NullReferenceException in ActiveHandlerTrackingEntry with 2.2.0 preview 3

[martincostello]

I don't yet have a repro, but I've just updated an ASP.NET Core web app that uses IHttpClientFactory to 2.2.0 preview 3, and I've got some in-memory self-hosted tests failing because of the following exception causing dotnet test to terminate:

[18:18:48][dotnet test] The active test run was aborted. Reason: Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
[18:18:48][dotnet test]    at Microsoft.Extensions.Http.ActiveHandlerTrackingEntry.Timer_Tick()
[18:18:48][dotnet test]    at Microsoft.Extensions.Http.ActiveHandlerTrackingEntry.<>c.<.cctor>b__22_0(Object s)
[18:18:48][dotnet test]    at System.Threading.TimerQueueTimer.CallCallback()
[18:18:48][dotnet test]    at System.Threading.TimerQueueTimer.Fire()
[18:18:48][dotnet test]    at System.Threading.TimerQueue.FireNextTimers()
[18:18:48][dotnet test]    at System.Threading.TimerQueue.AppDomainTimerCallback(Int32 id)

I guess that one of either _timer or _callback is somehow null:

HttpClientFactory/src/Microsoft.Extensions.Http/ActiveHandlerTrackingEntry.cs

Lines 75 to 87 in 5255a24

	private void Timer_Tick()
	{
	Debug.Assert(_callback != null);
	Debug.Assert(_timer != null);
	lock (_lock)
	{
	_timer.Dispose();
	_timer = null;
	_callback(this);
	}
	}

[martincostello]

It's also not a one-off, as we run these tests in batches based on categories to get some parallelism across TeamCity jobs, and 4 different jobs (of 7) have all failed with the same exception.

[rynowak]

Ah yes. It's probably the case that we need to null check the timer when we access it. This was a 2.2.0 change.

[palmtech]

Is there any workaround for this? All of our preview 3 containers terminate every few minutes with this error.

[martincostello]

Looking at the code, I think the only workaround is to make the messsge handler lifetimes either Infinite or really long (~the usual lifetime of your containers).

[rynowak]

Fixed this for 2.2 - sorry I don't think there's a workaround here. This is just broken. What happens is that an additional timer is started each time you request an HTTP client within the lifetime of a single handler.

[palmtech]

Ok, might be worth adding it to the known issues. Preview 3 is probably a skip for most microservice-ish apps.

[rynowak]

Yeah that's a fair point, will do.

[rynowak]

Added aspnet/Announcements#323