@@ -41,6 +41,7 @@ var (
 
 	// Flags
 	sftpAskPassword = flags.BoolP("sftp-ask-password", "", false, "Allow asking for SFTP password when needed.")
+	sshPathOverride = flags.StringP("ssh-path-override", "", "", "Override path used by SSH connection.")
 )
 
 func init() {
// Package sftp provides a filesystem interface using github.com/pkg/sftp

// +build !plan9,go1.8

package sftp

import (
	"bytes"
	"context"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/user"
	"path"
	"regexp"
	"strings"
	"sync"
	"time"

	"github.com/ncw/rclone/fs"
	"github.com/ncw/rclone/fs/config"
	"github.com/ncw/rclone/fs/config/flags"
	"github.com/ncw/rclone/fs/config/obscure"
	"github.com/ncw/rclone/fs/fshttp"
	"github.com/ncw/rclone/fs/hash"
	"github.com/ncw/rclone/lib/readers"
	"github.com/pkg/errors"
	"github.com/pkg/sftp"
	"github.com/xanzy/ssh-agent"
	"golang.org/x/crypto/ssh"
	"golang.org/x/time/rate"
)

const (
	connectionsPerSecond = 10 // don't make more than this many ssh connections/s
)

var (
	currentUser = readCurrentUser()

	// Flags
	sftpAskPassword = flags.BoolP("sftp-ask-password", "", false, "Allow asking for SFTP password when needed.")
)

func init() {
	fsi := &fs.RegInfo{
		Name:        "sftp",
		Description: "SSH/SFTP Connection",
		NewFs:       NewFs,
		Options: []fs.Option{{
			Name:     "host",
			Help:     "SSH host to connect to",
			Optional: false,
			Examples: []fs.OptionExample{{
				Value: "example.com",
				Help:  "Connect to example.com",
			}},
		}, {
			Name:     "user",
			Help:     "SSH username, leave blank for current username, " + currentUser,
			Optional: true,
		}, {
			Name:     "port",
			Help:     "SSH port, leave blank to use default (22)",
			Optional: true,
		}, {
			Name:       "pass",
			Help:       "SSH password, leave blank to use ssh-agent.",
			Optional:   true,
			IsPassword: true,
		}, {
			Name:     "key_file",
			Help:     "Path to unencrypted PEM-encoded private key file, leave blank to use ssh-agent.",
			Optional: true,
		}, {
			Name:     "use_insecure_cipher",
			Help:     "Enable the user of the aes128-cbc cipher. This cipher is insecure and may allow plaintext data to be recovered by an attacker.",
			Optional: true,
			Examples: []fs.OptionExample{
				{
					Value: "false",
					Help:  "Use default Cipher list.",
				}, {
					Value: "true",
					Help:  "Enables the use of the aes128-cbc cipher.",
				},
			},
		}, {
			Name:     "disable_hashcheck",
			Help:     "Disable the execution of SSH commands to determine if remote file hashing is available. Leave blank or set to false to enable hashing (recommended), set to true to disable hashing.",
			Optional: true,
		}},
	}
	fs.Register(fsi)
}

// Fs stores the interface to the remote SFTP files
type Fs struct {
	name              string
	root              string
	features          *fs.Features // optional features
	config            *ssh.ClientConfig
	host              string
	port              string
	url               string
	mkdirLock         *stringLock
	cachedHashes      *hash.Set
	hashcheckDisabled bool
	setModtime        bool
	poolMu            sync.Mutex
	pool              []*conn
	connLimit         *rate.Limiter // for limiting number of connections per second
}

// Object is a remote SFTP file that has been stat'd (so it exists, but is not necessarily open for reading)
type Object struct {
	fs      *Fs
	remote  string
	size    int64       // size of the object
	modTime time.Time   // modification time of the object
	mode    os.FileMode // mode bits from the file
	md5sum  *string     // Cached MD5 checksum
	sha1sum *string     // Cached SHA1 checksum
}

// ObjectReader holds the sftp.File interface to a remote SFTP file opened for reading
type ObjectReader struct {
	object   *Object
	sftpFile *sftp.File
}

// readCurrentUser finds the current user name or "" if not found
func readCurrentUser() (userName string) {
	usr, err := user.Current()
	if err == nil {
		return usr.Username
	}
	// Fall back to reading $USER then $LOGNAME
	userName = os.Getenv("USER")
	if userName != "" {
		return userName
	}
	return os.Getenv("LOGNAME")
}

// Dial starts a client connection to the given SSH server. It is a
// convenience function that connects to the given network address,
// initiates the SSH handshake, and then sets up a Client.
func Dial(network, addr string, sshConfig *ssh.ClientConfig) (*ssh.Client, error) {
	dialer := fshttp.NewDialer(fs.Config)
	conn, err := dialer.Dial(network, addr)
	if err != nil {
		return nil, err
	}
	c, chans, reqs, err := ssh.NewClientConn(conn, addr, sshConfig)
	if err != nil {
		return nil, err
	}
	return ssh.NewClient(c, chans, reqs), nil
}

// conn encapsulates an ssh client and corresponding sftp client
type conn struct {
	sshClient  *ssh.Client
	sftpClient *sftp.Client
	err        chan error
}

// Wait for connection to close
func (c *conn) wait() {
	c.err <- c.sshClient.Conn.Wait()
}

// Closes the connection
func (c *conn) close() error {
	sftpErr := c.sftpClient.Close()
	sshErr := c.sshClient.Close()
	if sftpErr != nil {
		return sftpErr
	}
	return sshErr
}

// Returns an error if closed
func (c *conn) closed() error {
	select {
	case err := <-c.err:
		return err
	default:
	}
	return nil
}

// Open a new connection to the SFTP server.
func (f *Fs) sftpConnection() (c *conn, err error) {
	// Rate limit rate of new connections
	err = f.connLimit.Wait(context.Background())
	if err != nil {
		return nil, errors.Wrap(err, "limiter failed in connect")
	}
	c = &conn{
		err: make(chan error, 1),
	}
	c.sshClient, err = Dial("tcp", f.host+":"+f.port, f.config)
	if err != nil {
		return nil, errors.Wrap(err, "couldn't connect SSH")
	}
	c.sftpClient, err = sftp.NewClient(c.sshClient)
	if err != nil {
		_ = c.sshClient.Close()
		return nil, errors.Wrap(err, "couldn't initialise SFTP")
	}
	go c.wait()
	return c, nil
}

// Get an SFTP connection from the pool, or open a new one
func (f *Fs) getSftpConnection() (c *conn, err error) {
	f.poolMu.Lock()
	for len(f.pool) > 0 {
		c = f.pool[0]
		f.pool = f.pool[1:]
		err := c.closed()
		if err == nil {
			break
		}
		fs.Errorf(f, "Discarding closed SSH connection: %v", err)
		c = nil
	}
	f.poolMu.Unlock()
	if c != nil {
		return c, nil
	}
	return f.sftpConnection()
}

// Return an SFTP connection to the pool
//
// It nils the pointed to connection out so it can't be reused
//
// if err is not nil then it checks the connection is alive using a
// Getwd request
func (f *Fs) putSftpConnection(pc **conn, err error) {
	c := *pc
	*pc = nil
	if err != nil {
		// work out if this is an expected error
		underlyingErr := errors.Cause(err)
		isRegularError := false
		switch underlyingErr {
		case os.ErrNotExist:
			isRegularError = true
		default:
			switch underlyingErr.(type) {
			case *sftp.StatusError, *os.PathError:
				isRegularError = true
			}
		}
		// If not a regular SFTP error code then check the connection
		if !isRegularError {
			_, nopErr := c.sftpClient.Getwd()
			if nopErr != nil {
				fs.Debugf(f, "Connection failed, closing: %v", nopErr)
				_ = c.close()
				return
			}
			fs.Debugf(f, "Connection OK after error: %v", err)
		}
	}
	f.poolMu.Lock()
	f.pool = append(f.pool, c)
	f.poolMu.Unlock()
}

// NewFs creates a new Fs object from the name and root. It connects to
// the host specified in the config file.
func NewFs(name, root string) (fs.Fs, error) {
	user := config.FileGet(name, "user")
	host := config.FileGet(name, "host")
	port := config.FileGet(name, "port")
	pass := config.FileGet(name, "pass")
	keyFile := config.FileGet(name, "key_file")
	insecureCipher := config.FileGetBool(name, "use_insecure_cipher")
	hashcheckDisabled := config.FileGetBool(name, "disable_hashcheck")
	setModtime := config.FileGetBool(name, "set_modtime", true)
	if user == "" {
		user = currentUser
	}
	if port == "" {
		port = "22"
	}
	sshConfig := &ssh.ClientConfig{
		User:            user,
		Auth:            []ssh.AuthMethod{},
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
		Timeout:         fs.Config.ConnectTimeout,
	}

	if insecureCipher {
		sshConfig.Config.SetDefaults()
		sshConfig.Config.Ciphers = append(sshConfig.Config.Ciphers, "aes128-cbc")
	}

	// Add ssh agent-auth if no password or file specified
	if pass == "" && keyFile == "" {
		sshAgentClient, _, err := sshagent.New()
		if err != nil {
			return nil, errors.Wrap(err, "couldn't connect to ssh-agent")
		}
		signers, err := sshAgentClient.Signers()
		if err != nil {
			return nil, errors.Wrap(err, "couldn't read ssh agent signers")
		}
		sshConfig.Auth = append(sshConfig.Auth, ssh.PublicKeys(signers...))
	}

	// Load key file if specified
	if keyFile != "" {
		key, err := ioutil.ReadFile(keyFile)
		if err != nil {
			return nil, errors.Wrap(err, "failed to read private key file")
		}
		signer, err := ssh.ParsePrivateKey(key)
		if err != nil {
			return nil, errors.Wrap(err, "failed to parse private key file")
		}
		sshConfig.Auth = append(sshConfig.Auth, ssh.PublicKeys(signer))
	}

	// Auth from password if specified
	if pass != "" {
		clearpass, err := obscure.Reveal(pass)
		if err != nil {
			return nil, err
		}
		sshConfig.Auth = append(sshConfig.Auth, ssh.Password(clearpass))
	}

	// Ask for password if none was defined and we're allowed to
	if pass == "" && *sftpAskPassword {
		fmt.Fprint(os.Stderr, "Enter SFTP password: ")
		clearpass := config.ReadPassword()
		sshConfig.Auth = append(sshConfig.Auth, ssh.Password(clearpass))
	}

	f := &Fs{
		name:              name,
		root:              root,
		config:            sshConfig,
		host:              host,
		port:              port,
		url:               "sftp://" + user + "@" + host + ":" + port + "/" + root,
		hashcheckDisabled: hashcheckDisabled,
		setModtime:        setModtime,
		mkdirLock:         newStringLock(),
		connLimit:         rate.NewLimiter(rate.Limit(connectionsPerSecond), 1),
	}
	f.features = (&fs.Features{
		CanHaveEmptyDirectories: true,
	}).Fill(f)
	// Make a connection and pool it to return errors early
	c, err := f.getSftpConnection()
	if err != nil {
		return nil, errors.Wrap(err, "NewFs")
	}
	f.putSftpConnection(&c, nil)
	if root != "" {
		// Check to see if the root actually an existing file
		remote := path.Base(root)
		f.root = path.Dir(root)
		if f.root == "." {
			f.root = ""
		}
		_, err := f.NewObject(remote)
		if err != nil {
			if err == fs.ErrorObjectNotFound || errors.Cause(err) == fs.ErrorNotAFile {
				// File doesn't exist so return old f
				f.root = root
				return f, nil
			}
			return nil, err
		}
		// return an error with an fs which points to the parent
		return f, fs.ErrorIsFile
	}
	return f, nil
}

// Name returns the configured name of the file system
func (f *Fs) Name() string {
	return f.name
}

// Root returns the root for the filesystem
func (f *Fs) Root() string {
	return f.root
}

// String returns the URL for the filesystem
func (f *Fs) String() string {
	return f.url
}

// Features returns the optional features of this Fs
func (f *Fs) Features() *fs.Features {
	return f.features
}

// Precision is the remote sftp file system's modtime precision, which we have no way of knowing. We estimate at 1s
func (f *Fs) Precision() time.Duration {
	return time.Second
}

// NewObject creates a new remote sftp file object
func (f *Fs) NewObject(remote string) (fs.Object, error) {
	o := &Object{
		fs:     f,
		remote: remote,
	}
	err := o.stat()
	if err != nil {
		return nil, err
	}
	return o, nil
}

// dirExists returns true,nil if the directory exists, false, nil if
// it doesn't or false, err
func (f *Fs) dirExists(dir string) (bool, error) {
	if dir == "" {
		dir = "."
	}
	c, err := f.getSftpConnection()
	if err != nil {
		return false, errors.Wrap(err, "dirExists")
	}
	info, err := c.sftpClient.Stat(dir)
	f.putSftpConnection(&c, err)
	if err != nil {
		if os.IsNotExist(err) {
			return false, nil
		}
		return false, errors.Wrap(err, "dirExists stat failed")
	}
	if !info.IsDir() {
		return false, fs.ErrorIsFile
	}
	return true, nil
}

// List the objects and directories in dir into entries.  The
// entries can be returned in any order but should be for a
// complete directory.
//
// dir should be "" to list the root, and should not have
// trailing slashes.
//
// This should return ErrDirNotFound if the directory isn't
// found.
func (f *Fs) List(dir string) (entries fs.DirEntries, err error) {
	root := path.Join(f.root, dir)
	ok, err := f.dirExists(root)
	if err != nil {
		return nil, errors.Wrap(err, "List failed")
	}
	if !ok {
		return nil, fs.ErrorDirNotFound
	}
	sftpDir := root
	if sftpDir == "" {
		sftpDir = "."
	}
	c, err := f.getSftpConnection()
	if err != nil {
		return nil, errors.Wrap(err, "List")
	}
	infos, err := c.sftpClient.ReadDir(sftpDir)
	f.putSftpConnection(&c, err)
	if err != nil {
		return nil, errors.Wrapf(err, "error listing %q", dir)
	}
	for _, info := range infos {
		remote := path.Join(dir, info.Name())
		// If file is a symlink (not a regular file is the best cross platform test we can do), do a stat to
		// pick up the size and type of the destination, instead of the size and type of the symlink.
		if !info.Mode().IsRegular() {
			info, err = f.stat(remote)
			if err != nil {
				return nil, errors.Wrap(err, "stat of non-regular file/dir failed")
			}
		}
		if info.IsDir() {
			d := fs.NewDir(remote, info.ModTime())
			entries = append(entries, d)
		} else {
			o := &Object{
				fs:     f,
				remote: remote,
			}
			o.setMetadata(info)
			entries = append(entries, o)
		}
	}
	return entries, nil
}

// Put data from <in> into a new remote sftp file object described by <src.Remote()> and <src.ModTime()>
func (f *Fs) Put(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
	err := f.mkParentDir(src.Remote())
	if err != nil {
		return nil, errors.Wrap(err, "Put mkParentDir failed")
	}
	// Temporary object under construction
	o := &Object{
		fs:     f,
		remote: src.Remote(),
	}
	err = o.Update(in, src, options...)
	if err != nil {
		return nil, err
	}
	return o, nil
}

// PutStream uploads to the remote path with the modTime given of indeterminate size
func (f *Fs) PutStream(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
	return f.Put(in, src, options...)
}

// mkParentDir makes the parent of remote if necessary and any
// directories above that
func (f *Fs) mkParentDir(remote string) error {
	parent := path.Dir(remote)
	return f.mkdir(path.Join(f.root, parent))
}

// mkdir makes the directory and parents using native paths
func (f *Fs) mkdir(dirPath string) error {
	f.mkdirLock.Lock(dirPath)
	defer f.mkdirLock.Unlock(dirPath)
	if dirPath == "." || dirPath == "/" {
		return nil
	}
	ok, err := f.dirExists(dirPath)
	if err != nil {
		return errors.Wrap(err, "mkdir dirExists failed")
	}
	if ok {
		return nil
	}
	parent := path.Dir(dirPath)
	err = f.mkdir(parent)
	if err != nil {
		return err
	}
	c, err := f.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "mkdir")
	}
	err = c.sftpClient.Mkdir(dirPath)
	f.putSftpConnection(&c, err)
	if err != nil {
		return errors.Wrapf(err, "mkdir %q failed", dirPath)
	}
	return nil
}

// Mkdir makes the root directory of the Fs object
func (f *Fs) Mkdir(dir string) error {
	root := path.Join(f.root, dir)
	return f.mkdir(root)
}

// Rmdir removes the root directory of the Fs object
func (f *Fs) Rmdir(dir string) error {
	root := path.Join(f.root, dir)
	c, err := f.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "Rmdir")
	}
	err = c.sftpClient.Remove(root)
	f.putSftpConnection(&c, err)
	return err
}

// Move renames a remote sftp file object
func (f *Fs) Move(src fs.Object, remote string) (fs.Object, error) {
	srcObj, ok := src.(*Object)
	if !ok {
		fs.Debugf(src, "Can't move - not same remote type")
		return nil, fs.ErrorCantMove
	}
	err := f.mkParentDir(remote)
	if err != nil {
		return nil, errors.Wrap(err, "Move mkParentDir failed")
	}
	c, err := f.getSftpConnection()
	if err != nil {
		return nil, errors.Wrap(err, "Move")
	}
	err = c.sftpClient.Rename(
		srcObj.path(),
		path.Join(f.root, remote),
	)
	f.putSftpConnection(&c, err)
	if err != nil {
		return nil, errors.Wrap(err, "Move Rename failed")
	}
	dstObj, err := f.NewObject(remote)
	if err != nil {
		return nil, errors.Wrap(err, "Move NewObject failed")
	}
	return dstObj, nil
}

// DirMove moves src, srcRemote to this remote at dstRemote
// using server side move operations.
//
// Will only be called if src.Fs().Name() == f.Name()
//
// If it isn't possible then return fs.ErrorCantDirMove
//
// If destination exists then return fs.ErrorDirExists
func (f *Fs) DirMove(src fs.Fs, srcRemote, dstRemote string) error {
	srcFs, ok := src.(*Fs)
	if !ok {
		fs.Debugf(srcFs, "Can't move directory - not same remote type")
		return fs.ErrorCantDirMove
	}
	srcPath := path.Join(srcFs.root, srcRemote)
	dstPath := path.Join(f.root, dstRemote)

	// Check if destination exists
	ok, err := f.dirExists(dstPath)
	if err != nil {
		return errors.Wrap(err, "DirMove dirExists dst failed")
	}
	if ok {
		return fs.ErrorDirExists
	}

	// Make sure the parent directory exists
	err = f.mkdir(path.Dir(dstPath))
	if err != nil {
		return errors.Wrap(err, "DirMove mkParentDir dst failed")
	}

	// Do the move
	c, err := f.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "DirMove")
	}
	err = c.sftpClient.Rename(
		srcPath,
		dstPath,
	)
	f.putSftpConnection(&c, err)
	if err != nil {
		return errors.Wrapf(err, "DirMove Rename(%q,%q) failed", srcPath, dstPath)
	}
	return nil
}

// Hashes returns the supported hash types of the filesystem
func (f *Fs) Hashes() hash.Set {
	if f.cachedHashes != nil {
		return *f.cachedHashes
	}

	if f.hashcheckDisabled {
		return hash.Set(hash.None)
	}

	c, err := f.getSftpConnection()
	if err != nil {
		fs.Errorf(f, "Couldn't get SSH connection to figure out Hashes: %v", err)
		return hash.Set(hash.None)
	}
	defer f.putSftpConnection(&c, err)
	session, err := c.sshClient.NewSession()
	if err != nil {
		return hash.Set(hash.None)
	}
	sha1Output, _ := session.Output("echo 'abc' | sha1sum")
	expectedSha1 := "03cfd743661f07975fa2f1220c5194cbaff48451"
	_ = session.Close()

	session, err = c.sshClient.NewSession()
	if err != nil {
		return hash.Set(hash.None)
	}
	md5Output, _ := session.Output("echo 'abc' | md5sum")
	expectedMd5 := "0bee89b07a248e27c83fc3d5951213c1"
	_ = session.Close()

	sha1Works := parseHash(sha1Output) == expectedSha1
	md5Works := parseHash(md5Output) == expectedMd5

	set := hash.NewHashSet()
	if !sha1Works && !md5Works {
		set.Add(hash.None)
	}
	if sha1Works {
		set.Add(hash.SHA1)
	}
	if md5Works {
		set.Add(hash.MD5)
	}

	_ = session.Close()
	f.cachedHashes = &set
	return set
}

// Fs is the filesystem this remote sftp file object is located within
func (o *Object) Fs() fs.Info {
	return o.fs
}

// String returns the URL to the remote SFTP file
func (o *Object) String() string {
	if o == nil {
		return "<nil>"
	}
	return o.remote
}

// Remote the name of the remote SFTP file, relative to the fs root
func (o *Object) Remote() string {
	return o.remote
}

// Hash returns the selected checksum of the file
// If no checksum is available it returns ""
func (o *Object) Hash(r hash.Type) (string, error) {
	var hashCmd string
	if r == hash.MD5 {
		if o.md5sum != nil {
			return *o.md5sum, nil
		}
		hashCmd = "md5sum"
	} else if r == hash.SHA1 {
		if o.sha1sum != nil {
			return *o.sha1sum, nil
		}
		hashCmd = "sha1sum"
	} else {
		return "", hash.ErrUnsupported
	}

	c, err := o.fs.getSftpConnection()
	if err != nil {
		return "", errors.Wrap(err, "Hash get SFTP connection")
	}
	session, err := c.sshClient.NewSession()
	o.fs.putSftpConnection(&c, err)
	if err != nil {
		return "", errors.Wrap(err, "Hash put SFTP connection")
	}

	var stdout, stderr bytes.Buffer
	session.Stdout = &stdout
	session.Stderr = &stderr
	escapedPath := shellEscape(o.path())
	err = session.Run(hashCmd + " " + escapedPath)
	if err != nil {
		_ = session.Close()
		fs.Debugf(o, "Failed to calculate %v hash: %v (%s)", r, err, bytes.TrimSpace(stderr.Bytes()))
		return "", nil
	}

	_ = session.Close()
	str := parseHash(stdout.Bytes())
	if r == hash.MD5 {
		o.md5sum = &str
	} else if r == hash.SHA1 {
		o.sha1sum = &str
	}
	return str, nil
}

var shellEscapeRegex = regexp.MustCompile(`[^A-Za-z0-9_.,:/@\n-]`)

// Escape a string s.t. it cannot cause unintended behavior
// when sending it to a shell.
func shellEscape(str string) string {
	safe := shellEscapeRegex.ReplaceAllString(str, `\$0`)
	return strings.Replace(safe, "\n", "'\n'", -1)
}

// Converts a byte array from the SSH session returned by
// an invocation of md5sum/sha1sum to a hash string
// as expected by the rest of this application
func parseHash(bytes []byte) string {
	return strings.Split(string(bytes), " ")[0] // Split at hash / filename separator
}

// Size returns the size in bytes of the remote sftp file
func (o *Object) Size() int64 {
	return o.size
}

// ModTime returns the modification time of the remote sftp file
func (o *Object) ModTime() time.Time {
	return o.modTime
}

// path returns the native path of the object
func (o *Object) path() string {
	return path.Join(o.fs.root, o.remote)
}

// setMetadata updates the info in the object from the stat result passed in
func (o *Object) setMetadata(info os.FileInfo) {
	o.modTime = info.ModTime()
	o.size = info.Size()
	o.mode = info.Mode()
}

// statRemote stats the file or directory at the remote given
func (f *Fs) stat(remote string) (info os.FileInfo, err error) {
	c, err := f.getSftpConnection()
	if err != nil {
		return nil, errors.Wrap(err, "stat")
	}
	absPath := path.Join(f.root, remote)
	info, err = c.sftpClient.Stat(absPath)
	f.putSftpConnection(&c, err)
	return info, err
}

// stat updates the info in the Object
func (o *Object) stat() error {
	info, err := o.fs.stat(o.remote)
	if err != nil {
		if os.IsNotExist(err) {
			return fs.ErrorObjectNotFound
		}
		return errors.Wrap(err, "stat failed")
	}
	if info.IsDir() {
		return errors.Wrapf(fs.ErrorNotAFile, "%q", o.remote)
	}
	o.setMetadata(info)
	return nil
}

// SetModTime sets the modification and access time to the specified time
//
// it also updates the info field
func (o *Object) SetModTime(modTime time.Time) error {
	c, err := o.fs.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "SetModTime")
	}
	if o.fs.setModtime {
		err = c.sftpClient.Chtimes(o.path(), modTime, modTime)
		o.fs.putSftpConnection(&c, err)
		if err != nil {
			return errors.Wrap(err, "SetModTime failed")
		}
	}
	err = o.stat()
	if err != nil {
		return errors.Wrap(err, "SetModTime stat failed")
	}
	return nil
}

// Storable returns whether the remote sftp file is a regular file (not a directory, symbolic link, block device, character device, named pipe, etc)
func (o *Object) Storable() bool {
	return o.mode.IsRegular()
}

// Read from a remote sftp file object reader
func (file *ObjectReader) Read(p []byte) (n int, err error) {
	n, err = file.sftpFile.Read(p)
	return n, err
}

// Close a reader of a remote sftp file
func (file *ObjectReader) Close() (err error) {
	err = file.sftpFile.Close()
	return err
}

// Open a remote sftp file object for reading. Seek is supported
func (o *Object) Open(options ...fs.OpenOption) (in io.ReadCloser, err error) {
	var offset, limit int64 = 0, -1
	for _, option := range options {
		switch x := option.(type) {
		case *fs.SeekOption:
			offset = x.Offset
		case *fs.RangeOption:
			offset, limit = x.Decode(o.Size())
		default:
			if option.Mandatory() {
				fs.Logf(o, "Unsupported mandatory option: %v", option)
			}
		}
	}
	c, err := o.fs.getSftpConnection()
	if err != nil {
		return nil, errors.Wrap(err, "Open")
	}
	sftpFile, err := c.sftpClient.Open(o.path())
	o.fs.putSftpConnection(&c, err)
	if err != nil {
		return nil, errors.Wrap(err, "Open failed")
	}
	if offset > 0 {
		off, err := sftpFile.Seek(offset, io.SeekStart)
		if err != nil || off != offset {
			return nil, errors.Wrap(err, "Open Seek failed")
		}
	}
	in = readers.NewLimitedReadCloser(&ObjectReader{
		object:   o,
		sftpFile: sftpFile,
	}, limit)
	return in, nil
}

// Update a remote sftp file using the data <in> and ModTime from <src>
func (o *Object) Update(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) error {
	// Clear the hash cache since we are about to update the object
	o.md5sum = nil
	o.sha1sum = nil
	c, err := o.fs.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "Update")
	}
	file, err := c.sftpClient.Create(o.path())
	o.fs.putSftpConnection(&c, err)
	if err != nil {
		return errors.Wrap(err, "Update Create failed")
	}
	// remove the file if upload failed
	remove := func() {
		c, removeErr := o.fs.getSftpConnection()
		if removeErr != nil {
			fs.Debugf(src, "Failed to open new SSH connection for delete: %v", removeErr)
			return
		}
		removeErr = c.sftpClient.Remove(o.path())
		o.fs.putSftpConnection(&c, removeErr)
		if removeErr != nil {
			fs.Debugf(src, "Failed to remove: %v", removeErr)
		} else {
			fs.Debugf(src, "Removed after failed upload: %v", err)
		}
	}
	_, err = file.ReadFrom(in)
	if err != nil {
		remove()
		return errors.Wrap(err, "Update ReadFrom failed")
	}
	err = file.Close()
	if err != nil {
		remove()
		return errors.Wrap(err, "Update Close failed")
	}
	err = o.SetModTime(src.ModTime())
	if err != nil {
		return errors.Wrap(err, "Update SetModTime failed")
	}
	return nil
}

// Remove a remote sftp file object
func (o *Object) Remove() error {
	c, err := o.fs.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "Remove")
	}
	err = c.sftpClient.Remove(o.path())
	o.fs.putSftpConnection(&c, err)
	return err
}

// Check the interfaces are satisfied
var (
	_ fs.Fs          = &Fs{}
	_ fs.PutStreamer = &Fs{}
	_ fs.Mover       = &Fs{}
	_ fs.DirMover    = &Fs{}
	_ fs.Object      = &Object{}
)

I think this should probably be a config variable rather than a command line flag as it is something you'd always want set for your synology isn't it?