Add REST command

DearFTP/Connection/Commands/CommandsDispatcher.cs

@@ -25,6 +25,7 @@ namespace DearFTP.Connection.Commands
             new QuitCommand(),
             new RenameCommand(),
             new RetrieveCommand(),
+            new RestartCommand(),
             new SiteCommand(),
             new SizeCommand(),
             new StoreCommand(),

DearFTP/Connection/Commands/RestartCommand.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace DearFTP.Connection.Commands
+{
+    class RestartCommand : ICommand
+    {
+        public string[] Aliases { get; } = new string[]
+        {
+            "REST"
+        };
+
+        public void Execute(Session session, FtpStream stream, string alias, string argument)
+        {
+            if (uint.TryParse(argument, out uint position))
+            {
+                session.RestartPosition = (int)position;
+                stream.Send(ResponseCode.PendingFurtherInformation, $"Restart position set to {position}.");
+            }
+            else
+            {
+                stream.Send(ResponseCode.ArgumentsError, "Specified position is not a valid number.");
+            }
+        }
+    }
+}

DearFTP/Connection/Commands/RetrieveCommand.cs

@@ -40,28 +40,44 @@ namespace DearFTP.Connection.Commands
 
             stream.Send(ResponseCode.FileStatusOK, "File coming.");
 
-            SendFile(dataConnection.Stream, realPath);
+            int restartPosition = session.RestartPosition;
+            session.RestartPosition = 0;
+
+            SendFile(dataConnection.Stream, realPath, restartPosition);
 
             dataConnection.Close();
 
             stream.Send(ResponseCode.CloseDataConnection, "File sent.");
         }
 
-        private void SendFile(Stream stream, string path)
+        private void SendFile(Stream stream, string path, int offset)
         {
             using (var file = File.OpenRead(path))
             {
+                if (offset != 0)
+                {
+                    file.Seek(offset, SeekOrigin.Begin);
+                }
+
                 Span<byte> buffer = stackalloc byte[BufferSize];
                 long bytesToWrite = Math.Min(file.Length - file.Position, BufferSize);
 
                 while (file.Read(buffer) > 0)
                 {
-                    if (bytesToWrite < BufferSize)
+                    try
                     {
-                        stream.Write(buffer.Slice(0, (int)bytesToWrite));
-                        break;
+                        if (bytesToWrite < BufferSize)
+                        {
+                            stream.Write(buffer.Slice(0, (int)bytesToWrite));
+                            break;
+                        }
+
+                        stream.Write(buffer);
+                    }
+                    catch (Exception)
+                    {
+                        return;
                     }
-                    stream.Write(buffer);
 
                     bytesToWrite = Math.Min(file.Length - file.Position, BufferSize);
                 }

DearFTP/Connection/Session.cs

@@ -19,11 +19,12 @@ namespace DearFTP.Connection
         public User User { get; set; }
         public Share[] Shares { get; set; }
         public Share[] WritablesShares { get; set; }
-        public FtpStream FtpStream { get; }
         public NavigablePath NavigablePath { get; set; }
+        public FtpStream FtpStream { get; }
+        public DataConnection DataConnection { get; set; }
+        public int RestartPosition { get; set; }
         public string CurrentWorkingDirectory => NavigablePath.CurrentDirectory;
         public string IP => ((IPEndPoint)_client.Client.LocalEndPoint).Address.ToString();
-        public DataConnection DataConnection { get; set; }
 
         private TcpClient _client;
         private NetworkStream _networkStream;
@@ -39,6 +40,7 @@ namespace DearFTP.Connection
             CommandsDispatcher = FtpServer.Instance.CommandsDispatcher;
             Logger = FtpServer.Instance.Logger;
             DataConnection = new DataConnection();
+            RestartPosition = 0;
         }
 
         public void Start()