DearFTP/DearFTP/Connection/Commands/RetrieveCommand.cs

using System;
using System.IO;

namespace DearFTP.Connection.Commands
{
    class RetrieveCommand : ICommand
    {
        public const int BufferSize = 4096;

        public string[] Aliases { get; } = new string[]
        {
            "RETR"
        };

        public void Execute(Session session, FtpStream stream, string alias, string argument)
        {
            var dataConnection = session.DataConnection;

            if (!dataConnection.IsTlsProtected && !dataConnection.IsAvailable)
            {
                stream.Send(ResponseCode.DataConnectionOpenError, "Passive mode not activated.");
                return;
            }

            var (_, realPath, isDirectory) = session.NavigablePath.GetSystemFilePath(argument);

            if (realPath == null)
            {
                stream.Send(ResponseCode.FileUnavailable, "Requested file does not exist.");
                return;
            }

            if (isDirectory)
            {
                stream.Send(ResponseCode.FileUnavailable, "Requested file is a directory.");
                return;
            }

            stream.Send(ResponseCode.FileStatusOK, "File coming.");

            if (dataConnection.IsTlsProtected && !dataConnection.IsAvailable)
            {
                stream.Send(ResponseCode.DataConnectionOpenError, "Passive mode not activated.");
                return;
            }

            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, 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)
                {
                    try
                    {
                        if (bytesToWrite < BufferSize)
                        {
                            stream.Write(buffer.Slice(0, (int)bytesToWrite));
                            break;
                        }

                        stream.Write(buffer);
                    }
                    catch (Exception)
                    {
                        return;
                    }

                    bytesToWrite = Math.Min(file.Length - file.Position, BufferSize);
                }
            }
        }
    }
}