server.go - external/github.com/pkg/sftp - Git at Google

 package sftp

 // sftp server counterpart

 import (
 	"encoding"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strconv"
 	"sync"
 	"syscall"
 	"time"

 	"github.com/pkg/errors"
 )

 const (
 	// SftpServerWorkerCount defines the number of workers for the SFTP server
 	SftpServerWorkerCount = 8
 )

 // Server is an SSH File Transfer Protocol (sftp) server.
 // This is intended to provide the sftp subsystem to an ssh server daemon.
 // This implementation currently supports most of sftp server protocol version 3,
 // as specified at http://tools.ietf.org/html/draft-ietf-secsh-filexfer-02
 type Server struct {
 	*serverConn
 	debugStream   io.Writer
 	readOnly      bool
 	pktMgr        *packetManager
 	openFiles     map[string]*os.File
 	openFilesLock sync.RWMutex
 	handleCount   int
 }

 func (svr *Server) nextHandle(f *os.File) string {
 	svr.openFilesLock.Lock()
 	defer svr.openFilesLock.Unlock()
 	svr.handleCount++
 	handle := strconv.Itoa(svr.handleCount)
 	svr.openFiles[handle] = f
 	return handle
 }

 func (svr *Server) closeHandle(handle string) error {
 	svr.openFilesLock.Lock()
 	defer svr.openFilesLock.Unlock()
 	if f, ok := svr.openFiles[handle]; ok {
 		delete(svr.openFiles, handle)
 		return f.Close()
 	}

 	return EBADF
 }

 func (svr *Server) getHandle(handle string) (*os.File, bool) {
 	svr.openFilesLock.RLock()
 	defer svr.openFilesLock.RUnlock()
 	f, ok := svr.openFiles[handle]
 	return f, ok
 }

 type serverRespondablePacket interface {
 	encoding.BinaryUnmarshaler
 	id() uint32
 	respond(svr *Server) responsePacket
 }

 // NewServer creates a new Server instance around the provided streams, serving
 // content from the root of the filesystem.  Optionally, ServerOption
 // functions may be specified to further configure the Server.
 //
 // A subsequent call to Serve() is required to begin serving files over SFTP.
 func NewServer(rwc io.ReadWriteCloser, options ...ServerOption) (*Server, error) {
 	svrConn := &serverConn{
 		conn: conn{
 			Reader:      rwc,
 			WriteCloser: rwc,
 		},
 	}
 	s := &Server{
 		serverConn:  svrConn,
 		debugStream: ioutil.Discard,
 		pktMgr:      newPktMgr(svrConn),
 		openFiles:   make(map[string]*os.File),
 	}

 	for _, o := range options {
 		if err := o(s); err != nil {
 			return nil, err
 		}
 	}

 	return s, nil
 }

 // A ServerOption is a function which applies configuration to a Server.
 type ServerOption func(*Server) error

 // WithDebug enables Server debugging output to the supplied io.Writer.
 func WithDebug(w io.Writer) ServerOption {
 	return func(s *Server) error {
 		s.debugStream = w
 		return nil
 	}
 }

 // ReadOnly configures a Server to serve files in read-only mode.
 func ReadOnly() ServerOption {
 	return func(s *Server) error {
 		s.readOnly = true
 		return nil
 	}
 }

 // WithAllocator enable the allocator.
 // After processing a packet we keep in memory the allocated slices
 // and we reuse them for new packets.
 // The allocator is experimental
 func WithAllocator() ServerOption {
 	return func(s *Server) error {
 		alloc := newAllocator()
 		s.pktMgr.alloc = alloc
 		s.conn.alloc = alloc
 		return nil
 	}
 }

 type rxPacket struct {
 	pktType  fxp
 	pktBytes []byte
 }

 // Up to N parallel servers
 func (svr *Server) sftpServerWorker(pktChan chan orderedRequest) error {
 	for pkt := range pktChan {
 		// readonly checks
 		readonly := true
 		switch pkt := pkt.requestPacket.(type) {
 		case notReadOnly:
 			readonly = false
 		case *sshFxpOpenPacket:
 			readonly = pkt.readonly()
 		case *sshFxpExtendedPacket:
 			readonly = pkt.readonly()
 		}

 		// If server is operating read-only and a write operation is requested,
 		// return permission denied
 		if !readonly && svr.readOnly {
 			svr.pktMgr.readyPacket(
 				svr.pktMgr.newOrderedResponse(statusFromError(pkt.id(), syscall.EPERM), pkt.orderID()),
 			)
 			continue
 		}

 		if err := handlePacket(svr, pkt); err != nil {
 			return err
 		}
 	}
 	return nil
 }

 func handlePacket(s *Server, p orderedRequest) error {
 	var rpkt responsePacket
 	orderID := p.orderID()
 	switch p := p.requestPacket.(type) {
 	case *sshFxInitPacket:
 		rpkt = &sshFxVersionPacket{
 			Version:    sftpProtocolVersion,
 			Extensions: sftpExtensions,
 		}
 	case *sshFxpStatPacket:
 		// stat the requested file
 		info, err := os.Stat(p.Path)
 		rpkt = &sshFxpStatResponse{
 			ID:   p.ID,
 			info: info,
 		}
 		if err != nil {
 			rpkt = statusFromError(p.ID, err)
 		}
 	case *sshFxpLstatPacket:
 		// stat the requested file
 		info, err := os.Lstat(p.Path)
 		rpkt = &sshFxpStatResponse{
 			ID:   p.ID,
 			info: info,
 		}
 		if err != nil {
 			rpkt = statusFromError(p.ID, err)
 		}
 	case *sshFxpFstatPacket:
 		f, ok := s.getHandle(p.Handle)
 		var err error = EBADF
 		var info os.FileInfo
 		if ok {
 			info, err = f.Stat()
 			rpkt = &sshFxpStatResponse{
 				ID:   p.ID,
 				info: info,
 			}
 		}
 		if err != nil {
 			rpkt = statusFromError(p.ID, err)
 		}
 	case *sshFxpMkdirPacket:
 		// TODO FIXME: ignore flags field
 		err := os.Mkdir(p.Path, 0755)
 		rpkt = statusFromError(p.ID, err)
 	case *sshFxpRmdirPacket:
 		err := os.Remove(p.Path)
 		rpkt = statusFromError(p.ID, err)
 	case *sshFxpRemovePacket:
 		err := os.Remove(p.Filename)
 		rpkt = statusFromError(p.ID, err)
 	case *sshFxpRenamePacket:
 		err := os.Rename(p.Oldpath, p.Newpath)
 		rpkt = statusFromError(p.ID, err)
 	case *sshFxpSymlinkPacket:
 		err := os.Symlink(p.Targetpath, p.Linkpath)
 		rpkt = statusFromError(p.ID, err)
 	case *sshFxpClosePacket:
 		rpkt = statusFromError(p.ID, s.closeHandle(p.Handle))
 	case *sshFxpReadlinkPacket:
 		f, err := os.Readlink(p.Path)
 		rpkt = &sshFxpNamePacket{
 			ID: p.ID,
 			NameAttrs: []*sshFxpNameAttr{
 				{
 					Name:     f,
 					LongName: f,
 					Attrs:    emptyFileStat,
 				},
 			},
 		}
 		if err != nil {
 			rpkt = statusFromError(p.ID, err)
 		}
 	case *sshFxpRealpathPacket:
 		f, err := filepath.Abs(p.Path)
 		f = cleanPath(f)
 		rpkt = &sshFxpNamePacket{
 			ID: p.ID,
 			NameAttrs: []*sshFxpNameAttr{
 				{
 					Name:     f,
 					LongName: f,
 					Attrs:    emptyFileStat,
 				},
 			},
 		}
 		if err != nil {
 			rpkt = statusFromError(p.ID, err)
 		}
 	case *sshFxpOpendirPacket:
 		if stat, err := os.Stat(p.Path); err != nil {
 			rpkt = statusFromError(p.ID, err)
 		} else if !stat.IsDir() {
 			rpkt = statusFromError(p.ID, &os.PathError{
 				Path: p.Path, Err: syscall.ENOTDIR})
 		} else {
 			rpkt = (&sshFxpOpenPacket{
 				ID:     p.ID,
 				Path:   p.Path,
 				Pflags: sshFxfRead,
 			}).respond(s)
 		}
 	case *sshFxpReadPacket:
 		var err error = EBADF
 		f, ok := s.getHandle(p.Handle)
 		if ok {
 			err = nil
 			data := p.getDataSlice(s.pktMgr.alloc, orderID)
 			n, _err := f.ReadAt(data, int64(p.Offset))
 			if _err != nil && (_err != io.EOF || n == 0) {
 				err = _err
 			}
 			rpkt = &sshFxpDataPacket{
 				ID:     p.ID,
 				Length: uint32(n),
 				Data:   data[:n],
 				// do not use data[:n:n] here to clamp the capacity, we allocated extra capacity above to avoid reallocations
 			}
 		}
 		if err != nil {
 			rpkt = statusFromError(p.ID, err)
 		}

 	case *sshFxpWritePacket:
 		f, ok := s.getHandle(p.Handle)
 		var err error = EBADF
 		if ok {
 			_, err = f.WriteAt(p.Data, int64(p.Offset))
 		}
 		rpkt = statusFromError(p.ID, err)
 	case *sshFxpExtendedPacket:
 		if p.SpecificPacket == nil {
 			rpkt = statusFromError(p.ID, ErrSSHFxOpUnsupported)
 		} else {
 			rpkt = p.respond(s)
 		}
 	case serverRespondablePacket:
 		rpkt = p.respond(s)
 	default:
 		return errors.Errorf("unexpected packet type %T", p)
 	}

 	s.pktMgr.readyPacket(s.pktMgr.newOrderedResponse(rpkt, orderID))
 	return nil
 }

 // Serve serves SFTP connections until the streams stop or the SFTP subsystem
 // is stopped.
 func (svr *Server) Serve() error {
 	defer func() {
 		if svr.pktMgr.alloc != nil {
 			svr.pktMgr.alloc.Free()
 		}
 	}()
 	var wg sync.WaitGroup
 	runWorker := func(ch chan orderedRequest) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
 			if err := svr.sftpServerWorker(ch); err != nil {
 				svr.conn.Close() // shuts down recvPacket
 			}
 		}()
 	}
 	pktChan := svr.pktMgr.workerChan(runWorker)

 	var err error
 	var pkt requestPacket
 	var pktType uint8
 	var pktBytes []byte
 	for {
 		pktType, pktBytes, err = svr.serverConn.recvPacket(svr.pktMgr.getNextOrderID())
 		if err != nil {
 			// we don't care about releasing allocated pages here, the server will quit and the allocator freed
 			break
 		}

 		pkt, err = makePacket(rxPacket{fxp(pktType), pktBytes})
 		if err != nil {
 			switch errors.Cause(err) {
 			case errUnknownExtendedPacket:
 				//if err := svr.serverConn.sendError(pkt, ErrSshFxOpUnsupported); err != nil {
 				//	debug("failed to send err packet: %v", err)
 				//	svr.conn.Close() // shuts down recvPacket
 				//	break
 				//}
 			default:
 				debug("makePacket err: %v", err)
 				svr.conn.Close() // shuts down recvPacket
 				break
 			}
 		}

 		pktChan <- svr.pktMgr.newOrderedRequest(pkt)
 	}

 	close(pktChan) // shuts down sftpServerWorkers
 	wg.Wait()      // wait for all workers to exit

 	// close any still-open files
 	for handle, file := range svr.openFiles {
 		fmt.Fprintf(svr.debugStream, "sftp server file with handle %q left open: %v\n", handle, file.Name())
 		file.Close()
 	}
 	return err // error from recvPacket
 }

 type ider interface {
 	id() uint32
 }

 // The init packet has no ID, so we just return a zero-value ID
 func (p *sshFxInitPacket) id() uint32 { return 0 }

 type sshFxpStatResponse struct {
 	ID   uint32
 	info os.FileInfo
 }

 func (p *sshFxpStatResponse) marshalPacket() ([]byte, []byte, error) {
 	l := 4 + 1 + 4 // uint32(length) + byte(type) + uint32(id)

 	b := make([]byte, 4, l)
 	b = append(b, sshFxpAttrs)
 	b = marshalUint32(b, p.ID)

 	var payload []byte
 	payload = marshalFileInfo(payload, p.info)

 	return b, payload, nil
 }

 func (p *sshFxpStatResponse) MarshalBinary() ([]byte, error) {
 	header, payload, err := p.marshalPacket()
 	return append(header, payload...), err
 }

 var emptyFileStat = []interface{}{uint32(0)}

 func (p *sshFxpOpenPacket) readonly() bool {
 	return !p.hasPflags(sshFxfWrite)
 }

 func (p *sshFxpOpenPacket) hasPflags(flags ...uint32) bool {
 	for _, f := range flags {
 		if p.Pflags&f == 0 {
 			return false
 		}
 	}
 	return true
 }

 func (p *sshFxpOpenPacket) respond(svr *Server) responsePacket {
 	var osFlags int
 	if p.hasPflags(sshFxfRead, sshFxfWrite) {
 		osFlags |= os.O_RDWR
 	} else if p.hasPflags(sshFxfWrite) {
 		osFlags |= os.O_WRONLY
 	} else if p.hasPflags(sshFxfRead) {
 		osFlags |= os.O_RDONLY
 	} else {
 		// how are they opening?
 		return statusFromError(p.ID, syscall.EINVAL)
 	}

 	// Don't use O_APPEND flag as it conflicts with WriteAt.
 	// The sshFxfAppend flag is a no-op here as the client sends the offsets.

 	if p.hasPflags(sshFxfCreat) {
 		osFlags |= os.O_CREATE
 	}
 	if p.hasPflags(sshFxfTrunc) {
 		osFlags |= os.O_TRUNC
 	}
 	if p.hasPflags(sshFxfExcl) {
 		osFlags |= os.O_EXCL
 	}

 	f, err := os.OpenFile(p.Path, osFlags, 0644)
 	if err != nil {
 		return statusFromError(p.ID, err)
 	}

 	handle := svr.nextHandle(f)
 	return &sshFxpHandlePacket{ID: p.ID, Handle: handle}
 }

 func (p *sshFxpReaddirPacket) respond(svr *Server) responsePacket {
 	f, ok := svr.getHandle(p.Handle)
 	if !ok {
 		return statusFromError(p.ID, EBADF)
 	}

 	dirname := f.Name()
 	dirents, err := f.Readdir(128)
 	if err != nil {
 		return statusFromError(p.ID, err)
 	}

 	ret := &sshFxpNamePacket{ID: p.ID}
 	for _, dirent := range dirents {
 		ret.NameAttrs = append(ret.NameAttrs, &sshFxpNameAttr{
 			Name:     dirent.Name(),
 			LongName: runLs(dirname, dirent),
 			Attrs:    []interface{}{dirent},
 		})
 	}
 	return ret
 }

 func (p *sshFxpSetstatPacket) respond(svr *Server) responsePacket {
 	// additional unmarshalling is required for each possibility here
 	b := p.Attrs.([]byte)
 	var err error

 	debug("setstat name \"%s\"", p.Path)
 	if (p.Flags & sshFileXferAttrSize) != 0 {
 		var size uint64
 		if size, b, err = unmarshalUint64Safe(b); err == nil {
 			err = os.Truncate(p.Path, int64(size))
 		}
 	}
 	if (p.Flags & sshFileXferAttrPermissions) != 0 {
 		var mode uint32
 		if mode, b, err = unmarshalUint32Safe(b); err == nil {
 			err = os.Chmod(p.Path, os.FileMode(mode))
 		}
 	}
 	if (p.Flags & sshFileXferAttrACmodTime) != 0 {
 		var atime uint32
 		var mtime uint32
 		if atime, b, err = unmarshalUint32Safe(b); err != nil {
 		} else if mtime, b, err = unmarshalUint32Safe(b); err != nil {
 		} else {
 			atimeT := time.Unix(int64(atime), 0)
 			mtimeT := time.Unix(int64(mtime), 0)
 			err = os.Chtimes(p.Path, atimeT, mtimeT)
 		}
 	}
 	if (p.Flags & sshFileXferAttrUIDGID) != 0 {
 		var uid uint32
 		var gid uint32
 		if uid, b, err = unmarshalUint32Safe(b); err != nil {
 		} else if gid, _, err = unmarshalUint32Safe(b); err != nil {
 		} else {
 			err = os.Chown(p.Path, int(uid), int(gid))
 		}
 	}

 	return statusFromError(p.ID, err)
 }

 func (p *sshFxpFsetstatPacket) respond(svr *Server) responsePacket {
 	f, ok := svr.getHandle(p.Handle)
 	if !ok {
 		return statusFromError(p.ID, EBADF)
 	}

 	// additional unmarshalling is required for each possibility here
 	b := p.Attrs.([]byte)
 	var err error

 	debug("fsetstat name \"%s\"", f.Name())
 	if (p.Flags & sshFileXferAttrSize) != 0 {
 		var size uint64
 		if size, b, err = unmarshalUint64Safe(b); err == nil {
 			err = f.Truncate(int64(size))
 		}
 	}
 	if (p.Flags & sshFileXferAttrPermissions) != 0 {
 		var mode uint32
 		if mode, b, err = unmarshalUint32Safe(b); err == nil {
 			err = f.Chmod(os.FileMode(mode))
 		}
 	}
 	if (p.Flags & sshFileXferAttrACmodTime) != 0 {
 		var atime uint32
 		var mtime uint32
 		if atime, b, err = unmarshalUint32Safe(b); err != nil {
 		} else if mtime, b, err = unmarshalUint32Safe(b); err != nil {
 		} else {
 			atimeT := time.Unix(int64(atime), 0)
 			mtimeT := time.Unix(int64(mtime), 0)
 			err = os.Chtimes(f.Name(), atimeT, mtimeT)
 		}
 	}
 	if (p.Flags & sshFileXferAttrUIDGID) != 0 {
 		var uid uint32
 		var gid uint32
 		if uid, b, err = unmarshalUint32Safe(b); err != nil {
 		} else if gid, _, err = unmarshalUint32Safe(b); err != nil {
 		} else {
 			err = f.Chown(int(uid), int(gid))
 		}
 	}

 	return statusFromError(p.ID, err)
 }

 func statusFromError(id uint32, err error) *sshFxpStatusPacket {
 	ret := &sshFxpStatusPacket{
 		ID: id,
 		StatusError: StatusError{
 			// sshFXOk               = 0
 			// sshFXEOF              = 1
 			// sshFXNoSuchFile       = 2 ENOENT
 			// sshFXPermissionDenied = 3
 			// sshFXFailure          = 4
 			// sshFXBadMessage       = 5
 			// sshFXNoConnection     = 6
 			// sshFXConnectionLost   = 7
 			// sshFXOPUnsupported    = 8
 			Code: sshFxOk,
 		},
 	}
 	if err == nil {
 		return ret
 	}

 	debug("statusFromError: error is %T %#v", err, err)
 	ret.StatusError.Code = sshFxFailure
 	ret.StatusError.msg = err.Error()

 	if os.IsNotExist(err) {
 		ret.StatusError.Code = sshFxNoSuchFile
 		return ret
 	}
 	if code, ok := translateSyscallError(err); ok {
 		ret.StatusError.Code = code
 		return ret
 	}

 	switch e := err.(type) {
 	case fxerr:
 		ret.StatusError.Code = uint32(e)
 	default:
 		if e == io.EOF {
 			ret.StatusError.Code = sshFxEOF
 		}
 	}

 	return ret
 }

 func clamp(v, max uint32) uint32 {
 	if v > max {
 		return max
 	}
 	return v
 }

 func runLsTypeWord(dirent os.FileInfo) string {
 	// find first character, the type char
 	// b     Block special file.
 	// c     Character special file.
 	// d     Directory.
 	// l     Symbolic link.
 	// s     Socket link.
 	// p     FIFO.
 	// -     Regular file.
 	tc := '-'
 	mode := dirent.Mode()
 	if (mode & os.ModeDir) != 0 {
 		tc = 'd'
 	} else if (mode & os.ModeDevice) != 0 {
 		tc = 'b'
 		if (mode & os.ModeCharDevice) != 0 {
 			tc = 'c'
 		}
 	} else if (mode & os.ModeSymlink) != 0 {
 		tc = 'l'
 	} else if (mode & os.ModeSocket) != 0 {
 		tc = 's'
 	} else if (mode & os.ModeNamedPipe) != 0 {
 		tc = 'p'
 	}

 	// owner
 	orc := '-'
 	if (mode & 0400) != 0 {
 		orc = 'r'
 	}
 	owc := '-'
 	if (mode & 0200) != 0 {
 		owc = 'w'
 	}
 	oxc := '-'
 	ox := (mode & 0100) != 0
 	setuid := (mode & os.ModeSetuid) != 0
 	if ox && setuid {
 		oxc = 's'
 	} else if setuid {
 		oxc = 'S'
 	} else if ox {
 		oxc = 'x'
 	}

 	// group
 	grc := '-'
 	if (mode & 040) != 0 {
 		grc = 'r'
 	}
 	gwc := '-'
 	if (mode & 020) != 0 {
 		gwc = 'w'
 	}
 	gxc := '-'
 	gx := (mode & 010) != 0
 	setgid := (mode & os.ModeSetgid) != 0
 	if gx && setgid {
 		gxc = 's'
 	} else if setgid {
 		gxc = 'S'
 	} else if gx {
 		gxc = 'x'
 	}

 	// all / others
 	arc := '-'
 	if (mode & 04) != 0 {
 		arc = 'r'
 	}
 	awc := '-'
 	if (mode & 02) != 0 {
 		awc = 'w'
 	}
 	axc := '-'
 	ax := (mode & 01) != 0
 	sticky := (mode & os.ModeSticky) != 0
 	if ax && sticky {
 		axc = 't'
 	} else if sticky {
 		axc = 'T'
 	} else if ax {
 		axc = 'x'
 	}

 	return fmt.Sprintf("%c%c%c%c%c%c%c%c%c%c", tc, orc, owc, oxc, grc, gwc, gxc, arc, awc, axc)
 }
	package sftp

	// sftp server counterpart

	import (
	"encoding"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"strconv"
	"sync"
	"syscall"
	"time"

	"github.com/pkg/errors"
	)

	const (
	// SftpServerWorkerCount defines the number of workers for the SFTP server
	SftpServerWorkerCount = 8
	)

	// Server is an SSH File Transfer Protocol (sftp) server.
	// This is intended to provide the sftp subsystem to an ssh server daemon.
	// This implementation currently supports most of sftp server protocol version 3,
	// as specified at http://tools.ietf.org/html/draft-ietf-secsh-filexfer-02
	type Server struct {
	*serverConn
	debugStream io.Writer
	readOnly bool
	pktMgr *packetManager
	openFiles map[string]*os.File
	openFilesLock sync.RWMutex
	handleCount int
	}

	func (svr Server) nextHandle(f os.File) string {
	svr.openFilesLock.Lock()
	defer svr.openFilesLock.Unlock()
	svr.handleCount++
	handle := strconv.Itoa(svr.handleCount)
	svr.openFiles[handle] = f
	return handle
	}

	func (svr *Server) closeHandle(handle string) error {
	svr.openFilesLock.Lock()
	defer svr.openFilesLock.Unlock()
	if f, ok := svr.openFiles[handle]; ok {
	delete(svr.openFiles, handle)
	return f.Close()
	}

	return EBADF
	}

	func (svr Server) getHandle(handle string) (os.File, bool) {
	svr.openFilesLock.RLock()
	defer svr.openFilesLock.RUnlock()
	f, ok := svr.openFiles[handle]
	return f, ok
	}

	type serverRespondablePacket interface {
	encoding.BinaryUnmarshaler
	id() uint32
	respond(svr *Server) responsePacket
	}

	// NewServer creates a new Server instance around the provided streams, serving
	// content from the root of the filesystem. Optionally, ServerOption
	// functions may be specified to further configure the Server.
	//
	// A subsequent call to Serve() is required to begin serving files over SFTP.
	func NewServer(rwc io.ReadWriteCloser, options ...ServerOption) (*Server, error) {
	svrConn := &serverConn{
	conn: conn{
	Reader: rwc,
	WriteCloser: rwc,
	},
	}
	s := &Server{
	serverConn: svrConn,
	debugStream: ioutil.Discard,
	pktMgr: newPktMgr(svrConn),
	openFiles: make(map[string]*os.File),
	}

	for _, o := range options {
	if err := o(s); err != nil {
	return nil, err
	}
	}

	return s, nil
	}

	// A ServerOption is a function which applies configuration to a Server.
	type ServerOption func(*Server) error

	// WithDebug enables Server debugging output to the supplied io.Writer.
	func WithDebug(w io.Writer) ServerOption {
	return func(s *Server) error {
	s.debugStream = w
	return nil
	}
	}

	// ReadOnly configures a Server to serve files in read-only mode.
	func ReadOnly() ServerOption {
	return func(s *Server) error {
	s.readOnly = true
	return nil
	}
	}

	// WithAllocator enable the allocator.
	// After processing a packet we keep in memory the allocated slices
	// and we reuse them for new packets.
	// The allocator is experimental
	func WithAllocator() ServerOption {
	return func(s *Server) error {
	alloc := newAllocator()
	s.pktMgr.alloc = alloc
	s.conn.alloc = alloc
	return nil
	}
	}

	type rxPacket struct {
	pktType fxp
	pktBytes []byte
	}

	// Up to N parallel servers
	func (svr *Server) sftpServerWorker(pktChan chan orderedRequest) error {
	for pkt := range pktChan {
	// readonly checks
	readonly := true
	switch pkt := pkt.requestPacket.(type) {
	case notReadOnly:
	readonly = false
	case *sshFxpOpenPacket:
	readonly = pkt.readonly()
	case *sshFxpExtendedPacket:
	readonly = pkt.readonly()
	}

	// If server is operating read-only and a write operation is requested,
	// return permission denied
	if !readonly && svr.readOnly {
	svr.pktMgr.readyPacket(
	svr.pktMgr.newOrderedResponse(statusFromError(pkt.id(), syscall.EPERM), pkt.orderID()),
	)
	continue
	}

	if err := handlePacket(svr, pkt); err != nil {
	return err
	}
	}
	return nil
	}

	func handlePacket(s *Server, p orderedRequest) error {
	var rpkt responsePacket
	orderID := p.orderID()
	switch p := p.requestPacket.(type) {
	case *sshFxInitPacket:
	rpkt = &sshFxVersionPacket{
	Version: sftpProtocolVersion,
	Extensions: sftpExtensions,
	}
	case *sshFxpStatPacket:
	// stat the requested file
	info, err := os.Stat(p.Path)
	rpkt = &sshFxpStatResponse{
	ID: p.ID,
	info: info,
	}
	if err != nil {
	rpkt = statusFromError(p.ID, err)
	}
	case *sshFxpLstatPacket:
	// stat the requested file
	info, err := os.Lstat(p.Path)
	rpkt = &sshFxpStatResponse{
	ID: p.ID,
	info: info,
	}
	if err != nil {
	rpkt = statusFromError(p.ID, err)
	}
	case *sshFxpFstatPacket:
	f, ok := s.getHandle(p.Handle)
	var err error = EBADF
	var info os.FileInfo
	if ok {
	info, err = f.Stat()
	rpkt = &sshFxpStatResponse{
	ID: p.ID,
	info: info,
	}
	}
	if err != nil {
	rpkt = statusFromError(p.ID, err)
	}
	case *sshFxpMkdirPacket:
	// TODO FIXME: ignore flags field
	err := os.Mkdir(p.Path, 0755)
	rpkt = statusFromError(p.ID, err)
	case *sshFxpRmdirPacket:
	err := os.Remove(p.Path)
	rpkt = statusFromError(p.ID, err)
	case *sshFxpRemovePacket:
	err := os.Remove(p.Filename)
	rpkt = statusFromError(p.ID, err)
	case *sshFxpRenamePacket:
	err := os.Rename(p.Oldpath, p.Newpath)
	rpkt = statusFromError(p.ID, err)
	case *sshFxpSymlinkPacket:
	err := os.Symlink(p.Targetpath, p.Linkpath)
	rpkt = statusFromError(p.ID, err)
	case *sshFxpClosePacket:
	rpkt = statusFromError(p.ID, s.closeHandle(p.Handle))
	case *sshFxpReadlinkPacket:
	f, err := os.Readlink(p.Path)
	rpkt = &sshFxpNamePacket{
	ID: p.ID,
	NameAttrs: []*sshFxpNameAttr{
	{
	Name: f,
	LongName: f,
	Attrs: emptyFileStat,
	},
	},
	}
	if err != nil {
	rpkt = statusFromError(p.ID, err)
	}
	case *sshFxpRealpathPacket:
	f, err := filepath.Abs(p.Path)
	f = cleanPath(f)
	rpkt = &sshFxpNamePacket{
	ID: p.ID,
	NameAttrs: []*sshFxpNameAttr{
	{
	Name: f,
	LongName: f,
	Attrs: emptyFileStat,
	},
	},
	}
	if err != nil {
	rpkt = statusFromError(p.ID, err)
	}
	case *sshFxpOpendirPacket:
	if stat, err := os.Stat(p.Path); err != nil {
	rpkt = statusFromError(p.ID, err)
	} else if !stat.IsDir() {
	rpkt = statusFromError(p.ID, &os.PathError{
	Path: p.Path, Err: syscall.ENOTDIR})
	} else {
	rpkt = (&sshFxpOpenPacket{
	ID: p.ID,
	Path: p.Path,
	Pflags: sshFxfRead,
	}).respond(s)
	}
	case *sshFxpReadPacket:
	var err error = EBADF
	f, ok := s.getHandle(p.Handle)
	if ok {
	err = nil
	data := p.getDataSlice(s.pktMgr.alloc, orderID)
	n, _err := f.ReadAt(data, int64(p.Offset))
	if _err != nil && (_err != io.EOF \|\| n == 0) {
	err = _err
	}
	rpkt = &sshFxpDataPacket{
	ID: p.ID,
	Length: uint32(n),
	Data: data[:n],
	// do not use data[:n:n] here to clamp the capacity, we allocated extra capacity above to avoid reallocations
	}
	}
	if err != nil {
	rpkt = statusFromError(p.ID, err)
	}

	case *sshFxpWritePacket:
	f, ok := s.getHandle(p.Handle)
	var err error = EBADF
	if ok {
	_, err = f.WriteAt(p.Data, int64(p.Offset))
	}
	rpkt = statusFromError(p.ID, err)
	case *sshFxpExtendedPacket:
	if p.SpecificPacket == nil {
	rpkt = statusFromError(p.ID, ErrSSHFxOpUnsupported)
	} else {
	rpkt = p.respond(s)
	}
	case serverRespondablePacket:
	rpkt = p.respond(s)
	default:
	return errors.Errorf("unexpected packet type %T", p)
	}

	s.pktMgr.readyPacket(s.pktMgr.newOrderedResponse(rpkt, orderID))
	return nil
	}

	// Serve serves SFTP connections until the streams stop or the SFTP subsystem
	// is stopped.
	func (svr *Server) Serve() error {
	defer func() {
	if svr.pktMgr.alloc != nil {
	svr.pktMgr.alloc.Free()
	}
	}()
	var wg sync.WaitGroup
	runWorker := func(ch chan orderedRequest) {
	wg.Add(1)
	go func() {
	defer wg.Done()
	if err := svr.sftpServerWorker(ch); err != nil {
	svr.conn.Close() // shuts down recvPacket
	}
	}()
	}
	pktChan := svr.pktMgr.workerChan(runWorker)

	var err error
	var pkt requestPacket
	var pktType uint8
	var pktBytes []byte
	for {
	pktType, pktBytes, err = svr.serverConn.recvPacket(svr.pktMgr.getNextOrderID())
	if err != nil {
	// we don't care about releasing allocated pages here, the server will quit and the allocator freed
	break
	}

	pkt, err = makePacket(rxPacket{fxp(pktType), pktBytes})
	if err != nil {
	switch errors.Cause(err) {
	case errUnknownExtendedPacket:
	//if err := svr.serverConn.sendError(pkt, ErrSshFxOpUnsupported); err != nil {
	// debug("failed to send err packet: %v", err)
	// svr.conn.Close() // shuts down recvPacket
	// break
	//}
	default:
	debug("makePacket err: %v", err)
	svr.conn.Close() // shuts down recvPacket
	break
	}
	}

	pktChan <- svr.pktMgr.newOrderedRequest(pkt)
	}

	close(pktChan) // shuts down sftpServerWorkers
	wg.Wait() // wait for all workers to exit

	// close any still-open files
	for handle, file := range svr.openFiles {
	fmt.Fprintf(svr.debugStream, "sftp server file with handle %q left open: %v\n", handle, file.Name())
	file.Close()
	}
	return err // error from recvPacket
	}

	type ider interface {
	id() uint32
	}

	// The init packet has no ID, so we just return a zero-value ID
	func (p *sshFxInitPacket) id() uint32 { return 0 }

	type sshFxpStatResponse struct {
	ID uint32
	info os.FileInfo
	}

	func (p *sshFxpStatResponse) marshalPacket() ([]byte, []byte, error) {
	l := 4 + 1 + 4 // uint32(length) + byte(type) + uint32(id)

	b := make([]byte, 4, l)
	b = append(b, sshFxpAttrs)
	b = marshalUint32(b, p.ID)

	var payload []byte
	payload = marshalFileInfo(payload, p.info)

	return b, payload, nil
	}

	func (p *sshFxpStatResponse) MarshalBinary() ([]byte, error) {
	header, payload, err := p.marshalPacket()
	return append(header, payload...), err
	}

	var emptyFileStat = []interface{}{uint32(0)}

	func (p *sshFxpOpenPacket) readonly() bool {
	return !p.hasPflags(sshFxfWrite)
	}

	func (p *sshFxpOpenPacket) hasPflags(flags ...uint32) bool {
	for _, f := range flags {
	if p.Pflags&f == 0 {
	return false
	}
	}
	return true
	}

	func (p sshFxpOpenPacket) respond(svr Server) responsePacket {
	var osFlags int
	if p.hasPflags(sshFxfRead, sshFxfWrite) {
	osFlags \|= os.O_RDWR
	} else if p.hasPflags(sshFxfWrite) {
	osFlags \|= os.O_WRONLY
	} else if p.hasPflags(sshFxfRead) {
	osFlags \|= os.O_RDONLY
	} else {
	// how are they opening?
	return statusFromError(p.ID, syscall.EINVAL)
	}

	// Don't use O_APPEND flag as it conflicts with WriteAt.
	// The sshFxfAppend flag is a no-op here as the client sends the offsets.

	if p.hasPflags(sshFxfCreat) {
	osFlags \|= os.O_CREATE
	}
	if p.hasPflags(sshFxfTrunc) {
	osFlags \|= os.O_TRUNC
	}
	if p.hasPflags(sshFxfExcl) {
	osFlags \|= os.O_EXCL
	}

	f, err := os.OpenFile(p.Path, osFlags, 0644)
	if err != nil {
	return statusFromError(p.ID, err)
	}

	handle := svr.nextHandle(f)
	return &sshFxpHandlePacket{ID: p.ID, Handle: handle}
	}

	func (p sshFxpReaddirPacket) respond(svr Server) responsePacket {
	f, ok := svr.getHandle(p.Handle)
	if !ok {
	return statusFromError(p.ID, EBADF)
	}

	dirname := f.Name()
	dirents, err := f.Readdir(128)
	if err != nil {
	return statusFromError(p.ID, err)
	}

	ret := &sshFxpNamePacket{ID: p.ID}
	for _, dirent := range dirents {
	ret.NameAttrs = append(ret.NameAttrs, &sshFxpNameAttr{
	Name: dirent.Name(),
	LongName: runLs(dirname, dirent),
	Attrs: []interface{}{dirent},
	})
	}
	return ret
	}

	func (p sshFxpSetstatPacket) respond(svr Server) responsePacket {
	// additional unmarshalling is required for each possibility here
	b := p.Attrs.([]byte)
	var err error

	debug("setstat name \"%s\"", p.Path)
	if (p.Flags & sshFileXferAttrSize) != 0 {
	var size uint64
	if size, b, err = unmarshalUint64Safe(b); err == nil {
	err = os.Truncate(p.Path, int64(size))
	}
	}
	if (p.Flags & sshFileXferAttrPermissions) != 0 {
	var mode uint32
	if mode, b, err = unmarshalUint32Safe(b); err == nil {
	err = os.Chmod(p.Path, os.FileMode(mode))
	}
	}
	if (p.Flags & sshFileXferAttrACmodTime) != 0 {
	var atime uint32
	var mtime uint32
	if atime, b, err = unmarshalUint32Safe(b); err != nil {
	} else if mtime, b, err = unmarshalUint32Safe(b); err != nil {
	} else {
	atimeT := time.Unix(int64(atime), 0)
	mtimeT := time.Unix(int64(mtime), 0)
	err = os.Chtimes(p.Path, atimeT, mtimeT)
	}
	}
	if (p.Flags & sshFileXferAttrUIDGID) != 0 {
	var uid uint32
	var gid uint32
	if uid, b, err = unmarshalUint32Safe(b); err != nil {
	} else if gid, _, err = unmarshalUint32Safe(b); err != nil {
	} else {
	err = os.Chown(p.Path, int(uid), int(gid))
	}
	}

	return statusFromError(p.ID, err)
	}

	func (p sshFxpFsetstatPacket) respond(svr Server) responsePacket {
	f, ok := svr.getHandle(p.Handle)
	if !ok {
	return statusFromError(p.ID, EBADF)
	}

	// additional unmarshalling is required for each possibility here
	b := p.Attrs.([]byte)
	var err error

	debug("fsetstat name \"%s\"", f.Name())
	if (p.Flags & sshFileXferAttrSize) != 0 {
	var size uint64
	if size, b, err = unmarshalUint64Safe(b); err == nil {
	err = f.Truncate(int64(size))
	}
	}
	if (p.Flags & sshFileXferAttrPermissions) != 0 {
	var mode uint32
	if mode, b, err = unmarshalUint32Safe(b); err == nil {
	err = f.Chmod(os.FileMode(mode))
	}
	}
	if (p.Flags & sshFileXferAttrACmodTime) != 0 {
	var atime uint32
	var mtime uint32
	if atime, b, err = unmarshalUint32Safe(b); err != nil {
	} else if mtime, b, err = unmarshalUint32Safe(b); err != nil {
	} else {
	atimeT := time.Unix(int64(atime), 0)
	mtimeT := time.Unix(int64(mtime), 0)
	err = os.Chtimes(f.Name(), atimeT, mtimeT)
	}
	}
	if (p.Flags & sshFileXferAttrUIDGID) != 0 {
	var uid uint32
	var gid uint32
	if uid, b, err = unmarshalUint32Safe(b); err != nil {
	} else if gid, _, err = unmarshalUint32Safe(b); err != nil {
	} else {
	err = f.Chown(int(uid), int(gid))
	}
	}

	return statusFromError(p.ID, err)
	}

	func statusFromError(id uint32, err error) *sshFxpStatusPacket {
	ret := &sshFxpStatusPacket{
	ID: id,
	StatusError: StatusError{
	// sshFXOk = 0
	// sshFXEOF = 1
	// sshFXNoSuchFile = 2 ENOENT
	// sshFXPermissionDenied = 3
	// sshFXFailure = 4
	// sshFXBadMessage = 5
	// sshFXNoConnection = 6
	// sshFXConnectionLost = 7
	// sshFXOPUnsupported = 8
	Code: sshFxOk,
	},
	}
	if err == nil {
	return ret
	}

	debug("statusFromError: error is %T %#v", err, err)
	ret.StatusError.Code = sshFxFailure
	ret.StatusError.msg = err.Error()

	if os.IsNotExist(err) {
	ret.StatusError.Code = sshFxNoSuchFile
	return ret
	}
	if code, ok := translateSyscallError(err); ok {
	ret.StatusError.Code = code
	return ret
	}

	switch e := err.(type) {
	case fxerr:
	ret.StatusError.Code = uint32(e)
	default:
	if e == io.EOF {
	ret.StatusError.Code = sshFxEOF
	}
	}

	return ret
	}

	func clamp(v, max uint32) uint32 {
	if v > max {
	return max
	}
	return v
	}

	func runLsTypeWord(dirent os.FileInfo) string {
	// find first character, the type char
	// b Block special file.
	// c Character special file.
	// d Directory.
	// l Symbolic link.
	// s Socket link.
	// p FIFO.
	// - Regular file.
	tc := '-'
	mode := dirent.Mode()
	if (mode & os.ModeDir) != 0 {
	tc = 'd'
	} else if (mode & os.ModeDevice) != 0 {
	tc = 'b'
	if (mode & os.ModeCharDevice) != 0 {
	tc = 'c'
	}
	} else if (mode & os.ModeSymlink) != 0 {
	tc = 'l'
	} else if (mode & os.ModeSocket) != 0 {
	tc = 's'
	} else if (mode & os.ModeNamedPipe) != 0 {
	tc = 'p'
	}

	// owner
	orc := '-'
	if (mode & 0400) != 0 {
	orc = 'r'
	}
	owc := '-'
	if (mode & 0200) != 0 {
	owc = 'w'
	}
	oxc := '-'
	ox := (mode & 0100) != 0
	setuid := (mode & os.ModeSetuid) != 0
	if ox && setuid {
	oxc = 's'
	} else if setuid {
	oxc = 'S'
	} else if ox {
	oxc = 'x'
	}

	// group
	grc := '-'
	if (mode & 040) != 0 {
	grc = 'r'
	}
	gwc := '-'
	if (mode & 020) != 0 {
	gwc = 'w'
	}
	gxc := '-'
	gx := (mode & 010) != 0
	setgid := (mode & os.ModeSetgid) != 0
	if gx && setgid {
	gxc = 's'
	} else if setgid {
	gxc = 'S'
	} else if gx {
	gxc = 'x'
	}

	// all / others
	arc := '-'
	if (mode & 04) != 0 {
	arc = 'r'
	}
	awc := '-'
	if (mode & 02) != 0 {
	awc = 'w'
	}
	axc := '-'
	ax := (mode & 01) != 0
	sticky := (mode & os.ModeSticky) != 0
	if ax && sticky {
	axc = 't'
	} else if sticky {
	axc = 'T'
	} else if ax {
	axc = 'x'
	}

	return fmt.Sprintf("%c%c%c%c%c%c%c%c%c%c", tc, orc, owc, oxc, grc, gwc, gxc, arc, awc, axc)
	}