UnobservedTaskException thrown on configuration reload about nlog.targets.syslog HOT 3 CLOSED

The issue reported by @sanderweltje highlights how easy it is to write bug-free concurrent code.

The continuation used in the ProcessQueueAsync(MessageBuilder messageBuilder) method might seem the proper solution to take care of any exception raised in the ProcessQueueAsync(MessageBuilder messageBuilder, TaskCompletionSource<object> tcs) method:

private void ProcessQueueAsync(MessageBuilder messageBuilder)
{
    ProcessQueueAsync(messageBuilder, new TaskCompletionSource<object>())
        .ContinueWith(t =>
        {
            InternalLogger.Warn(t.Exception?.GetBaseException(), "[Syslog] ProcessQueueAsync faulted within try");
            ProcessQueueAsync(messageBuilder);
        }, token, TaskContinuationOptions.ExecuteSynchronously | TaskContinuationOptions.OnlyOnFaulted, TaskScheduler.Current);
}

Unfortunately due to the fact that the same CancellationToken is used everywhere, the ContinueWith does not execute the continuation function if the token has been cancelled.

from nlog.targets.syslog.

If the CancellationToken is used to cancel all processing, the reuse of the token is correct. If the CancellationToken is also used for other purposes, a second CancellationToken might be a better solution. In case you want to cancel all processing and an exception is thrown, you probably don't want to restart ProcessQueueAsync(messageBuilder)

from nlog.targets.syslog.

I already use multiple tokens for different purposes, but the issue was caused by the one used to stop processing, because it was passed to all ContinueWith, preventing exceptions from being observed if thrown after a cancellation.

Considering the logical workflow it does not make sense to continue processing when a cancellation is triggered, but, due to the way ContinueWith interacts with CancellationToken (here some tests), relying on the first successful detection of the cancellation i.e. if (token.IsCancellationRequested), allows to achieve the expected graceful termination.

from nlog.targets.syslog.