Queries hang in WebApi actions

[tamasvajk]

Issue raised through NuGet e-mail.

I tried the following, and it works without problem:

public async Task<IEnumerable<string>> Get()
{
    var scanner = new Scanner(ConfigurationManager.AppSettings["apiKey"]);
    var fromPlace = (await scanner.QueryLocation("London")).First();
    var toPlace = (await scanner.QueryLocation("New York")).First();

    //Query flights
    var itineraries = await scanner.QueryFlight(
      new FlightQuerySettings(
        new FlightRequestSettings(
          fromPlace, toPlace,
          new LocalDate(2016, 06, 19), new LocalDate(2016, 06, 25)),
        new FlightResponseSettings(SortType.Price, SortOrder.Ascending)));

    var itinerary = itineraries.First();
    var estimatedPrice = itinerary.PricingOptions.Min(option => option.Price);

    //Query booking
    var booking = await scanner.QueryBooking(itinerary);
    var actualPrice = booking.BookingOptions
      .Select(option => option.BookingItems.Sum(item => item.Price))
      .Min();

    ...
}

[KirillShlenskiy]

I obviously haven't seen the original e-mail, but this sounds like a deadlock caused by the client code blocking on one of the Tasks returned by a SkyScanner API method. This will occur (at least partly) because you're not using ConfigureAwait(false) on any of the Tasks awaited inside the current code base.

As an illustration, new Scanner(ConfigurationManager.AppSettings["apiKey"]).QueryFlight(flightSettings).Result will deadlock in Web API.

You won't reproduce this in a console or test project as they run with SynchronizationContext.Current == null and so all awaitable objects will have ConfigureAwait(false) semantics by default.

[tamasvajk]

Thanks @KirillShlenskiy. I tried to repro this in a WebAPI project and couldn't. But the .ConfigureAwait(false) makes sense.

[filippobottega]

Hello,
this code in a console application hangs at line Dim itineraries = queryFlightTask.Result

Best regards,
Filippo Bottega.

Imports NodaTime
Imports SkyScanner.Services
Imports SkyScanner.Settings

Module Module1

  Sub Main()
    Dim scanner = New Scanner("APIKey"))
    Dim queryLocationTask = scanner.QueryLocation("Venezia")
    queryLocationTask.ConfigureAwait(False)
    Dim fromPlace = (queryLocationTask.Result).First()
    Console.WriteLine($"{NameOf(fromPlace)}: {fromPlace.ToString}")
    queryLocationTask = scanner.QueryLocation("London")
    queryLocationTask.ConfigureAwait(False)
    Dim toPlace = (queryLocationTask.Result).First()
    Console.WriteLine($"{NameOf(toPlace)}: {toPlace.ToString}")

    'Query flights
    Dim queryFlightTask = scanner.QueryFlight(New FlightQuerySettings(New FlightRequestSettings(fromPlace, toPlace, New LocalDate(2016, 12, 23), New LocalDate(2016, 12, 31)), New FlightResponseSettings(SortType.Price, SortOrder.Ascending)))
    queryFlightTask.ConfigureAwait(False)
    **Dim itineraries = queryFlightTask.Result**

    Dim itinerary = itineraries.First()
    Console.WriteLine($"{NameOf(itinerary)}: {itinerary.ToString}")

    Dim estimatedPrice = itinerary.PricingOptions.Min(Function([option]) [option].Price)
    Console.WriteLine($"{NameOf(estimatedPrice)}: {estimatedPrice.ToString}")

    'Query booking
    Dim queryBookingTask = scanner.QueryBooking(itinerary)
    queryBookingTask.ConfigureAwait(False)
    Dim booking = queryBookingTask.Result
    Dim actualPrice = booking.BookingOptions.[Select](Function([option]) [option].BookingItems.Sum(Function(item) item.Price)).Min()
    Console.WriteLine($"{NameOf(actualPrice)}: {actualPrice.ToString}")
  End Sub

End Module

[filippobottega]

I found this:
Which services can I access and what are the rate limits?

Please note access to our Hotels API and Flights Live Pricing service are currently suspended. Apologies, but we are unable to give a date for these services to be re-launched, but please do check back regularly for updates.

If I use my API key I get:

{
  "status": 429,
  "statusText": "OK",
  "debugInformation": {
    "ValidationErrors": [
      {
        "Message": "Rate limit has been exceeded: 0 PerMinute for PricingSession"
      }
    ]
  }
}

Maybe this is the reason of the application's hang ...

Filippo.

[tamasvajk]

Thanks for investigating this further. You're right, I haven't tested this scenario. It's a shame that SkyScanner has shut down the service. I hope it's only temporary. I'll try to contact them to get more information.