Happy Eyeballs for HTTP requests.

Fixes #38
Thanks to #38

SS14.Launcher/HappyEyeballsHttp.cs

@@ -0,0 +1,104 @@
+using System.IO;
+using System.Net;
+using System.Net.Http;
+using System.Net.Sockets;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace SS14.Launcher;
+
+public static class HappyEyeballsHttp
+{
+    // .NET does not implement Happy Eyeballs at the time of writing.
+    // https://github.com/space-wizards/SS14.Launcher/issues/38
+    // This is the workaround.
+    //
+    // Implementation taken from https://github.com/ppy/osu-framework/pull/4191/files
+    public static HttpClient CreateHttpClient()
+    {
+        var handler = new SocketsHttpHandler
+        {
+            ConnectCallback = OnConnect,
+            AutomaticDecompression = DecompressionMethods.All
+        };
+
+        return new HttpClient(handler);
+    }
+
+    /// <summary>
+    /// Whether IPv6 should be preferred. Value may change based on runtime failures.
+    /// </summary>
+    private static bool _useIPv6 = Socket.OSSupportsIPv6;
+
+    /// <summary>
+    /// Whether the initial IPv6 check has been performed (to determine whether v6 is available or not).
+    /// </summary>
+    private static bool _hasResolvedIPv6Availability;
+
+    private const int FirstTryTimeout = 2000;
+
+    private static async ValueTask<Stream> OnConnect(
+        SocketsHttpConnectionContext context,
+        CancellationToken cancellationToken)
+    {
+        if (_useIPv6)
+        {
+            try
+            {
+                var localToken = cancellationToken;
+
+                if (!_hasResolvedIPv6Availability)
+                {
+                    // to make things move fast, use a very low timeout for the initial ipv6 attempt.
+                    var quickFailCts = new CancellationTokenSource(FirstTryTimeout);
+                    var linkedTokenSource =
+                        CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, quickFailCts.Token);
+
+                    localToken = linkedTokenSource.Token;
+                }
+
+                return await AttemptConnection(AddressFamily.InterNetworkV6, context, localToken);
+            }
+            catch
+            {
+                // very naively fallback to ipv4 permanently for this execution based on the response of the first connection attempt.
+                // note that this may cause users to eventually get switched to ipv4 (on a random failure when they are switching networks, for instance)
+                // but in the interest of keeping this implementation simple, this is acceptable.
+                _useIPv6 = false;
+            }
+            finally
+            {
+                _hasResolvedIPv6Availability = true;
+            }
+        }
+
+        // fallback to IPv4.
+        return await AttemptConnection(AddressFamily.InterNetwork, context, cancellationToken);
+    }
+
+    private static async ValueTask<Stream> AttemptConnection(
+        AddressFamily addressFamily,
+        SocketsHttpConnectionContext context,
+        CancellationToken cancellationToken)
+    {
+        // The following socket constructor will create a dual-mode socket on systems where IPV6 is available.
+        var socket = new Socket(addressFamily, SocketType.Stream, ProtocolType.Tcp)
+        {
+            // Turn off Nagle's algorithm since it degrades performance in most HttpClient scenarios.
+            NoDelay = true
+        };
+
+        try
+        {
+            await socket.ConnectAsync(context.DnsEndPoint, cancellationToken).ConfigureAwait(false);
+            // The stream should take the ownership of the underlying socket,
+            // closing it when it's disposed.
+            return new NetworkStream(socket, ownsSocket: true);
+        }
+        catch
+        {
+            socket.Dispose();
+            throw;
+        }
+    }
+}

SS14.Launcher/Program.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Net.Http;
 using System.Net.Http.Headers;
 using System.Runtime.InteropServices;
 using System.Threading.Tasks;
@@ -78,7 +77,7 @@ public static void Main(string[] args)
         cfg.Load();
         Locator.CurrentMutable.RegisterConstant(cfg);
 
-        var http = new HttpClient();
+        var http = HappyEyeballsHttp.CreateHttpClient();
         http.DefaultRequestHeaders.UserAgent.Add(
             new ProductInfoHeaderValue(LauncherVersion.Name, LauncherVersion.Version?.ToString()));
         http.DefaultRequestHeaders.Add("SS14-Launcher-Fingerprint", cfg.Fingerprint.ToString());