(#109) XmppTcpConnection: cancellable TCP client connections

SharpXMPP.Shared/Compat/TcpClientEx.cs

@@ -0,0 +1,59 @@
+using System.Net;
+using System.Net.Sockets;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace SharpXMPP.Compat
+{
+    internal static class TcpClientEx
+    {
+#if !NET5_0_OR_GREATER
+        private static async Task AbandonOnCancel(this Task task, CancellationToken cancellationToken)
+        {
+            // See https://devblogs.microsoft.com/pfxteam/how-do-i-cancel-non-cancelable-async-operations/ for details.
+            var tcs = new TaskCompletionSource<bool>();
+            using (cancellationToken.Register(() => tcs.TrySetResult(true)))
+            {
+                if (task != await Task.WhenAny(task, tcs.Task))
+                {
+                    throw new OperationCanceledException(cancellationToken);
+                }
+            }
+        }
+#endif
+
+        // Unfortunately, only .NET 5+ supports TcpClient connection cancellation. We'll do the best effort here,
+        // though.
+        //
+        // TcpClient uses Socket::BeginConnect under the covers for all the connection methods on .NET Framework, which
+        // is documented to be cancelled on Close(). So, ideally, if the caller eventually disposes the client, then all
+        // the resources will be freed upon its destruction. Which means we are free to just abandon the task in
+        // question.
+
+        public static Task ConnectWithCancellationAsync(
+            this TcpClient tcpClient,
+            IPAddress address,
+            int port,
+            CancellationToken cancellationToken)
+        {
+#if NET5_0_OR_GREATER
+            return tcpClient.ConnectAsync(address, port, cancellationToken).AsTask();
+#else
+            return tcpClient.ConnectAsync(address, port).AbandonOnCancel(cancellationToken);
+#endif
+        }
+
+        public static Task ConnectWithCancellationAsync(
+            this TcpClient tcpClient,
+            IPAddress[] addresses,
+            int port,
+            CancellationToken cancellationToken)
+        {
+#if NET5_0_OR_GREATER
+            return tcpClient.ConnectAsync(addresses, port, cancellationToken).AsTask();
+#else
+            return tcpClient.ConnectAsync(addresses, port).AbandonOnCancel(cancellationToken);
+#endif
+        }
+    }
+}

SharpXMPP.Shared/Resolver.cs

@@ -4,7 +4,9 @@
 using System.Net;
 using System.Net.Sockets;
 using System.Text;
+using System.Threading;
 using System.Threading.Tasks;
+using SharpXMPP.Compat;
 
 namespace SharpXMPP
 {
@@ -18,12 +20,14 @@ public struct SRVRecord
 
     public static class Resolver
     {
-        public async static Task<List<SRVRecord>> ResolveXMPPClient(string domain)
+        public static Task<List<SRVRecord>> ResolveXMPPClient(string domain) =>
+            ResolveXMPPClient(domain, default);
+
+        public static async Task<List<SRVRecord>> ResolveXMPPClient(string domain, CancellationToken cancellationToken)
         {
-            var result = new List<SRVRecord>();
-            var client = new TcpClient();
-            await client.ConnectAsync(IPAddress.Parse("1.1.1.1"), 53);
-            var stream = client.GetStream();
+            using var client = new TcpClient();
+            await client.ConnectWithCancellationAsync(IPAddress.Parse("1.1.1.1"), 53, cancellationToken);
+            using var stream = client.GetStream();
             var message = EncodeQuery(domain);
             var lengthPrefix = IPAddress.HostToNetworkOrder((short)message.Length);
             var lengthPrefixBytes = BitConverter.GetBytes(lengthPrefix);
@@ -34,8 +38,7 @@ public async static Task<List<SRVRecord>> ResolveXMPPClient(string domain)
             stream.Read(responseLengthBytes, 0, 2);
             var responseMessage = new byte[IPAddress.NetworkToHostOrder(BitConverter.ToInt16(responseLengthBytes, 0))];
             stream.Read(responseMessage, 0, responseMessage.Length);
-            result = Decode(responseMessage);
-            return result;
+            return Decode(responseMessage);
         }
 
         private static byte[] EncodeQuery(string domain)

SharpXMPP.Shared/XmppTcpConnection.cs

@@ -8,6 +8,7 @@
 using System.Threading.Tasks;
 using System.Xml;
 using System.Xml.Linq;
+using SharpXMPP.Compat;
 using SharpXMPP.Errors;
 using SharpXMPP.XMPP;
 using SharpXMPP.XMPP.Bind;
@@ -143,8 +144,8 @@ public void SessionLoop()
 
         public override async Task ConnectAsync(CancellationToken token)
         {
-            List<IPAddress> HostAddresses = await ResolveHostAddresses();
-            await ConnectOverTcp(HostAddresses);
+            List<IPAddress> HostAddresses = await ResolveHostAddresses(token);
+            await ConnectOverTcp(HostAddresses, token);
 
             RestartXmlStreams();
 
@@ -201,11 +202,23 @@ public Task SessionLoopAsync(CancellationToken token)
 
         private Features GetServerFeatures() => Stanza.Parse<Features>(NextElement());
 
-        private async Task ConnectOverTcp(List<IPAddress> HostAddresses)
+        private async Task ConnectOverTcp(List<IPAddress> HostAddresses, CancellationToken cancellationToken)
         {
+            ((IDisposable)_client)?.Dispose();
+
             _client = new TcpClient();
-            await _client.ConnectAsync(HostAddresses.ToArray(), TcpPort); // TODO: check ports
-            ConnectionStream = _client.GetStream();
+            try
+            {
+                // TODO: check ports
+                await _client.ConnectWithCancellationAsync(HostAddresses.ToArray(), TcpPort, cancellationToken);
+                ConnectionStream = _client.GetStream();
+            }
+            catch
+            {
+                ((IDisposable)_client).Dispose();
+                _client = null;
+                throw;
+            }
         }
 
         private Task StartAuthentication(Features features)
@@ -244,21 +257,23 @@ private Task StartAuthentication(Features features)
         }
 
 
-        private async Task<List<IPAddress>> ResolveHostAddresses()
+        private async Task<List<IPAddress>> ResolveHostAddresses(CancellationToken cancellationToken)
         {
             List<IPAddress> HostAddresses = new List<IPAddress>();
 
-            var srvs = await Resolver.ResolveXMPPClient(Jid.Domain);
+            var srvs = await Resolver.ResolveXMPPClient(Jid.Domain, cancellationToken);
             if (srvs.Any())
             {
                 foreach (var srv in srvs)
                 {
+                    cancellationToken.ThrowIfCancellationRequested();
                     var addresses = await Dns.GetHostAddressesAsync(srv.Host);
                     HostAddresses.AddRange(addresses);
                 }
             }
             else
             {
+                cancellationToken.ThrowIfCancellationRequested();
                 HostAddresses.AddRange(await Dns.GetHostAddressesAsync(Jid.Domain));
             }