Make DNS lookup threadsafe, also still work if IPv4 resolution fails

Author: UnknownShadow200

fCraft/Network/HttpUtil.cs

@@ -1,53 +1,53 @@
 // Part of fCraft | Copyright 2009-2015 Matvei Stefarov <me@matvei.org> | BSD-3 | See LICENSE.txt //Copyright (c) 2011-2013 Jon Baker, Glenn Marien and Lao Tszy <Jonty800@gmail.com> //Copyright (c) <2012-2014> <LeChosenOne, DingusBungus> | ProCraft Copyright 2014-2016 Joseph Beauvais <123DMWM@gmail.com>
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Net;
 using System.Net.Sockets;
 using JetBrains.Annotations;
 
-namespace fCraft
-{
+namespace fCraft {
     /// <summary> Static class for assisting with making web requests. </summary>
-    public static class HttpUtil
-    {
+    public static class HttpUtil {
+        
         // Dns lookup, to make sure that IPv4 is preferred for connections
-        static readonly Dictionary<string, IPAddress> TargetAddresses = new Dictionary<string, IPAddress>();
+        static readonly Dictionary<string, IPAddress> cache = new Dictionary<string, IPAddress>();
         static DateTime nextDnsLookup = DateTime.MinValue;
         static readonly TimeSpan DnsRefreshInterval = TimeSpan.FromMinutes(30);
+        static readonly object dnsLock = new object();
 
-        static IPAddress RefreshTargetAddress([NotNull] Uri requestUri)
-        {
-            if (requestUri == null) throw new ArgumentNullException("requestUri");
-
-            string hostName = requestUri.Host.ToLowerInvariant();
-            IPAddress targetAddress;
-            if (!TargetAddresses.TryGetValue(hostName, out targetAddress) || DateTime.UtcNow >= nextDnsLookup) {
-                IPAddress[] allAddresses = null;
+        static IPAddress LookupIPv4([NotNull] Uri uri) {
+            if (uri == null) throw new ArgumentNullException("requestUri");
+            string hostName = uri.Host.ToLowerInvariant();
+            IPAddress ip;
+            
+            lock (dnsLock) {
+                if (cache.TryGetValue(hostName, out ip) && DateTime.UtcNow < nextDnsLookup) return ip;
+            }
+            
+            try {
+                // Perform a DNS lookup on given host. Throws SocketException if no host found.
+                IPAddress[] allIPs = null;
                 try {
-                    // Perform a DNS lookup on given host. Throws SocketException if no host found.
-                    try {
-                        allAddresses = Dns.GetHostAddresses(requestUri.Host);
-                    } catch {
-                        return null;
-                    }
-                    // Find a suitable IPv4 address. Throws InvalidOperationException if none found.
-                    targetAddress = allAddresses.First(ip => ip.AddressFamily == AddressFamily.InterNetwork);
-                } catch (SocketException ex) {
-                    Logger.Log(LogType.Error,
-                        "HttpUtil.RefreshTargetAddress: Error looking up server URL: {0}", ex);
-                } catch (InvalidOperationException) {
-                    Logger.Log(LogType.Warning, "HttpUtil.RefreshTargetAddress: {0} does not have an IPv4 address!",
-                        requestUri.Host);
-                } catch (UriFormatException) {
-                    Logger.Log(LogType.Warning, "Invalid URI: The hostname could not be parsed.");
-                    
+                    allIPs = Dns.GetHostAddresses(uri.Host);
+                } catch {
+                    return null;
                 }
-                TargetAddresses[hostName] = targetAddress;
-                nextDnsLookup = DateTime.UtcNow + DnsRefreshInterval;
+                
+                // Find a suitable IPv4 address. Throws InvalidOperationException if none found.
+                ip = allIPs.First(address => address.AddressFamily == AddressFamily.InterNetwork);
+            } catch (SocketException ex) {
+                Logger.Log(LogType.Error, "HttpUtil.LookupIPv4: Error looking up server URL: {0}", ex);
+            } catch (InvalidOperationException) {
+                Logger.Log(LogType.Warning, "HttpUtil.LookupIPv4: {0} does not have an IPv4 address!", uri.Host);
+            } catch (UriFormatException) {
+                Logger.Log(LogType.Warning, "Invalid URI: The hostname could not be parsed.");
+                
             }
-            return targetAddress;
+            
+            lock (dnsLock) { cache[hostName] = ip; }
+            nextDnsLookup = DateTime.UtcNow + DnsRefreshInterval;
+            return ip;
         }
 
 
@@ -61,9 +61,12 @@ public static HttpWebRequest CreateRequest([NotNull] Uri uri, TimeSpan timeout)
             req.ServicePoint.BindIPEndPointDelegate = Server.BindIPEndPointCallback;
             req.Timeout = (int)timeout.TotalMilliseconds;
             req.UserAgent = Updater.UserAgent;
-            
+
             if (uri.Scheme == "http") {
-                req.Proxy = new WebProxy("http://" + RefreshTargetAddress(uri) + ":" + uri.Port);
+                IPAddress ipv4 = LookupIPv4(uri);
+                if (ipv4 != null) {
+                    req.Proxy = new WebProxy("http://" + LookupIPv4(uri) + ":" + uri.Port);
+                }
             }
             return req;
         }