NancyFX - Return custom error response on uncaught exception

Question

I'm trying to get my self-hosted service using Nancy to return json formatted errors upon an uncaught exception. However, I am always getting the response:

{"readyState":4,"status":404,"statusText":"error"}

(below is the amalgamation of several examples across the net).

My bootstrapper contains the following:

        pipelines.OnError.AddItemToEndOfPipeline((ctx, exc) =>
        {
            if (exc is Exception)
            {
                // this is always executed upon failure to handle an exception.

                Log.Error("Unhandled error on request: " + context.Request.Url + " : " + exc.Message, exc);

                JsonResponse response = new JsonResponse(string.Format("{0}:{1}", exc, exc.Message), new DefaultJsonSerializer());
                response.StatusCode = HttpStatusCode.InternalServerError;

                return response;
            }

            return HttpStatusCode.InternalServerError;
        });

I have a StatusCodeHandler:

public class JsonErrorStatusCodeHandler : IStatusCodeHandler
{
    public bool HandlesStatusCode(HttpStatusCode statusCode, NancyContext context)
    {
        return statusCode == HttpStatusCode.InternalServerError;
    }


    public void Handle(HttpStatusCode statusCode, NancyContext context)
    {
        var exception = context.GetException();

        if (exception != null)
        {
            // never executed
        }

        // this is executed

        JsonResponse response = new JsonResponse("wtf"), new DefaultJsonSerializer());
        response.StatusCode = HttpStatusCode.InternalServerError;

        context.Response = response;
    }

Although I've verified that the code in OnError and Handle is executed (see comments), my clients still receive 404. I've also tried using

        var exception = context.Items[NancyEngine.ERROR_EXCEPTION] as Exception;

instead of

        var exception = context.GetException();

with no luck.

Answer 1

Gah, so this was a CORS issue.

I'm automatically adding the CORS headers to the response:

    protected override void RequestStartup(TinyIoCContainer container, IPipelines pipelines, NancyContext context)
    {
        pipelines.AfterRequest.AddItemToEndOfPipeline((ctx) =>
        {
            ctx.Response.WithHeader("Access-Control-Allow-Origin", "*")
                .WithHeader("Access-Control-Allow-Methods", "POST,GET")
                .WithHeader("Access-Control-Allow-Headers", "Accept, Origin, Content-type");
        });

        pipelines.OnError.AddItemToEndOfPipeline((ctx, exc) =>
        {
            if (exc != null)
            {
                throw exc;
            }

            return HttpStatusCode.InternalServerError;
        });

        base.RequestStartup(container, pipelines, context);
    }

But when the response is replaced in my status code handler I need to set these headers again:

public class JsonErrorStatusCodeHandler : IStatusCodeHandler
{
    public bool HandlesStatusCode(HttpStatusCode statusCode, NancyContext context)
    {
        if (statusCode != HttpStatusCode.InternalServerError)
        {
            return false;
        }

        var exception = context.GetException();

        return exception != null;
    }


    public void Handle(HttpStatusCode statusCode, NancyContext context)
    {
        var exception = context.GetException();

        JsonResponse response = new JsonResponse(string.Format("{0}:{1}", exception, exception.Message), new DefaultJsonSerializer());

        response.StatusCode = HttpStatusCode.InternalServerError;

        context.Response = response;

        context.Response.WithHeader("Access-Control-Allow-Origin", "*")
            .WithHeader("Access-Control-Allow-Methods", "POST,GET")
            .WithHeader("Access-Control-Allow-Headers", "Accept, Origin, Content-type");
    }
}