _delayStart.TrySetException(ex);
             }
+            _serviceStopped.TrySetResult(null);
         }
 
         public Task StopAsync(CancellationToken cancellationToken)
         {
-            // Avoid deadlock where host waits for StopAsync before firing ApplicationStopped,
-            // and Stop waits for ApplicationStopped.
+            // Stop will cause the ServiceBase.Run method to complete and return, which completes _serviceStopped.
             Task.Run(Stop, CancellationToken.None);
-            return Task.CompletedTask;
+            return _serviceStopped.Task;
         }
 
         // Called by base.Run when the service is ready to start.

src/libraries/System.ServiceProcess.ServiceController/src/System/ServiceProcess/ServiceBase.cs

@@ -647,6 +647,12 @@ public static void Run(ServiceBase service)
 
         public void Stop()
         {
+            if (_status.currentState == ServiceControlStatus.STATE_STOPPED || _status.currentState == default)
+            {
+                // nothing to do if the service is already stopped or never started
+                return;
+            }
+
             DeferredStop();
         }