Remove calls to Socket.Poll and use SocketShutdown.Both (#1706)

The message loop currently sits in a call to Poll until the socket has data to read or
it is closed. This is unnecessary - it can equally just sit in the call to Receive.

The call to Poll in Session.IsConnected is also unnecessary - we can instead just call
Socket.Connected. This only returns the connection state as of the last operation, but
we are always performing operations in the message loop (or else we are not connected),
so it should work equally well while being cheaper.

Lastly, when shutting down the socket, shut down both sides rather than just the sending
side (SocketShutdown.Both rather than SocketShutdown.Send) - at this point we do not care
about reading anything else. This makes it (more) certain that we will break out of the
Receive call in the message loop, as has been noted in #355 for whatever remaining issues
still exist there.

Author: Rob-Hague

src/Renci.SshNet/Abstractions/SocketAbstraction.cs

@@ -12,34 +12,6 @@ namespace Renci.SshNet.Abstractions
 {
     internal static partial class SocketAbstraction
     {
-        public static bool CanRead(Socket socket)
-        {
-            if (socket.Connected)
-            {
-                return socket.Poll(-1, SelectMode.SelectRead) && socket.Available > 0;
-            }
-
-            return false;
-        }
-
-        /// <summary>
-        /// Returns a value indicating whether the specified <see cref="Socket"/> can be used
-        /// to send data.
-        /// </summary>
-        /// <param name="socket">The <see cref="Socket"/> to check.</param>
-        /// <returns>
-        /// <see langword="true"/> if <paramref name="socket"/> can be written to; otherwise, <see langword="false"/>.
-        /// </returns>
-        public static bool CanWrite(Socket socket)
-        {
-            if (socket != null && socket.Connected)
-            {
-                return socket.Poll(-1, SelectMode.SelectWrite);
-            }
-
-            return false;
-        }
-
         public static Socket Connect(IPEndPoint remoteEndpoint, TimeSpan connectTimeout)
         {
             var socket = new Socket(remoteEndpoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) { NoDelay = true };

src/Renci.SshNet/Common/Extensions.cs

@@ -10,7 +10,6 @@
 using System.Runtime.CompilerServices;
 using System.Threading;
 
-using Renci.SshNet.Abstractions;
 using Renci.SshNet.Messages;
 
 namespace Renci.SshNet.Common
@@ -319,16 +318,6 @@ public static byte[] Concat(this byte[] first, byte[] second)
             return concat;
         }
 
-        internal static bool CanRead(this Socket socket)
-        {
-            return SocketAbstraction.CanRead(socket);
-        }
-
-        internal static bool CanWrite(this Socket socket)
-        {
-            return SocketAbstraction.CanWrite(socket);
-        }
-
         internal static bool IsConnected(this Socket socket)
         {
             if (socket is null)