rclone/backend/sftp/sftp.go

// Package sftp provides a filesystem interface using github.com/pkg/sftp

// +build !plan9

package sftp

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

	"github.com/pkg/errors"
	"github.com/pkg/sftp"
	"github.com/rclone/rclone/fs"
	"github.com/rclone/rclone/fs/config"
	"github.com/rclone/rclone/fs/config/configmap"
	"github.com/rclone/rclone/fs/config/configstruct"
	"github.com/rclone/rclone/fs/config/obscure"
	"github.com/rclone/rclone/fs/fshttp"
	"github.com/rclone/rclone/fs/hash"
	"github.com/rclone/rclone/lib/env"
	"github.com/rclone/rclone/lib/pacer"
	"github.com/rclone/rclone/lib/readers"
	sshagent "github.com/xanzy/ssh-agent"
	"golang.org/x/crypto/ssh"
)

const (
	hashCommandNotSupported = "none"
	minSleep                = 100 * time.Millisecond
	maxSleep                = 2 * time.Second
	decayConstant           = 2 // bigger for slower decay, exponential
)

var (
	currentUser = readCurrentUser()
)

func init() {
	fsi := &fs.RegInfo{
		Name:        "sftp",
		Description: "SSH/SFTP Connection",
		NewFs:       NewFs,
		Options: []fs.Option{{
			Name:     "host",
			Help:     "SSH host to connect to",
			Required: true,
			Examples: []fs.OptionExample{{
				Value: "example.com",
				Help:  "Connect to example.com",
			}},
		}, {
			Name: "user",
			Help: "SSH username, leave blank for current username, " + currentUser,
		}, {
			Name: "port",
			Help: "SSH port, leave blank to use default (22)",
		}, {
			Name:       "pass",
			Help:       "SSH password, leave blank to use ssh-agent.",
			IsPassword: true,
		}, {
			Name: "key_file",
			Help: "Path to PEM-encoded private key file, leave blank or set key-use-agent to use ssh-agent.",
		}, {
			Name: "key_file_pass",
			Help: `The passphrase to decrypt the PEM-encoded private key file.

Only PEM encrypted key files (old OpenSSH format) are supported. Encrypted keys
in the new OpenSSH format can't be used.`,
			IsPassword: true,
		}, {
			Name: "key_use_agent",
			Help: `When set forces the usage of the ssh-agent.

When key-file is also set, the ".pub" file of the specified key-file is read and only the associated key is
requested from the ssh-agent. This allows to avoid ` + "`Too many authentication failures for *username*`" + ` errors
when the ssh-agent contains many keys.`,
			Default: false,
		}, {
			Name: "use_insecure_cipher",
			Help: `Enable the use of insecure ciphers and key exchange methods. 

This enables the use of the the following insecure ciphers and key exchange methods:

- aes128-cbc
- aes192-cbc
- aes256-cbc
- 3des-cbc
- diffie-hellman-group-exchange-sha256
- diffie-hellman-group-exchange-sha1

Those algorithms are insecure and may allow plaintext data to be recovered by an attacker.`,
			Default: false,
			Examples: []fs.OptionExample{
				{
					Value: "false",
					Help:  "Use default Cipher list.",
				}, {
					Value: "true",
					Help:  "Enables the use of the aes128-cbc cipher and diffie-hellman-group-exchange-sha256, diffie-hellman-group-exchange-sha1 key exchange.",
				},
			},
		}, {
			Name:    "disable_hashcheck",
			Default: false,
			Help:    "Disable the execution of SSH commands to determine if remote file hashing is available.\nLeave blank or set to false to enable hashing (recommended), set to true to disable hashing.",
		}, {
			Name:    "ask_password",
			Default: false,
			Help: `Allow asking for SFTP password when needed.

If this is set and no password is supplied then rclone will:
- ask for a password
- not contact the ssh agent
`,
			Advanced: true,
		}, {
			Name:    "path_override",
			Default: "",
			Help: `Override path used by SSH connection.

This allows checksum calculation when SFTP and SSH paths are
different. This issue affects among others Synology NAS boxes.

Shared folders can be found in directories representing volumes

    rclone sync /home/local/directory remote:/directory --ssh-path-override /volume2/directory

Home directory can be found in a shared folder called "home"

    rclone sync /home/local/directory remote:/home/directory --ssh-path-override /volume1/homes/USER/directory`,
			Advanced: true,
		}, {
			Name:     "set_modtime",
			Default:  true,
			Help:     "Set the modified time on the remote if set.",
			Advanced: true,
		}, {
			Name:     "md5sum_command",
			Default:  "",
			Help:     "The command used to read md5 hashes. Leave blank for autodetect.",
			Advanced: true,
		}, {
			Name:     "sha1sum_command",
			Default:  "",
			Help:     "The command used to read sha1 hashes. Leave blank for autodetect.",
			Advanced: true,
		}, {
			Name:     "skip_links",
			Default:  false,
			Help:     "Set to skip any symlinks and any other non regular files.",
			Advanced: true,
		}},
	}
	fs.Register(fsi)
}

// Options defines the configuration for this backend
type Options struct {
	Host              string `config:"host"`
	User              string `config:"user"`
	Port              string `config:"port"`
	Pass              string `config:"pass"`
	KeyFile           string `config:"key_file"`
	KeyFilePass       string `config:"key_file_pass"`
	KeyUseAgent       bool   `config:"key_use_agent"`
	UseInsecureCipher bool   `config:"use_insecure_cipher"`
	DisableHashCheck  bool   `config:"disable_hashcheck"`
	AskPassword       bool   `config:"ask_password"`
	PathOverride      string `config:"path_override"`
	SetModTime        bool   `config:"set_modtime"`
	Md5sumCommand     string `config:"md5sum_command"`
	Sha1sumCommand    string `config:"sha1sum_command"`
	SkipLinks         bool   `config:"skip_links"`
}

// Fs stores the interface to the remote SFTP files
type Fs struct {
	name         string
	root         string
	opt          Options          // parsed options
	m            configmap.Mapper // config
	features     *fs.Features     // optional features
	config       *ssh.ClientConfig
	url          string
	mkdirLock    *stringLock
	cachedHashes *hash.Set
	poolMu       sync.Mutex
	pool         []*conn
	pacer        *fs.Pacer // pacer for operations
}

// 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
}

// 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 (f *Fs) 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
	}
	fs.Debugf(f, "New connection %s->%s to %q", c.LocalAddr(), c.RemoteAddr(), c.ServerVersion())
	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
	c = &conn{
		err: make(chan error, 1),
	}
	c.sshClient, err = f.dial("tcp", f.opt.Host+":"+f.opt.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
	}
	err = f.pacer.Call(func() (bool, error) {
		c, err = f.sftpConnection()
		if err != nil {
			return true, err
		}
		return false, nil
	})
	return c, err
}

// 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, m configmap.Mapper) (fs.Fs, error) {
	ctx := context.Background()
	// Parse config into Options struct
	opt := new(Options)
	err := configstruct.Set(m, opt)
	if err != nil {
		return nil, err
	}
	if opt.User == "" {
		opt.User = currentUser
	}
	if opt.Port == "" {
		opt.Port = "22"
	}
	sshConfig := &ssh.ClientConfig{
		User:            opt.User,
		Auth:            []ssh.AuthMethod{},
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
		Timeout:         fs.Config.ConnectTimeout,
		ClientVersion:   "SSH-2.0-" + fs.Config.UserAgent,
	}

	if opt.UseInsecureCipher {
		sshConfig.Config.SetDefaults()
		sshConfig.Config.Ciphers = append(sshConfig.Config.Ciphers, "aes128-cbc", "aes192-cbc", "aes256-cbc", "3des-cbc")
		sshConfig.Config.KeyExchanges = append(sshConfig.Config.KeyExchanges, "diffie-hellman-group-exchange-sha1", "diffie-hellman-group-exchange-sha256")
	}

	keyFile := env.ShellExpand(opt.KeyFile)
	// Add ssh agent-auth if no password or file specified
	if (opt.Pass == "" && keyFile == "" && !opt.AskPassword) || opt.KeyUseAgent {
		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")
		}
		if keyFile != "" {
			pubBytes, err := ioutil.ReadFile(keyFile + ".pub")
			if err != nil {
				return nil, errors.Wrap(err, "failed to read public key file")
			}
			pub, _, _, _, err := ssh.ParseAuthorizedKey(pubBytes)
			if err != nil {
				return nil, errors.Wrap(err, "failed to parse public key file")
			}
			pubM := pub.Marshal()
			found := false
			for _, s := range signers {
				if bytes.Equal(pubM, s.PublicKey().Marshal()) {
					sshConfig.Auth = append(sshConfig.Auth, ssh.PublicKeys(s))
					found = true
					break
				}
			}
			if !found {
				return nil, errors.New("private key not found in the ssh-agent")
			}
		} else {
			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")
		}
		clearpass := ""
		if opt.KeyFilePass != "" {
			clearpass, err = obscure.Reveal(opt.KeyFilePass)
			if err != nil {
				return nil, err
			}
		}
		signer, err := ssh.ParsePrivateKeyWithPassphrase(key, []byte(clearpass))
		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 opt.Pass != "" {
		clearpass, err := obscure.Reveal(opt.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 opt.Pass == "" && opt.AskPassword {
		_, _ = fmt.Fprint(os.Stderr, "Enter SFTP password: ")
		clearpass := config.ReadPassword()
		sshConfig.Auth = append(sshConfig.Auth, ssh.Password(clearpass))
	}

	return NewFsWithConnection(ctx, name, root, m, opt, sshConfig)
}

// NewFsWithConnection creates a new Fs object from the name and root and a ssh.ClientConfig. It connects to
// the host specified in the ssh.ClientConfig
func NewFsWithConnection(ctx context.Context, name string, root string, m configmap.Mapper, opt *Options, sshConfig *ssh.ClientConfig) (fs.Fs, error) {
	f := &Fs{
		name:      name,
		root:      root,
		opt:       *opt,
		m:         m,
		config:    sshConfig,
		url:       "sftp://" + opt.User + "@" + opt.Host + ":" + opt.Port + "/" + root,
		mkdirLock: newStringLock(),
		pacer:     fs.NewPacer(pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
	}
	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(ctx, 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(ctx context.Context, 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(ctx context.Context, 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.IsDir() {
			if f.opt.SkipLinks {
				// skip non regular file if SkipLinks is set
				continue
			}
			oldInfo := info
			info, err = f.stat(remote)
			if err != nil {
				if !os.IsNotExist(err) {
					fs.Errorf(remote, "stat of non-regular file failed: %v", err)
				}
				info = oldInfo
			}
		}
		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(ctx)>
func (f *Fs) Put(ctx context.Context, 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(ctx, 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(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
	return f.Put(ctx, 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(ctx context.Context, 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(ctx context.Context, dir string) error {
	// Check to see if directory is empty as some servers will
	// delete recursively with RemoveDirectory
	entries, err := f.List(ctx, dir)
	if err != nil {
		return errors.Wrap(err, "Rmdir")
	}
	if len(entries) != 0 {
		return fs.ErrorDirectoryNotEmpty
	}
	// Remove the directory
	root := path.Join(f.root, dir)
	c, err := f.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "Rmdir")
	}
	err = c.sftpClient.RemoveDirectory(root)
	f.putSftpConnection(&c, err)
	return err
}

// Move renames a remote sftp file object
func (f *Fs) Move(ctx context.Context, 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(ctx, 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(ctx context.Context, 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
}

// run runds cmd on the remote end returning standard output
func (f *Fs) run(cmd string) ([]byte, error) {
	c, err := f.getSftpConnection()
	if err != nil {
		return nil, errors.Wrap(err, "run: get SFTP connection")
	}
	defer f.putSftpConnection(&c, err)

	session, err := c.sshClient.NewSession()
	if err != nil {
		return nil, errors.Wrap(err, "run: get SFTP sessiion")
	}
	defer func() {
		_ = session.Close()
	}()

	var stdout, stderr bytes.Buffer
	session.Stdout = &stdout
	session.Stderr = &stderr

	err = session.Run(cmd)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to run %q: %s", cmd, stderr.Bytes())
	}

	return stdout.Bytes(), nil
}

// Hashes returns the supported hash types of the filesystem
func (f *Fs) Hashes() hash.Set {
	if f.opt.DisableHashCheck {
		return hash.Set(hash.None)
	}

	if f.cachedHashes != nil {
		return *f.cachedHashes
	}

	// look for a hash command which works
	checkHash := func(commands []string, expected string, hashCommand *string, changed *bool) bool {
		if *hashCommand == hashCommandNotSupported {
			return false
		}
		if *hashCommand != "" {
			return true
		}
		*changed = true
		for _, command := range commands {
			output, err := f.run(command)
			if err != nil {
				continue
			}
			output = bytes.TrimSpace(output)
			fs.Debugf(f, "checking %q command: %q", command, output)
			if parseHash(output) == expected {
				*hashCommand = command
				return true
			}
		}
		*hashCommand = hashCommandNotSupported
		return false
	}

	changed := false
	md5Works := checkHash([]string{"md5sum", "md5 -r"}, "d41d8cd98f00b204e9800998ecf8427e", &f.opt.Md5sumCommand, &changed)
	sha1Works := checkHash([]string{"sha1sum", "sha1 -r"}, "da39a3ee5e6b4b0d3255bfef95601890afd80709", &f.opt.Sha1sumCommand, &changed)

	if changed {
		f.m.Set("md5sum_command", f.opt.Md5sumCommand)
		f.m.Set("sha1sum_command", f.opt.Sha1sumCommand)
	}

	set := hash.NewHashSet()
	if sha1Works {
		set.Add(hash.SHA1)
	}
	if md5Works {
		set.Add(hash.MD5)
	}

	f.cachedHashes = &set
	return set
}

// About gets usage stats
func (f *Fs) About(ctx context.Context) (*fs.Usage, error) {
	escapedPath := shellEscape(f.root)
	if f.opt.PathOverride != "" {
		escapedPath = shellEscape(path.Join(f.opt.PathOverride, f.root))
	}
	if len(escapedPath) == 0 {
		escapedPath = "/"
	}
	stdout, err := f.run("df -k " + escapedPath)
	if err != nil {
		return nil, errors.Wrap(err, "your remote may not support About")
	}

	usageTotal, usageUsed, usageAvail := parseUsage(stdout)
	usage := &fs.Usage{}
	if usageTotal >= 0 {
		usage.Total = fs.NewUsageValue(usageTotal)
	}
	if usageUsed >= 0 {
		usage.Used = fs.NewUsageValue(usageUsed)
	}
	if usageAvail >= 0 {
		usage.Free = fs.NewUsageValue(usageAvail)
	}
	return usage, nil
}

// 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(ctx context.Context, r hash.Type) (string, error) {
	if o.fs.opt.DisableHashCheck {
		return "", nil
	}
	_ = o.fs.Hashes()

	var hashCmd string
	if r == hash.MD5 {
		if o.md5sum != nil {
			return *o.md5sum, nil
		}
		hashCmd = o.fs.opt.Md5sumCommand
	} else if r == hash.SHA1 {
		if o.sha1sum != nil {
			return *o.sha1sum, nil
		}
		hashCmd = o.fs.opt.Sha1sumCommand
	} else {
		return "", hash.ErrUnsupported
	}
	if hashCmd == "" || hashCmd == hashCommandNotSupported {
		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())
	if o.fs.opt.PathOverride != "" {
		escapedPath = shellEscape(path.Join(o.fs.opt.PathOverride, o.remote))
	}
	err = session.Run(hashCmd + " " + escapedPath)
	fs.Debugf(nil, "sftp cmd = %s", 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()
	b := stdout.Bytes()
	fs.Debugf(nil, "sftp output = %q", b)
	str := parseHash(b)
	fs.Debugf(nil, "sftp hash = %q", str)
	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_.,:/\\@\u0080-\uFFFFFFFF\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 {
	// For strings with backslash *sum writes a leading \
	// https://unix.stackexchange.com/q/313733/94054
	return strings.Split(strings.TrimLeft(string(bytes), "\\"), " ")[0] // Split at hash / filename separator
}

// Parses the byte array output from the SSH session
// returned by an invocation of df into
// the disk size, used space, and avaliable space on the disk, in that order.
// Only works when `df` has output info on only one disk
func parseUsage(bytes []byte) (spaceTotal int64, spaceUsed int64, spaceAvail int64) {
	spaceTotal, spaceUsed, spaceAvail = -1, -1, -1
	lines := strings.Split(string(bytes), "\n")
	if len(lines) < 2 {
		return
	}
	split := strings.Fields(lines[1])
	if len(split) < 6 {
		return
	}
	spaceTotal, err := strconv.ParseInt(split[1], 10, 64)
	if err != nil {
		spaceTotal = -1
	}
	spaceUsed, err = strconv.ParseInt(split[2], 10, 64)
	if err != nil {
		spaceUsed = -1
	}
	spaceAvail, err = strconv.ParseInt(split[3], 10, 64)
	if err != nil {
		spaceAvail = -1
	}
	return spaceTotal * 1024, spaceUsed * 1024, spaceAvail * 1024
}

// 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(ctx context.Context) 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(ctx context.Context, modTime time.Time) error {
	if !o.fs.opt.SetModTime {
		return nil
	}
	c, err := o.fs.getSftpConnection()
	if err != nil {
		return errors.Wrap(err, "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()
}

// objectReader represents a file open for reading on the SFTP server
type objectReader struct {
	sftpFile   *sftp.File
	pipeReader *io.PipeReader
	done       chan struct{}
}

func newObjectReader(sftpFile *sftp.File) *objectReader {
	pipeReader, pipeWriter := io.Pipe()
	file := &objectReader{
		sftpFile:   sftpFile,
		pipeReader: pipeReader,
		done:       make(chan struct{}),
	}

	go func() {
		// Use sftpFile.WriteTo to pump data so that it gets a
		// chance to build the window up.
		_, err := sftpFile.WriteTo(pipeWriter)
		// Close the pipeWriter so the pipeReader fails with
		// the same error or EOF if err == nil
		_ = pipeWriter.CloseWithError(err)
		// signal that we've finished
		close(file.done)
	}()

	return file
}

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

// Close a reader of a remote sftp file
func (file *objectReader) Close() (err error) {
	// Close the sftpFile - this will likely cause the WriteTo to error
	err = file.sftpFile.Close()
	// Close the pipeReader so writes to the pipeWriter fail
	_ = file.pipeReader.Close()
	// Wait for the background process to finish
	<-file.done
	return err
}

// Open a remote sftp file object for reading. Seek is supported
func (o *Object) Open(ctx context.Context, 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(newObjectReader(sftpFile), limit)
	return in, nil
}

// Update a remote sftp file using the data <in> and ModTime from <src>
func (o *Object) Update(ctx context.Context, 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(ctx, src.ModTime(ctx))
	if err != nil {
		return errors.Wrap(err, "Update SetModTime failed")
	}
	return nil
}

// Remove a remote sftp file object
func (o *Object) Remove(ctx context.Context) 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.Abouter     = &Fs{}
	_ fs.Object      = &Object{}
)