PeriodicTimer with TimeProvider assertion is not stable in .net 8

[fingers10]

I have a simple BackgroundService in my asp.net 8 web api which performs an hourly backup using an PeriodicTimer. Here is the code sample,

public class OfflineHourlyDatabaseBackup(
	TimeProvider timeProvider,
	IServiceProvider serviceProvider) : BackgroundService
{
	protected override async Task ExecuteAsync(CancellationToken stoppingToken)
	{
		var timer = new PeriodicTimer(TimeSpan.FromHours(1), timeProvider);

		while (await timer.WaitForNextTickAsync(stoppingToken) && !stoppingToken.IsCancellationRequested) 
		{
			Backup();
		}
	}

	private void Backup()
	{
		using var scope = serviceProvider.CreateScope();
		var offlineDatabaseBackupService = scope.ServiceProvider.GetRequiredService<IOfflineDatabaseBackupService>();
		offlineDatabaseBackupService.Backup();
	}
}

Now I have added unit test to assert the hourly backup behaviour as shown below,

[Fact]
public async Task OfflineHourlyDatabaseBackup_ExecuteAsync()
{
    // Arrange
    var serviceCollection = new ServiceCollection();
    var mockOfflineDatabaseBackupService = new Mock<IOfflineDatabaseBackupService>();
    serviceCollection.AddSingleton(_ => mockOfflineDatabaseBackupService.Object);
    var timeProvider = new FakeTimeProvider();
    var serviceProvider = serviceCollection.BuildServiceProvider();

    // Act
    using var offlineHourlyDatabaseBackup = new OfflineHourlyDatabaseBackup(timeProvider, serviceProvider);
    await offlineHourlyDatabaseBackup.StartAsync(default);
    timeProvider.Advance(TimeSpan.FromHours(1));
    timeProvider.Advance(TimeSpan.FromHours(1));
    await offlineHourlyDatabaseBackup.ExecuteTask!;

    // Assert
    offlineHourlyDatabaseBackup.ExecuteTask.IsCompletedSuccessfully.Should().BeTrue();
    mockOfflineDatabaseBackupService.Verify(x => x.Backup(), Times.AtLeast(2));
}

The test assertion is not stable. It passes sometimes and fails sometimes with the following error message

Moq.MockException : Expected invocation on the mock at least 2 times,
but was 1 times: x => x.Backup()

Please can you help me understand what I'm doing wrong here?

[BrennanConroy]

Doesn't that code hang forever on await offlineHourlyDatabaseBackup.ExecuteTask!;?

[fingers10]

@BrennanConroy yes it happens sometimes. Any inputs on how to resolve this? Please guide me on what I'm missing here..

[martincostello]

Don't you need to pass a CancellationToken to the service so that it will terminate the loop?

[fingers10]

@martincostello ExecuteTask doesn't have an option to supply CancellationToken

[martincostello]

No it doesn't, but the CancellationToken passed to StartAsync() passes it to the ExecuteAsync() method associated with that task: code

[fingers10]

I thought the default will take care of it in await offlineHourlyDatabaseBackup.StartAsync(default);. Let me explicitly create and pass and check and get back

[fingers10]

@martincostello no it doesnt work

[fingers10]

@martincostello any thoughts on this?