IHostedService with Monitor produces System.OperationCanceledException

Describe the bug

IHostedService with Monitor produces System.OperationCanceledException on some occasions but not on all occasions on termination.

To Reproduce

1. Using Target Framework netcoreapp3.1
2. Using Microsoft.Extensions.Hosting version 3.1.2
3. Run this code:

public class SyncService : IHostedService, IDisposable
{
	private static object _workerMonitor = new object();
	private Timer _timer;

	public Task StartAsync(CancellationToken cancellationToken)
	{
		_timer = new Timer(Callback, null, TimeSpan.FromSeconds(0), TimeSpan.FromSeconds(5));
		return Task.CompletedTask;
	}

	public Task StopAsync(CancellationToken cancellationToken)
	{
		Console.WriteLine("Stopping");
		_timer?.Change(0, 0);
		Monitor.Enter(_workerMonitor);
		Console.WriteLine("Stopped");
		return Task.CompletedTask;
	}

	public void Dispose()
	{
		Console.WriteLine("Disposing");
		_timer?.Dispose();
		Console.WriteLine("Disposed");
	}

	private void Callback(object state)
	{
		if (Monitor.TryEnter(_workerMonitor))
			try
			{
				Console.WriteLine("Cycle Start");
				Thread.Sleep(10000);
				Console.WriteLine("Cycle End");
			}
			catch (Exception e)
			{
				Console.WriteLine("Fatal Exception" + e);
			}
			finally
			{
				Monitor.Exit(_workerMonitor);
				Console.WriteLine("Exited");
			}
	}
}

class Program
{
	static void Main(string[] args)
	{
		try
		{
			var builder = new HostBuilder()
						  .ConfigureServices(collection => collection.AddHostedService<SyncService>())
				.Build();

			builder.Run();
		}
		catch (Exception ex)
		{
			Console.WriteLine(ex);
		}
	}
}

1. Terminate the program using CTRL+C. This triggers the StopAsync method.
2. See error

Expected behavior

The program should terminate without error message.

Actual behavior

The program terminates sometimes with error message sometimes without. I noticed that the exception shows up if I hit CTRL+C immediately after the “Cycle Start” message. If I wait for about 6/7 seconds the exception isn’t thrown.

Console Output if error shows up:

Cycle Start
Stopping
Cycle End
Exited
Stopped
Disposing
Disposed
System.OperationCanceledException: The operation was canceled.
   at System.Threading.CancellationToken.ThrowOperationCanceledException()
   at Microsoft.Extensions.Hosting.Internal.Host.StopAsync(CancellationToken cancellationToken)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.WaitForShutdownAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.Run(IHost host)
   at ConsoleApp1.Program.Main(String[] args) in C:\Users\...\ConsoleApp1\ConsoleApp1\Program.cs:line 67

Console output without error (expected):

Cycle Start
Stopping
Cycle End
Exited
Stopped
Disposing
Disposed

Additional context

Output of dotnet --info .NET Core SDK (reflecting any global.json): Version: 3.1.101 Commit: b377529961

Runtime Environment: OS Name: Windows OS Version: 10.0.17763 OS Platform: Windows RID: win10-x64 Base Path: C:\Program Files\dotnet\sdk\3.1.101\

Host (useful for support): Version: 3.1.1 Commit: a1388f194c

.NET Core SDKs installed: 2.2.402 [C:\Program Files\dotnet\sdk] 3.1.101 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed: Microsoft.AspNetCore.All 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.App 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.NETCore.App 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.WindowsDesktop.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[analogrelay]

It looks like we don’t have an extension method for it on IHostBuilder. Might just be something we missed when moving from WebHost to GenericHost, or it might have been an intentional omission. Feel free to file a new bug asking for the extension method to be supported on IHostBuilder! If there’s a reason we omitted it, we’ll let you know in the issue, but there’s no harm in suggesting it!

[klyse]

This solved my actual problem because I’m using a WebHost.

But my example uses a HostBuilder. Therefore UseShutdownTimeout is not available as a extension method. So I did some digging in the framework and found that the UseShutdownTimeout is eventually just a configuration with the keyword shutdownTimeoutSeconds

Link to code: https://github.com/dotnet/aspnetcore/blob/8cba1cbb231b37d56c352fad196619b11c5c6613/src/Hosting/Abstractions/src/HostingAbstractionsWebHostBuilderExtensions.cs#L179-L182

Then I refactored my create HostBuilder code:

var builder = new HostBuilder()
	.ConfigureAppConfiguration(configurationBuilder =>
	{
		configurationBuilder.AddInMemoryCollection(new Dictionary<string, string>
		{
			{ "shutdownTimeoutSeconds", "600" }
		});
	})
	.ConfigureServices(collection => collection.AddHostedService<SyncService>())
	.Build();

But this ain’t working… Just out of curiosity (and because I think somebody else might stumble over the same issue) how could I solve this when I have a HostBuilder?