Occasional Task Cancelled on .Result of HttpClient code

Question

I m a bit puzzled with this. I get an occasional task cancelled exception on the following piece of code. This is not because it times out - it happens too soon.

Assume the MakeRequest in the code below formulates the URL and simply returns with

return await _client.SendAsync(request, cancellationToken).ConfigureAwait(false);

The rest of the code gets the response and makes sure its successful otherwise throws.

    protected async Task<T> MakeRequestAndResponseAsync<T>(
        HttpRequestMessage request,
        CancellationToken cancellationToken = default(CancellationToken))
    {
        var response = await MakeRequest(request, cancellationToken).ConfigureAwait(false);
        return await GetResponse<T>(response, throwsOnNotFound).ConfigureAwait(false);
    }

    protected async Task<T> GetResponse<T>(HttpResponseMessage response, Boolean throwsOnNotFound = true)
    {
        String content = await EnsureSuccessfulResponse(response, throwsOnNotFound).ConfigureAwait(false);
        if (content == null && !throwsOnNotFound) return default(T);
        return JsonConvert.DeserializeObject<T>(content);
    }

    protected async Task<String> EnsureSuccessfulResponse(HttpResponseMessage response, Boolean throwsOnNotFound = true)
    {
        String content = string.Empty;
        if (response.Content != null)
        {
            content = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
        }

        if (response.IsSuccessStatusCode) return content;

        var error = JsonConvert.DeserializeObject<ApiErrorResponse>(content);
        throw new ApiErrorException(/*some internal stuff*/);
    }

The whole code is invoked from a Console app like this. This is a multithreaded application and so this code may be invoked multiple times simultaneously.

var result = MakeRequestAndResponseAsync(...).Result;
// do something with result

What I have crossed out so far:

The HttpClient is correctly initialised and re-used. It doesnt get disposed mid-way.
This error happens sometimes. Those requests happen in parallel so I thought the HttpClient SendAsync is not thread-safe but I discovered here that in fact it is thread-safe
It's not a timeout exception. I have a unique id per request and i can see in the logs when it starts and when it errors (approx 1-2 seconds)

What I don't know: - If the other service returns a non-200 I intentionally throw a custom exception (ApiErrorException). I suspect it may be related to that, i.e. that the task is cancelled when it throws. If thats the case I would expect to get back the ApiErrorException rather than a task cancelled exception. I unwrap all inner exceptions to the point there isn't anything left. Whats left is the TaskCancelled exception.

Any help would be greatly appreciated.

Thanks

Have you inspected the properties of the exception you're getting? — Paulo Morgado, Dec 18 '16 at 20:54
@Yannis - How many threads are doing that concurrently? Do they share an `HttpClient`? They _can_ - as you point out `HttpClient` is thread-safe - but there is a default limit of 2 concurrent connections per host that `HttpWebRequest` - which `HttpClient` uses, imposes. Perhaps many threads are having to wait. I'm not sure whether that waiting counts towards any timeouts, but might be worth investigating. — sellotape, Dec 18 '16 at 21:23
@PauloMorgado Unfortunately I cannot reproduce it locally. It only happens on production where the volume of requests is much higher. — Yannis, Dec 18 '16 at 21:48
@Yannis - are you able to see the value of `TaskCanceledException.CancellationToken.IsCancellationRequested` in the logs, and if so, what's its value? — sellotape, Dec 18 '16 at 21:59
@Yannis, I believe I am having a similar issue. Was there anything else you found out with this? — Chucky, Feb 21 '19 at 11:47

Occasional Task Cancelled on .Result of HttpClient code

0 Answers0