"Task was canceled" errors on Azure website

Question

As traffic to our Azure website has increased, we've been seeing an increasing number of "A task was canceled" errors. A typical trace might look like this:

Fatal web api error: Controller: CustomerUserEventsController; 
Url: http://app.payboard.com/api/organizations/9ddf55d1-e0c1-4a8f-9327-eef38682e090/addcustomeruserevent?callback=jQuery210035782216349616647_1398442710964&cookieId=05be2755-dc0d-414d-b0d2-ea1986a929c3&customerId=&customerName=&customerUserId=&customerUserFirstName=&customerUserLastName=&eventName=hr-index-GET&_=1398442710965; 
Error: A task was canceled. ( at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext())

We used to maybe get one a day; it's been increasing gradually, so that now we're getting a couple dozen.

You'll notice that there's only framework code in the stack trace - none of our code is in there at all. So I'm a bit at a loss for how to troubleshoot it. Any suggestions on how to approach tracking these down?

Answer 1

See the answer here:

ASP.NET Web API OperationCanceledException when browser cancels the request

And the WebAPI bug here:

http://aspnetwebstack.codeplex.com/workitem/1797

Basically, it's expected when an HTTP request is canceled, for instance, if the user closes the page that initiated the request. To keep it from showing up in your logs, basically, you just need to create a custom message handler that suppresses the error:

/// <summary>
/// See https://stackoverflow.com/questions/22157596/asp-net-web-api-operationcanceledexception-when-browser-cancels-the-request
/// </summary>
public class CancelledTaskBugWorkaroundMessageHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var response = await base.SendAsync(request, cancellationToken);

        // Try to suppress response content when the cancellation token has fired; ASP.NET will log to the Application event log if there's content in this case.
        if (cancellationToken.IsCancellationRequested)
        {
            return new HttpResponseMessage(HttpStatusCode.InternalServerError);
        }

        return response;
    }
}