text: // 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
  }
  