Get host entry async freezes

Question

I have a problem that occurs when i have deployed the code to the production server. It does not happen locally on my computer. When i run the line

var host = await Dns.GetHostEntryAsync(adresse);

The production environment freezes or awaits indefinitely. Why does this happen?

Here is the code:

    public async Task<string> HentAsync(string ip)
    {
        if (string.IsNullOrWhiteSpace(ip) || kLoopbackIp.Contains(ip))
        {
            return string.Empty;
        }

        if (IpHostname.ContainsKey(ip))
        {
            UpdateAndRemoveOldElements(ip);

            return IpHostname[ip].Item1;
        }

        try
        {
            IPAddress adresse = IPAddress.None;
            if (!IPAddress.TryParse(ip, out adresse))
            {
                return string.Empty;
            }

            var host = await Dns.GetHostEntryAsync(adresse);
            string hostname = host.HostName;

            IpHostname.AddOrUpdate(ip,
                                   new Tuple<string, DateTime>(hostname, DateTime.Now),
                                   (x, y) => new Tuple<string, DateTime>(hostname, DateTime.Now));
            return hostname;
        }
        catch (Exception)
        {
            // Tar i mot exception og returnerer tom string
            return string.Empty;
        }
    }

UPDATE: The async method is called like this from a sync method that is doing the same thing (could this be the culprit).

public string Hent(string ip)
{
    return HentAsync(ip).Result;
}

+ what is the difference between production environment and your environment ? OS ? installed .net version ? — ilansch, Jul 07 '14 at 12:57
The .net version is the same as my local environment. Only difference is that i am running IIS express and the production is runnin IIS — Stian Standahl, Jul 07 '14 at 12:58
Are you using `HentAsync(ip).Result` or some variation of it? If your synchronization context is blocked it could result in a deadlock. — Dirk, Jul 07 '14 at 13:02
Is your DNS server address set up correctly on the server machine? Can you resolve hostnames via commandline? — CodingIntrigue, Jul 07 '14 at 13:03
Yes @Dirk, i have updated the question with the code that is calling it. — Stian Standahl, Jul 07 '14 at 13:04
@RGraham - The sync version of the same call (Dns.GetHostEntry()) works fine. — Stian Standahl, Jul 07 '14 at 13:05
This is known "problem", I recently answered a question about it [here](https://stackoverflow.com/questions/24572908/using-async-and-await-to-break-up-database-call-with-dapper/24573088#24573088). — Dirk, Jul 07 '14 at 13:06
Is it possible to keep the code "DRY" with this deadlock problem? — Stian Standahl, Jul 07 '14 at 13:10
@StianStandahl You could try adding `.ConfigureAwait(false)` to your awaitables — spender, Jul 07 '14 at 13:17

score 2 · Accepted Answer · edited May 23 '17 at 11:58

The most likely cause is that the origin of this call is running on the UI thread; perhaps the call to Hent() updates a UI element?

The problem is well documented here: await works but calling task.Result hangs/deadlocks

Hent() calls HentAsync() and then await Dns.GetHostEntryAsync(). This last call spawns a new thread, but wants to return to the calling thread when it has finished, which in this case is the UI thread, but it cannot do this because .Result is squatting on the thread rather than yielding it temporarily like it would have with await.

As @spender and @Dirk point out, one solution is to add .ConfigureAwait(false) to await Dns.GetHostEntryAsync(). In fact you should add this to every await where you don't care what thread context the function continues in.

This approach is still quite fragile though, as any time you forget to do it or execution takes a path with an unmodified await, the problem will happen again. The best option is to make the top-level UI-threaded function async and then await every function call that could cause a delay. This is the canonical use-case for async / await and leads to flat, easy to follow code with very little chance of deadlock.

Get host entry async freezes

1 Answers1