Add async support to SftpClient and SftpFileStream (#819)

* Add FEATURE_TAP and net472 target
* Add TAP async support to SftpClient and SftpFileStream
* Add async support to DnsAbstraction and SocketAbstraction
* Add async support to *Connector and refactor the hierarchy
* Add ConnectAsync to BaseClient

src/Renci.SshNet/Abstractions/DnsAbstraction.cs

@@ -2,6 +2,10 @@
 using System.Net;
 using System.Net.Sockets;
 
+#if FEATURE_TAP
+using System.Threading.Tasks;
+#endif
+
 #if FEATURE_DNS_SYNC
 #elif FEATURE_DNS_APM
 using Renci.SshNet.Common;
@@ -87,5 +91,23 @@ public static IPAddress[] GetHostAddresses(string hostNameOrAddress)
 #endif // FEATURE_DEVICEINFORMATION_APM
 #endif
         }
+
+#if FEATURE_TAP
+        /// <summary>
+        /// Returns the Internet Protocol (IP) addresses for the specified host.
+        /// </summary>
+        /// <param name="hostNameOrAddress">The host name or IP address to resolve</param>
+        /// <returns>
+        /// A task with result of an array of type <see cref="IPAddress"/> that holds the IP addresses for the host that
+        /// is specified by the <paramref name="hostNameOrAddress"/> parameter.
+        /// </returns>
+        /// <exception cref="ArgumentNullException"><paramref name="hostNameOrAddress"/> is <c>null</c>.</exception>
+        /// <exception cref="SocketException">An error is encountered when resolving <paramref name="hostNameOrAddress"/>.</exception>
+        public static Task<IPAddress[]> GetHostAddressesAsync(string hostNameOrAddress)
+        {
+            return Dns.GetHostAddressesAsync(hostNameOrAddress);
+        }
+#endif
+
     }
 }

src/Renci.SshNet/Abstractions/SocketAbstraction.cs

@@ -3,6 +3,9 @@
 using System.Net;
 using System.Net.Sockets;
 using System.Threading;
+#if FEATURE_TAP
+using System.Threading.Tasks;
+#endif
 using Renci.SshNet.Common;
 using Renci.SshNet.Messages.Transport;
 
@@ -59,6 +62,13 @@ public static void Connect(Socket socket, IPEndPoint remoteEndpoint, TimeSpan co
             ConnectCore(socket, remoteEndpoint, connectTimeout, false);
         }
 
+#if FEATURE_TAP
+        public static Task ConnectAsync(Socket socket, IPEndPoint remoteEndpoint, CancellationToken cancellationToken)
+        {
+            return socket.ConnectAsync(remoteEndpoint, cancellationToken);
+        }
+#endif
+
         private static void ConnectCore(Socket socket, IPEndPoint remoteEndpoint, TimeSpan connectTimeout, bool ownsSocket)
         {
 #if FEATURE_SOCKET_EAP
@@ -317,6 +327,13 @@ public static byte[] Read(Socket socket, int size, TimeSpan timeout)
             return buffer;
         }
 
+#if FEATURE_TAP
+        public static Task<int> ReadAsync(Socket socket, byte[] buffer, int offset, int length, CancellationToken cancellationToken)
+        {
+            return socket.ReceiveAsync(buffer, offset, length, cancellationToken);
+        }
+#endif
+
         /// <summary>
         /// Receives data from a bound <see cref="Socket"/> into a receive buffer.
         /// </summary>

src/Renci.SshNet/Abstractions/SocketExtensions.cs

@@ -0,0 +1,119 @@
+#if FEATURE_TAP
+using System;
+using System.Net;
+using System.Net.Sockets;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Renci.SshNet.Abstractions
+{
+    // Async helpers based on https://devblogs.microsoft.com/pfxteam/awaiting-socket-operations/
+
+    internal static class SocketExtensions
+    {
+        sealed class SocketAsyncEventArgsAwaitable : SocketAsyncEventArgs, INotifyCompletion
+        {
+            private readonly static Action SENTINEL = () => { };
+
+            private bool isCancelled;
+            private Action continuationAction;
+
+            public SocketAsyncEventArgsAwaitable()
+            {
+                Completed += delegate { SetCompleted(); };
+            }
+
+            public SocketAsyncEventArgsAwaitable ExecuteAsync(Func<SocketAsyncEventArgs, bool> func)
+            {
+                if (!func(this))
+                {
+                    SetCompleted();
+                }
+                return this;
+            }
+
+            public void SetCompleted()
+            {
+                IsCompleted = true;
+                var continuation = continuationAction ?? Interlocked.CompareExchange(ref continuationAction, SENTINEL, null);
+                if (continuation != null)
+                {
+                    continuation();
+                }
+            }
+
+            public void SetCancelled()
+            {
+                isCancelled = true;
+                SetCompleted();
+            }
+
+            public SocketAsyncEventArgsAwaitable GetAwaiter() { return this; }
+
+            public bool IsCompleted { get; private set; }
+
+            void INotifyCompletion.OnCompleted(Action continuation)
+            {
+                if (continuationAction == SENTINEL || Interlocked.CompareExchange(ref continuationAction, continuation, null) == SENTINEL)
+                {
+                    // We have already completed; run continuation asynchronously
+                    Task.Run(continuation);
+                }
+            }
+
+            public void GetResult()
+            {
+                if (isCancelled)
+                {
+                    throw new TaskCanceledException();
+                }
+                else if (IsCompleted)
+                {
+                    if (SocketError != SocketError.Success)
+                    {
+                        throw new SocketException((int)SocketError);
+                    }
+                }
+                else
+                {
+                    // We don't support sync/async
+                    throw new InvalidOperationException("The asynchronous operation has not yet completed.");
+                }
+            }
+        }
+
+        public static async Task ConnectAsync(this Socket socket, IPEndPoint remoteEndpoint, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            using (var args = new SocketAsyncEventArgsAwaitable())
+            {
+                args.RemoteEndPoint = remoteEndpoint;
+
+                using (cancellationToken.Register(o => ((SocketAsyncEventArgsAwaitable)o).SetCancelled(), args, false))
+                {
+                    await args.ExecuteAsync(socket.ConnectAsync);
+                }
+            }
+        }
+
+        public static async Task<int> ReceiveAsync(this Socket socket, byte[] buffer, int offset, int length, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            using (var args = new SocketAsyncEventArgsAwaitable())
+            {
+                args.SetBuffer(buffer, offset, length);
+
+                using (cancellationToken.Register(o => ((SocketAsyncEventArgsAwaitable)o).SetCancelled(), args, false))
+                {
+                    await args.ExecuteAsync(socket.ReceiveAsync);
+                }
+
+                return args.BytesTransferred;
+            }
+        }
+    }
+}
+#endif

src/Renci.SshNet/BaseClient.cs

@@ -1,6 +1,9 @@
 using System;
 using System.Net.Sockets;
 using System.Threading;
+#if FEATURE_TAP
+using System.Threading.Tasks;
+#endif
 using Renci.SshNet.Abstractions;
 using Renci.SshNet.Common;
 using Renci.SshNet.Messages.Transport;
@@ -239,6 +242,63 @@ public void Connect()
             StartKeepAliveTimer();
         }
 
+#if FEATURE_TAP
+        /// <summary>
+        /// Asynchronously connects client to the server.
+        /// </summary>
+        /// <param name="cancellationToken">The <see cref="CancellationToken"/> to observe.</param>
+        /// <returns>A <see cref="Task"/> that represents the asynchronous connect operation.
+        /// </returns>
+        /// <exception cref="InvalidOperationException">The client is already connected.</exception>
+        /// <exception cref="ObjectDisposedException">The method was called after the client was disposed.</exception>
+        /// <exception cref="SocketException">Socket connection to the SSH server or proxy server could not be established, or an error occurred while resolving the hostname.</exception>
+        /// <exception cref="SshConnectionException">SSH session could not be established.</exception>
+        /// <exception cref="SshAuthenticationException">Authentication of SSH session failed.</exception>
+        /// <exception cref="ProxyException">Failed to establish proxy connection.</exception>
+        public async Task ConnectAsync(CancellationToken cancellationToken)
+        {
+            CheckDisposed();
+            cancellationToken.ThrowIfCancellationRequested();
+
+            // TODO (see issue #1758):
+            // we're not stopping the keep-alive timer and disposing the session here
+            // 
+            // we could do this but there would still be side effects as concrete
+            // implementations may still hang on to the original session
+            // 
+            // therefore it would be better to actually invoke the Disconnect method
+            // (and then the Dispose on the session) but even that would have side effects
+            // eg. it would remove all forwarded ports from SshClient
+            // 
+            // I think we should modify our concrete clients to better deal with a
+            // disconnect. In case of SshClient this would mean not removing the 
+            // forwarded ports on disconnect (but only on dispose ?) and link a
+            // forwarded port with a client instead of with a session
+            //
+            // To be discussed with Oleg (or whoever is interested)
+            if (IsSessionConnected())
+                throw new InvalidOperationException("The client is already connected.");
+
+            OnConnecting();
+
+            Session = await CreateAndConnectSessionAsync(cancellationToken).ConfigureAwait(false);
+            try
+            {
+                // Even though the method we invoke makes you believe otherwise, at this point only
+                // the SSH session itself is connected.
+                OnConnected();
+            }
+            catch
+            {
+                // Only dispose the session as Disconnect() would have side-effects (such as remove forwarded
+                // ports in SshClient).
+                DisposeSession();
+                throw;
+            }
+            StartKeepAliveTimer();
+        }
+#endif
+
         /// <summary>
         /// Disconnects client from the server.
         /// </summary>
@@ -473,6 +533,26 @@ private ISession CreateAndConnectSession()
             }
         }
 
+#if FEATURE_TAP
+        private async Task<ISession> CreateAndConnectSessionAsync(CancellationToken cancellationToken)
+        {
+            var session = _serviceFactory.CreateSession(ConnectionInfo, _serviceFactory.CreateSocketFactory());
+            session.HostKeyReceived += Session_HostKeyReceived;
+            session.ErrorOccured += Session_ErrorOccured;
+
+            try
+            {
+                await session.ConnectAsync(cancellationToken).ConfigureAwait(false);
+                return session;
+            }
+            catch
+            {
+                DisposeSession(session);
+                throw;
+            }
+        }
+#endif
+
         private void DisposeSession(ISession session)
         {
             session.ErrorOccured -= Session_ErrorOccured;

src/Renci.SshNet/Connection/ConnectorBase.cs

@@ -4,6 +4,11 @@
 using System;
 using System.Net;
 using System.Net.Sockets;
+using System.Threading;
+
+#if FEATURE_TAP
+using System.Threading.Tasks;
+#endif
 
 namespace Renci.SshNet.Connection
 {
@@ -21,6 +26,10 @@ protected ConnectorBase(ISocketFactory socketFactory)
 
         public abstract Socket Connect(IConnectionInfo connectionInfo);
 
+#if FEATURE_TAP
+        public abstract Task<Socket> ConnectAsync(IConnectionInfo connectionInfo, CancellationToken cancellationToken);
+#endif
+
         /// <summary>
         /// Establishes a socket connection to the specified host and port.
         /// </summary>
@@ -54,6 +63,42 @@ protected Socket SocketConnect(string host, int port, TimeSpan timeout)
             }
         }
 
+#if FEATURE_TAP
+        /// <summary>
+        /// Establishes a socket connection to the specified host and port.
+        /// </summary>
+        /// <param name="host">The host name of the server to connect to.</param>
+        /// <param name="port">The port to connect to.</param>
+        /// <param name="cancellationToken">The cancellation token to observe.</param>
+        /// <exception cref="SshOperationTimeoutException">The connection failed to establish within the configured <see cref="ConnectionInfo.Timeout"/>.</exception>
+        /// <exception cref="SocketException">An error occurred trying to establish the connection.</exception>
+        protected async Task<Socket> SocketConnectAsync(string host, int port, CancellationToken cancellationToken)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            var ipAddress = (await DnsAbstraction.GetHostAddressesAsync(host).ConfigureAwait(false))[0];
+            var ep = new IPEndPoint(ipAddress, port);
+
+            DiagnosticAbstraction.Log(string.Format("Initiating connection to '{0}:{1}'.", host, port));
+
+            var socket = SocketFactory.Create(ep.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
+            try
+            {
+                await SocketAbstraction.ConnectAsync(socket, ep, cancellationToken).ConfigureAwait(false);
+
+                const int socketBufferSize = 2 * Session.MaximumSshPacketSize;
+                socket.SendBufferSize = socketBufferSize;
+                socket.ReceiveBufferSize = socketBufferSize;
+                return socket;
+            }
+            catch (Exception)
+            {
+                socket.Dispose();
+                throw;
+            }
+        }
+#endif
+
         protected static byte SocketReadByte(Socket socket)
         {
             var buffer = new byte[1];

src/Renci.SshNet/Connection/DirectConnector.cs

@@ -1,8 +1,9 @@
 using System.Net.Sockets;
+using System.Threading;
 
 namespace Renci.SshNet.Connection
 {
-    internal class DirectConnector : ConnectorBase
+    internal sealed class DirectConnector : ConnectorBase
     {
         public DirectConnector(ISocketFactory socketFactory) : base(socketFactory)
         {
@@ -12,5 +13,12 @@ public override Socket Connect(IConnectionInfo connectionInfo)
         {
             return SocketConnect(connectionInfo.Host, connectionInfo.Port, connectionInfo.Timeout);
         }
+
+#if FEATURE_TAP
+        public override System.Threading.Tasks.Task<Socket> ConnectAsync(IConnectionInfo connectionInfo, CancellationToken cancellationToken)
+        {
+            return SocketConnectAsync(connectionInfo.Host, connectionInfo.Port, cancellationToken);
+        }
+#endif
     }
 }