src/archive/tar/writer.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package tar

 // TODO(dsymonds):
 // - catch more errors (no first header, etc.)

 import (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"path"
 	"sort"
 	"strconv"
 	"strings"
 	"time"
 )

 var (
 	ErrWriteTooLong    = errors.New("archive/tar: write too long")
 	ErrFieldTooLong    = errors.New("archive/tar: header field too long")
 	ErrWriteAfterClose = errors.New("archive/tar: write after close")
 	errInvalidHeader   = errors.New("archive/tar: header field too long or contains invalid values")
 )

 // A Writer provides sequential writing of a tar archive in POSIX.1 format.
 // A tar archive consists of a sequence of files.
 // Call WriteHeader to begin a new file, and then call Write to supply that file's data,
 // writing at most hdr.Size bytes in total.
 type Writer struct {
 	w          io.Writer
 	err        error
 	nb         int64 // number of unwritten bytes for current file entry
 	pad        int64 // amount of padding to write after current file entry
 	closed     bool
 	usedBinary bool            // whether the binary numeric field extension was used
 	preferPax  bool            // use pax header instead of binary numeric header
 	hdrBuff    [blockSize]byte // buffer to use in writeHeader when writing a regular header
 	paxHdrBuff [blockSize]byte // buffer to use in writeHeader when writing a pax header
 }

 type formatter struct {
 	err error // Last error seen
 }

 // NewWriter creates a new Writer writing to w.
 func NewWriter(w io.Writer) *Writer { return &Writer{w: w} }

 // Flush finishes writing the current file (optional).
 func (tw *Writer) Flush() error {
 	if tw.nb > 0 {
 		tw.err = fmt.Errorf("archive/tar: missed writing %d bytes", tw.nb)
 		return tw.err
 	}

 	n := tw.nb + tw.pad
 	for n > 0 && tw.err == nil {
 		nr := n
 		if nr > blockSize {
 			nr = blockSize
 		}
 		var nw int
 		nw, tw.err = tw.w.Write(zeroBlock[0:nr])
 		n -= int64(nw)
 	}
 	tw.nb = 0
 	tw.pad = 0
 	return tw.err
 }

 // Write s into b, terminating it with a NUL if there is room.
 func (f *formatter) formatString(b []byte, s string) {
 	if len(s) > len(b) {
 		f.err = ErrFieldTooLong
 		return
 	}
 	ascii := toASCII(s)
 	copy(b, ascii)
 	if len(ascii) < len(b) {
 		b[len(ascii)] = 0
 	}
 }

 // Encode x as an octal ASCII string and write it into b with leading zeros.
 func (f *formatter) formatOctal(b []byte, x int64) {
 	s := strconv.FormatInt(x, 8)
 	// leading zeros, but leave room for a NUL.
 	for len(s)+1 < len(b) {
 		s = "0" + s
 	}
 	f.formatString(b, s)
 }

 // fitsInBase256 reports whether x can be encoded into n bytes using base-256
 // encoding. Unlike octal encoding, base-256 encoding does not require that the
 // string ends with a NUL character. Thus, all n bytes are available for output.
 //
 // If operating in binary mode, this assumes strict GNU binary mode; which means
 // that the first byte can only be either 0x80 or 0xff. Thus, the first byte is
 // equivalent to the sign bit in two's complement form.
 func fitsInBase256(n int, x int64) bool {
 	var binBits = uint(n-1) * 8
 	return n >= 9 || (x >= -1<<binBits && x < 1<<binBits)
 }

 // Write x into b, as binary (GNUtar/star extension).
 func (f *formatter) formatNumeric(b []byte, x int64) {
 	if fitsInBase256(len(b), x) {
 		for i := len(b) - 1; i >= 0; i-- {
 			b[i] = byte(x)
 			x >>= 8
 		}
 		b[0] |= 0x80 // Highest bit indicates binary format
 		return
 	}

 	f.formatOctal(b, 0) // Last resort, just write zero
 	f.err = ErrFieldTooLong
 }

 var (
 	minTime = time.Unix(0, 0)
 	// There is room for 11 octal digits (33 bits) of mtime.
 	maxTime = minTime.Add((1<<33 - 1) * time.Second)
 )

 // WriteHeader writes hdr and prepares to accept the file's contents.
 // WriteHeader calls Flush if it is not the first header.
 // Calling after a Close will return ErrWriteAfterClose.
 func (tw *Writer) WriteHeader(hdr *Header) error {
 	return tw.writeHeader(hdr, true)
 }

 // WriteHeader writes hdr and prepares to accept the file's contents.
 // WriteHeader calls Flush if it is not the first header.
 // Calling after a Close will return ErrWriteAfterClose.
 // As this method is called internally by writePax header to allow it to
 // suppress writing the pax header.
 func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
 	if tw.closed {
 		return ErrWriteAfterClose
 	}
 	if tw.err == nil {
 		tw.Flush()
 	}
 	if tw.err != nil {
 		return tw.err
 	}

 	// a map to hold pax header records, if any are needed
 	paxHeaders := make(map[string]string)

 	// TODO(shanemhansen): we might want to use PAX headers for
 	// subsecond time resolution, but for now let's just capture
 	// too long fields or non ascii characters

 	var f formatter
 	var header []byte

 	// We need to select which scratch buffer to use carefully,
 	// since this method is called recursively to write PAX headers.
 	// If allowPax is true, this is the non-recursive call, and we will use hdrBuff.
 	// If allowPax is false, we are being called by writePAXHeader, and hdrBuff is
 	// already being used by the non-recursive call, so we must use paxHdrBuff.
 	header = tw.hdrBuff[:]
 	if !allowPax {
 		header = tw.paxHdrBuff[:]
 	}
 	copy(header, zeroBlock)
 	s := slicer(header)

 	// Wrappers around formatter that automatically sets paxHeaders if the
 	// argument extends beyond the capacity of the input byte slice.
 	var formatString = func(b []byte, s string, paxKeyword string) {
 		needsPaxHeader := paxKeyword != paxNone && len(s) > len(b) || !isASCII(s)
 		if needsPaxHeader {
 			paxHeaders[paxKeyword] = s
 			return
 		}
 		f.formatString(b, s)
 	}
 	var formatNumeric = func(b []byte, x int64, paxKeyword string) {
 		// Try octal first.
 		s := strconv.FormatInt(x, 8)
 		if len(s) < len(b) {
 			f.formatOctal(b, x)
 			return
 		}

 		// If it is too long for octal, and PAX is preferred, use a PAX header.
 		if paxKeyword != paxNone && tw.preferPax {
 			f.formatOctal(b, 0)
 			s := strconv.FormatInt(x, 10)
 			paxHeaders[paxKeyword] = s
 			return
 		}

 		tw.usedBinary = true
 		f.formatNumeric(b, x)
 	}

 	// keep a reference to the filename to allow to overwrite it later if we detect that we can use ustar longnames instead of pax
 	pathHeaderBytes := s.next(fileNameSize)

 	formatString(pathHeaderBytes, hdr.Name, paxPath)

 	// Handle out of range ModTime carefully.
 	var modTime int64
 	if !hdr.ModTime.Before(minTime) && !hdr.ModTime.After(maxTime) {
 		modTime = hdr.ModTime.Unix()
 	}

 	f.formatOctal(s.next(8), hdr.Mode)               // 100:108
 	formatNumeric(s.next(8), int64(hdr.Uid), paxUid) // 108:116
 	formatNumeric(s.next(8), int64(hdr.Gid), paxGid) // 116:124
 	formatNumeric(s.next(12), hdr.Size, paxSize)     // 124:136
 	formatNumeric(s.next(12), modTime, paxNone)      // 136:148 --- consider using pax for finer granularity
 	s.next(8)                                        // chksum (148:156)
 	s.next(1)[0] = hdr.Typeflag                      // 156:157

 	formatString(s.next(100), hdr.Linkname, paxLinkpath)

 	copy(s.next(8), []byte("ustar\x0000"))          // 257:265
 	formatString(s.next(32), hdr.Uname, paxUname)   // 265:297
 	formatString(s.next(32), hdr.Gname, paxGname)   // 297:329
 	formatNumeric(s.next(8), hdr.Devmajor, paxNone) // 329:337
 	formatNumeric(s.next(8), hdr.Devminor, paxNone) // 337:345

 	// keep a reference to the prefix to allow to overwrite it later if we detect that we can use ustar longnames instead of pax
 	prefixHeaderBytes := s.next(155)
 	formatString(prefixHeaderBytes, "", paxNone) // 345:500  prefix

 	// Use the GNU magic instead of POSIX magic if we used any GNU extensions.
 	if tw.usedBinary {
 		copy(header[257:265], []byte("ustar  \x00"))
 	}

 	_, paxPathUsed := paxHeaders[paxPath]
 	// try to use a ustar header when only the name is too long
 	if !tw.preferPax && len(paxHeaders) == 1 && paxPathUsed {
 		prefix, suffix, ok := splitUSTARPath(hdr.Name)
 		if ok {
 			// Since we can encode in USTAR format, disable PAX header.
 			delete(paxHeaders, paxPath)

 			// Update the path fields
 			formatString(pathHeaderBytes, suffix, paxNone)
 			formatString(prefixHeaderBytes, prefix, paxNone)
 		}
 	}

 	// The chksum field is terminated by a NUL and a space.
 	// This is different from the other octal fields.
 	chksum, _ := checksum(header)
 	f.formatOctal(header[148:155], chksum) // Never fails
 	header[155] = ' '

 	// Check if there were any formatting errors.
 	if f.err != nil {
 		tw.err = f.err
 		return tw.err
 	}

 	if allowPax {
 		for k, v := range hdr.Xattrs {
 			paxHeaders[paxXattr+k] = v
 		}
 	}

 	if len(paxHeaders) > 0 {
 		if !allowPax {
 			return errInvalidHeader
 		}
 		if err := tw.writePAXHeader(hdr, paxHeaders); err != nil {
 			return err
 		}
 	}
 	tw.nb = int64(hdr.Size)
 	tw.pad = (blockSize - (tw.nb % blockSize)) % blockSize

 	_, tw.err = tw.w.Write(header)
 	return tw.err
 }

 // splitUSTARPath splits a path according to USTAR prefix and suffix rules.
 // If the path is not splittable, then it will return ("", "", false).
 func splitUSTARPath(name string) (prefix, suffix string, ok bool) {
 	length := len(name)
 	if length <= fileNameSize || !isASCII(name) {
 		return "", "", false
 	} else if length > fileNamePrefixSize+1 {
 		length = fileNamePrefixSize + 1
 	} else if name[length-1] == '/' {
 		length--
 	}

 	i := strings.LastIndex(name[:length], "/")
 	nlen := len(name) - i - 1 // nlen is length of suffix
 	plen := i                 // plen is length of prefix
 	if i <= 0 || nlen > fileNameSize || nlen == 0 || plen > fileNamePrefixSize {
 		return "", "", false
 	}
 	return name[:i], name[i+1:], true
 }

 // writePaxHeader writes an extended pax header to the
 // archive.
 func (tw *Writer) writePAXHeader(hdr *Header, paxHeaders map[string]string) error {
 	// Prepare extended header
 	ext := new(Header)
 	ext.Typeflag = TypeXHeader
 	// Setting ModTime is required for reader parsing to
 	// succeed, and seems harmless enough.
 	ext.ModTime = hdr.ModTime
 	// The spec asks that we namespace our pseudo files
 	// with the current pid.  However, this results in differing outputs
 	// for identical inputs.  As such, the constant 0 is now used instead.
 	// golang.org/issue/12358
 	dir, file := path.Split(hdr.Name)
 	fullName := path.Join(dir, "PaxHeaders.0", file)

 	ascii := toASCII(fullName)
 	if len(ascii) > 100 {
 		ascii = ascii[:100]
 	}
 	ext.Name = ascii
 	// Construct the body
 	var buf bytes.Buffer

 	// Keys are sorted before writing to body to allow deterministic output.
 	var keys []string
 	for k := range paxHeaders {
 		keys = append(keys, k)
 	}
 	sort.Strings(keys)

 	for _, k := range keys {
 		fmt.Fprint(&buf, formatPAXRecord(k, paxHeaders[k]))
 	}

 	ext.Size = int64(len(buf.Bytes()))
 	if err := tw.writeHeader(ext, false); err != nil {
 		return err
 	}
 	if _, err := tw.Write(buf.Bytes()); err != nil {
 		return err
 	}
 	if err := tw.Flush(); err != nil {
 		return err
 	}
 	return nil
 }

 // formatPAXRecord formats a single PAX record, prefixing it with the
 // appropriate length.
 func formatPAXRecord(k, v string) string {
 	const padding = 3 // Extra padding for ' ', '=', and '\n'
 	size := len(k) + len(v) + padding
 	size += len(strconv.Itoa(size))
 	record := fmt.Sprintf("%d %s=%s\n", size, k, v)

 	// Final adjustment if adding size field increased the record size.
 	if len(record) != size {
 		size = len(record)
 		record = fmt.Sprintf("%d %s=%s\n", size, k, v)
 	}
 	return record
 }

 // Write writes to the current entry in the tar archive.
 // Write returns the error ErrWriteTooLong if more than
 // hdr.Size bytes are written after WriteHeader.
 func (tw *Writer) Write(b []byte) (n int, err error) {
 	if tw.closed {
 		err = ErrWriteAfterClose
 		return
 	}
 	overwrite := false
 	if int64(len(b)) > tw.nb {
 		b = b[0:tw.nb]
 		overwrite = true
 	}
 	n, err = tw.w.Write(b)
 	tw.nb -= int64(n)
 	if err == nil && overwrite {
 		err = ErrWriteTooLong
 		return
 	}
 	tw.err = err
 	return
 }

 // Close closes the tar archive, flushing any unwritten
 // data to the underlying writer.
 func (tw *Writer) Close() error {
 	if tw.err != nil || tw.closed {
 		return tw.err
 	}
 	tw.Flush()
 	tw.closed = true
 	if tw.err != nil {
 		return tw.err
 	}

 	// trailer: two zero blocks
 	for i := 0; i < 2; i++ {
 		_, tw.err = tw.w.Write(zeroBlock)
 		if tw.err != nil {
 			break
 		}
 	}
 	return tw.err
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package tar

	// TODO(dsymonds):
	// - catch more errors (no first header, etc.)

	import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"path"
	"sort"
	"strconv"
	"strings"
	"time"
	)

	var (
	ErrWriteTooLong = errors.New("archive/tar: write too long")
	ErrFieldTooLong = errors.New("archive/tar: header field too long")
	ErrWriteAfterClose = errors.New("archive/tar: write after close")
	errInvalidHeader = errors.New("archive/tar: header field too long or contains invalid values")
	)

	// A Writer provides sequential writing of a tar archive in POSIX.1 format.
	// A tar archive consists of a sequence of files.
	// Call WriteHeader to begin a new file, and then call Write to supply that file's data,
	// writing at most hdr.Size bytes in total.
	type Writer struct {
	w io.Writer
	err error
	nb int64 // number of unwritten bytes for current file entry
	pad int64 // amount of padding to write after current file entry
	closed bool
	usedBinary bool // whether the binary numeric field extension was used
	preferPax bool // use pax header instead of binary numeric header
	hdrBuff [blockSize]byte // buffer to use in writeHeader when writing a regular header
	paxHdrBuff [blockSize]byte // buffer to use in writeHeader when writing a pax header
	}

	type formatter struct {
	err error // Last error seen
	}

	// NewWriter creates a new Writer writing to w.
	func NewWriter(w io.Writer) *Writer { return &Writer{w: w} }

	// Flush finishes writing the current file (optional).
	func (tw *Writer) Flush() error {
	if tw.nb > 0 {
	tw.err = fmt.Errorf("archive/tar: missed writing %d bytes", tw.nb)
	return tw.err
	}

	n := tw.nb + tw.pad
	for n > 0 && tw.err == nil {
	nr := n
	if nr > blockSize {
	nr = blockSize
	}
	var nw int
	nw, tw.err = tw.w.Write(zeroBlock[0:nr])
	n -= int64(nw)
	}
	tw.nb = 0
	tw.pad = 0
	return tw.err
	}

	// Write s into b, terminating it with a NUL if there is room.
	func (f *formatter) formatString(b []byte, s string) {
	if len(s) > len(b) {
	f.err = ErrFieldTooLong
	return
	}
	ascii := toASCII(s)
	copy(b, ascii)
	if len(ascii) < len(b) {
	b[len(ascii)] = 0
	}
	}

	// Encode x as an octal ASCII string and write it into b with leading zeros.
	func (f *formatter) formatOctal(b []byte, x int64) {
	s := strconv.FormatInt(x, 8)
	// leading zeros, but leave room for a NUL.
	for len(s)+1 < len(b) {
	s = "0" + s
	}
	f.formatString(b, s)
	}

	// fitsInBase256 reports whether x can be encoded into n bytes using base-256
	// encoding. Unlike octal encoding, base-256 encoding does not require that the
	// string ends with a NUL character. Thus, all n bytes are available for output.
	//
	// If operating in binary mode, this assumes strict GNU binary mode; which means
	// that the first byte can only be either 0x80 or 0xff. Thus, the first byte is
	// equivalent to the sign bit in two's complement form.
	func fitsInBase256(n int, x int64) bool {
	var binBits = uint(n-1) * 8
	return n >= 9 \|\| (x >= -1<<binBits && x < 1<<binBits)
	}

	// Write x into b, as binary (GNUtar/star extension).
	func (f *formatter) formatNumeric(b []byte, x int64) {
	if fitsInBase256(len(b), x) {
	for i := len(b) - 1; i >= 0; i-- {
	b[i] = byte(x)
	x >>= 8
	}
	b[0] \|= 0x80 // Highest bit indicates binary format
	return
	}

	f.formatOctal(b, 0) // Last resort, just write zero
	f.err = ErrFieldTooLong
	}

	var (
	minTime = time.Unix(0, 0)
	// There is room for 11 octal digits (33 bits) of mtime.
	maxTime = minTime.Add((1<<33 - 1) * time.Second)
	)

	// WriteHeader writes hdr and prepares to accept the file's contents.
	// WriteHeader calls Flush if it is not the first header.
	// Calling after a Close will return ErrWriteAfterClose.
	func (tw Writer) WriteHeader(hdr Header) error {
	return tw.writeHeader(hdr, true)
	}

	// WriteHeader writes hdr and prepares to accept the file's contents.
	// WriteHeader calls Flush if it is not the first header.
	// Calling after a Close will return ErrWriteAfterClose.
	// As this method is called internally by writePax header to allow it to
	// suppress writing the pax header.
	func (tw Writer) writeHeader(hdr Header, allowPax bool) error {
	if tw.closed {
	return ErrWriteAfterClose
	}
	if tw.err == nil {
	tw.Flush()
	}
	if tw.err != nil {
	return tw.err
	}

	// a map to hold pax header records, if any are needed
	paxHeaders := make(map[string]string)

	// TODO(shanemhansen): we might want to use PAX headers for
	// subsecond time resolution, but for now let's just capture
	// too long fields or non ascii characters

	var f formatter
	var header []byte

	// We need to select which scratch buffer to use carefully,
	// since this method is called recursively to write PAX headers.
	// If allowPax is true, this is the non-recursive call, and we will use hdrBuff.
	// If allowPax is false, we are being called by writePAXHeader, and hdrBuff is
	// already being used by the non-recursive call, so we must use paxHdrBuff.
	header = tw.hdrBuff[:]
	if !allowPax {
	header = tw.paxHdrBuff[:]
	}
	copy(header, zeroBlock)
	s := slicer(header)

	// Wrappers around formatter that automatically sets paxHeaders if the
	// argument extends beyond the capacity of the input byte slice.
	var formatString = func(b []byte, s string, paxKeyword string) {
	needsPaxHeader := paxKeyword != paxNone && len(s) > len(b) \|\| !isASCII(s)
	if needsPaxHeader {
	paxHeaders[paxKeyword] = s
	return
	}
	f.formatString(b, s)
	}
	var formatNumeric = func(b []byte, x int64, paxKeyword string) {
	// Try octal first.
	s := strconv.FormatInt(x, 8)
	if len(s) < len(b) {
	f.formatOctal(b, x)
	return
	}

	// If it is too long for octal, and PAX is preferred, use a PAX header.
	if paxKeyword != paxNone && tw.preferPax {
	f.formatOctal(b, 0)
	s := strconv.FormatInt(x, 10)
	paxHeaders[paxKeyword] = s
	return
	}

	tw.usedBinary = true
	f.formatNumeric(b, x)
	}

	// keep a reference to the filename to allow to overwrite it later if we detect that we can use ustar longnames instead of pax
	pathHeaderBytes := s.next(fileNameSize)

	formatString(pathHeaderBytes, hdr.Name, paxPath)

	// Handle out of range ModTime carefully.
	var modTime int64
	if !hdr.ModTime.Before(minTime) && !hdr.ModTime.After(maxTime) {
	modTime = hdr.ModTime.Unix()
	}

	f.formatOctal(s.next(8), hdr.Mode) // 100:108
	formatNumeric(s.next(8), int64(hdr.Uid), paxUid) // 108:116
	formatNumeric(s.next(8), int64(hdr.Gid), paxGid) // 116:124
	formatNumeric(s.next(12), hdr.Size, paxSize) // 124:136
	formatNumeric(s.next(12), modTime, paxNone) // 136:148 --- consider using pax for finer granularity
	s.next(8) // chksum (148:156)
	s.next(1)[0] = hdr.Typeflag // 156:157

	formatString(s.next(100), hdr.Linkname, paxLinkpath)

	copy(s.next(8), []byte("ustar\x0000")) // 257:265
	formatString(s.next(32), hdr.Uname, paxUname) // 265:297
	formatString(s.next(32), hdr.Gname, paxGname) // 297:329
	formatNumeric(s.next(8), hdr.Devmajor, paxNone) // 329:337
	formatNumeric(s.next(8), hdr.Devminor, paxNone) // 337:345

	// keep a reference to the prefix to allow to overwrite it later if we detect that we can use ustar longnames instead of pax
	prefixHeaderBytes := s.next(155)
	formatString(prefixHeaderBytes, "", paxNone) // 345:500 prefix

	// Use the GNU magic instead of POSIX magic if we used any GNU extensions.
	if tw.usedBinary {
	copy(header[257:265], []byte("ustar \x00"))
	}

	_, paxPathUsed := paxHeaders[paxPath]
	// try to use a ustar header when only the name is too long
	if !tw.preferPax && len(paxHeaders) == 1 && paxPathUsed {
	prefix, suffix, ok := splitUSTARPath(hdr.Name)
	if ok {
	// Since we can encode in USTAR format, disable PAX header.
	delete(paxHeaders, paxPath)

	// Update the path fields
	formatString(pathHeaderBytes, suffix, paxNone)
	formatString(prefixHeaderBytes, prefix, paxNone)
	}
	}

	// The chksum field is terminated by a NUL and a space.
	// This is different from the other octal fields.
	chksum, _ := checksum(header)
	f.formatOctal(header[148:155], chksum) // Never fails
	header[155] = ' '

	// Check if there were any formatting errors.
	if f.err != nil {
	tw.err = f.err
	return tw.err
	}

	if allowPax {
	for k, v := range hdr.Xattrs {
	paxHeaders[paxXattr+k] = v
	}
	}

	if len(paxHeaders) > 0 {
	if !allowPax {
	return errInvalidHeader
	}
	if err := tw.writePAXHeader(hdr, paxHeaders); err != nil {
	return err
	}
	}
	tw.nb = int64(hdr.Size)
	tw.pad = (blockSize - (tw.nb % blockSize)) % blockSize

	_, tw.err = tw.w.Write(header)
	return tw.err
	}

	// splitUSTARPath splits a path according to USTAR prefix and suffix rules.
	// If the path is not splittable, then it will return ("", "", false).
	func splitUSTARPath(name string) (prefix, suffix string, ok bool) {
	length := len(name)
	if length <= fileNameSize \|\| !isASCII(name) {
	return "", "", false
	} else if length > fileNamePrefixSize+1 {
	length = fileNamePrefixSize + 1
	} else if name[length-1] == '/' {
	length--
	}

	i := strings.LastIndex(name[:length], "/")
	nlen := len(name) - i - 1 // nlen is length of suffix
	plen := i // plen is length of prefix
	if i <= 0 \|\| nlen > fileNameSize \|\| nlen == 0 \|\| plen > fileNamePrefixSize {
	return "", "", false
	}
	return name[:i], name[i+1:], true
	}

	// writePaxHeader writes an extended pax header to the
	// archive.
	func (tw Writer) writePAXHeader(hdr Header, paxHeaders map[string]string) error {
	// Prepare extended header
	ext := new(Header)
	ext.Typeflag = TypeXHeader
	// Setting ModTime is required for reader parsing to
	// succeed, and seems harmless enough.
	ext.ModTime = hdr.ModTime
	// The spec asks that we namespace our pseudo files
	// with the current pid. However, this results in differing outputs
	// for identical inputs. As such, the constant 0 is now used instead.
	// golang.org/issue/12358
	dir, file := path.Split(hdr.Name)
	fullName := path.Join(dir, "PaxHeaders.0", file)

	ascii := toASCII(fullName)
	if len(ascii) > 100 {
	ascii = ascii[:100]
	}
	ext.Name = ascii
	// Construct the body
	var buf bytes.Buffer

	// Keys are sorted before writing to body to allow deterministic output.
	var keys []string
	for k := range paxHeaders {
	keys = append(keys, k)
	}
	sort.Strings(keys)

	for _, k := range keys {
	fmt.Fprint(&buf, formatPAXRecord(k, paxHeaders[k]))
	}

	ext.Size = int64(len(buf.Bytes()))
	if err := tw.writeHeader(ext, false); err != nil {
	return err
	}
	if _, err := tw.Write(buf.Bytes()); err != nil {
	return err
	}
	if err := tw.Flush(); err != nil {
	return err
	}
	return nil
	}

	// formatPAXRecord formats a single PAX record, prefixing it with the
	// appropriate length.
	func formatPAXRecord(k, v string) string {
	const padding = 3 // Extra padding for ' ', '=', and '\n'
	size := len(k) + len(v) + padding
	size += len(strconv.Itoa(size))
	record := fmt.Sprintf("%d %s=%s\n", size, k, v)

	// Final adjustment if adding size field increased the record size.
	if len(record) != size {
	size = len(record)
	record = fmt.Sprintf("%d %s=%s\n", size, k, v)
	}
	return record
	}

	// Write writes to the current entry in the tar archive.
	// Write returns the error ErrWriteTooLong if more than
	// hdr.Size bytes are written after WriteHeader.
	func (tw *Writer) Write(b []byte) (n int, err error) {
	if tw.closed {
	err = ErrWriteAfterClose
	return
	}
	overwrite := false
	if int64(len(b)) > tw.nb {
	b = b[0:tw.nb]
	overwrite = true
	}
	n, err = tw.w.Write(b)
	tw.nb -= int64(n)
	if err == nil && overwrite {
	err = ErrWriteTooLong
	return
	}
	tw.err = err
	return
	}

	// Close closes the tar archive, flushing any unwritten
	// data to the underlying writer.
	func (tw *Writer) Close() error {
	if tw.err != nil \|\| tw.closed {
	return tw.err
	}
	tw.Flush()
	tw.closed = true
	if tw.err != nil {
	return tw.err
	}

	// trailer: two zero blocks
	for i := 0; i < 2; i++ {
	_, tw.err = tw.w.Write(zeroBlock)
	if tw.err != nil {
	break
	}
	}
	return tw.err
	}