Cancel long running operation on client cancel

Question

I started my first project with asp.net core and razor pages. On a client request, a long running database operation will be started. Now i want to recognize, when the user leaves the website, so the database operation can be cancelled.

I've tried it with a cancellationToken, but it will never be cancelled.

public async Task<JsonResult> OnPostReadAsync([DataSourceRequest] DataSourceRequest request, CancellationToken cancellationToken)
{
    var messages = await _logMessageService.GetLogMessagesAsync(request, cancellationToken);

    return new JsonResult(messages.ToDataSourceResult(request));
}

The function get called by an Telerik Kendo UI Grid. Can you tell me, why the cancellation token don't get cancelled, or what other options I have to detect a abortion by the client?

Edit 1

I pass the Token through to this function call of a NpgsqlCommand:

var dataReader = await command.ExecuteReaderAsync(cancellationToken);

Answer 1

To cancel IO bound ie task which is running long , following is code that you can do which i got form the C# with CLR book.

Design extension method for task as below.

private static async Task<TResult> WithCancellation<TResult>(this Task<TResult> originalTask,
CancellationToken ct) {
   // Create a Task that completes when the CancellationToken is canceled
   var cancelTask = new TaskCompletionSource<Void>();
   // When the CancellationToken is canceled, complete the Task
  using (ct.Register(
     t => ((TaskCompletionSource<Void>)t).TrySetResult(new Void()), cancelTask)) {
    // Create a Task that completes when either the original or
    // CancellationToken Task completes
    Task any = await Task.WhenAny(originalTask, cancelTask.Task);
    // If any Task completes due to CancellationToken, throw OperationCanceledException
     if (any == cancelTask.Task) ct.ThrowIfCancellationRequested();
  }
  // await original task (synchronously); if it failed, awaiting it
  // throws 1st inner exception instead of AggregateException
 return await originalTask;
}

as given in following example code you can cancel it by using extension method designed above.

public static async Task Go() {
   // Create a CancellationTokenSource that cancels itself after # milliseconds
   var cts = new CancellationTokenSource(5000); // To cancel sooner, call cts.Cancel()
   var ct = cts.Token;
   try {
    // I used Task.Delay for testing; replace this with another method that returns a Task
     await Task.Delay(10000).WithCancellation(ct);
     Console.WriteLine("Task completed");
   }
   catch (OperationCanceledException) {
    Console.WriteLine("Task cancelled");
  }
}

In this example cancellation done based on given time , but you can call cancellation by calling cancel method.

Answer 2

So I found the answer myself after some more research. The problem is a bug in IISExpress as mentioned here: https://github.com/aspnet/Mvc/issues/5239#issuecomment-323567952

I switched to Kestrel and now everything works as expected.