writer.go

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package zip
     6  
     7  import (
     8  	"bufio"
     9  	"encoding/binary"
    10  	"errors"
    11  	"hash"
    12  	"hash/crc32"
    13  	"io"
    14  	"io/fs"
    15  	"strings"
    16  	"unicode/utf8"
    17  )
    18  
    19  var (
    20  	errLongName  = errors.New("zip: FileHeader.Name too long")
    21  	errLongExtra = errors.New("zip: FileHeader.Extra too long")
    22  )
    23  
    24  // Writer implements a zip file writer.
    25  type Writer struct {
    26  	cw          *countWriter
    27  	dir         []*header
    28  	last        *fileWriter
    29  	closed      bool
    30  	compressors map[uint16]Compressor
    31  	comment     string
    32  
    33  	// testHookCloseSizeOffset if non-nil is called with the size
    34  	// of offset of the central directory at Close.
    35  	testHookCloseSizeOffset func(size, offset uint64)
    36  }
    37  
    38  type header struct {
    39  	*FileHeader
    40  	offset uint64
    41  	raw    bool
    42  }
    43  
    44  // NewWriter returns a new [Writer] writing a zip file to w.
    45  func NewWriter(w io.Writer) *Writer {
    46  	return &Writer{cw: &countWriter{w: bufio.NewWriter(w)}}
    47  }
    48  
    49  // SetOffset sets the offset of the beginning of the zip data within the
    50  // underlying writer. It should be used when the zip data is appended to an
    51  // existing file, such as a binary executable.
    52  // It must be called before any data is written.
    53  func (w *Writer) SetOffset(n int64) {
    54  	if w.cw.count != 0 {
    55  		panic("zip: SetOffset called after data was written")
    56  	}
    57  	w.cw.count = n
    58  }
    59  
    60  // Flush flushes any buffered data to the underlying writer.
    61  // Calling Flush is not normally necessary; calling Close is sufficient.
    62  func (w *Writer) Flush() error {
    63  	return w.cw.w.(*bufio.Writer).Flush()
    64  }
    65  
    66  // SetComment sets the end-of-central-directory comment field.
    67  // It can only be called before [Writer.Close].
    68  func (w *Writer) SetComment(comment string) error {
    69  	if len(comment) > uint16max {
    70  		return errors.New("zip: Writer.Comment too long")
    71  	}
    72  	w.comment = comment
    73  	return nil
    74  }
    75  
    76  // Close finishes writing the zip file by writing the central directory.
    77  // It does not close the underlying writer.
    78  func (w *Writer) Close() error {
    79  	if w.last != nil && !w.last.closed {
    80  		if err := w.last.close(); err != nil {
    81  			return err
    82  		}
    83  		w.last = nil
    84  	}
    85  	if w.closed {
    86  		return errors.New("zip: writer closed twice")
    87  	}
    88  	w.closed = true
    89  
    90  	// write central directory
    91  	start := w.cw.count
    92  	for _, h := range w.dir {
    93  		var buf [directoryHeaderLen]byte
    94  		b := writeBuf(buf[:])
    95  		b.uint32(uint32(directoryHeaderSignature))
    96  		b.uint16(h.CreatorVersion)
    97  		b.uint16(h.ReaderVersion)
    98  		b.uint16(h.Flags)
    99  		b.uint16(h.Method)
   100  		b.uint16(h.ModifiedTime)
   101  		b.uint16(h.ModifiedDate)
   102  		b.uint32(h.CRC32)
   103  		if h.isZip64() || h.offset >= uint32max {
   104  			// the file needs a zip64 header. store maxint in both
   105  			// 32 bit size fields (and offset later) to signal that the
   106  			// zip64 extra header should be used.
   107  			b.uint32(uint32max) // compressed size
   108  			b.uint32(uint32max) // uncompressed size
   109  
   110  			// append a zip64 extra block to Extra
   111  			var buf [28]byte // 2x uint16 + 3x uint64
   112  			eb := writeBuf(buf[:])
   113  			eb.uint16(zip64ExtraID)
   114  			eb.uint16(24) // size = 3x uint64
   115  			eb.uint64(h.UncompressedSize64)
   116  			eb.uint64(h.CompressedSize64)
   117  			eb.uint64(h.offset)
   118  			h.Extra = append(h.Extra, buf[:]...)
   119  		} else {
   120  			b.uint32(h.CompressedSize)
   121  			b.uint32(h.UncompressedSize)
   122  		}
   123  
   124  		b.uint16(uint16(len(h.Name)))
   125  		b.uint16(uint16(len(h.Extra)))
   126  		b.uint16(uint16(len(h.Comment)))
   127  		b = b[4:] // skip disk number start and internal file attr (2x uint16)
   128  		b.uint32(h.ExternalAttrs)
   129  		if h.offset > uint32max {
   130  			b.uint32(uint32max)
   131  		} else {
   132  			b.uint32(uint32(h.offset))
   133  		}
   134  		if _, err := w.cw.Write(buf[:]); err != nil {
   135  			return err
   136  		}
   137  		if _, err := io.WriteString(w.cw, h.Name); err != nil {
   138  			return err
   139  		}
   140  		if _, err := w.cw.Write(h.Extra); err != nil {
   141  			return err
   142  		}
   143  		if _, err := io.WriteString(w.cw, h.Comment); err != nil {
   144  			return err
   145  		}
   146  	}
   147  	end := w.cw.count
   148  
   149  	records := uint64(len(w.dir))
   150  	size := uint64(end - start)
   151  	offset := uint64(start)
   152  
   153  	if f := w.testHookCloseSizeOffset; f != nil {
   154  		f(size, offset)
   155  	}
   156  
   157  	if records >= uint16max || size >= uint32max || offset >= uint32max {
   158  		var buf [directory64EndLen + directory64LocLen]byte
   159  		b := writeBuf(buf[:])
   160  
   161  		// zip64 end of central directory record
   162  		b.uint32(directory64EndSignature)
   163  		b.uint64(directory64EndLen - 12) // length minus signature (uint32) and length fields (uint64)
   164  		b.uint16(zipVersion45)           // version made by
   165  		b.uint16(zipVersion45)           // version needed to extract
   166  		b.uint32(0)                      // number of this disk
   167  		b.uint32(0)                      // number of the disk with the start of the central directory
   168  		b.uint64(records)                // total number of entries in the central directory on this disk
   169  		b.uint64(records)                // total number of entries in the central directory
   170  		b.uint64(size)                   // size of the central directory
   171  		b.uint64(offset)                 // offset of start of central directory with respect to the starting disk number
   172  
   173  		// zip64 end of central directory locator
   174  		b.uint32(directory64LocSignature)
   175  		b.uint32(0)           // number of the disk with the start of the zip64 end of central directory
   176  		b.uint64(uint64(end)) // relative offset of the zip64 end of central directory record
   177  		b.uint32(1)           // total number of disks
   178  
   179  		if _, err := w.cw.Write(buf[:]); err != nil {
   180  			return err
   181  		}
   182  
   183  		// store max values in the regular end record to signal
   184  		// that the zip64 values should be used instead
   185  		records = uint16max
   186  		size = uint32max
   187  		offset = uint32max
   188  	}
   189  
   190  	// write end record
   191  	var buf [directoryEndLen]byte
   192  	b := writeBuf(buf[:])
   193  	b.uint32(uint32(directoryEndSignature))
   194  	b = b[4:]                        // skip over disk number and first disk number (2x uint16)
   195  	b.uint16(uint16(records))        // number of entries this disk
   196  	b.uint16(uint16(records))        // number of entries total
   197  	b.uint32(uint32(size))           // size of directory
   198  	b.uint32(uint32(offset))         // start of directory
   199  	b.uint16(uint16(len(w.comment))) // byte size of EOCD comment
   200  	if _, err := w.cw.Write(buf[:]); err != nil {
   201  		return err
   202  	}
   203  	if _, err := io.WriteString(w.cw, w.comment); err != nil {
   204  		return err
   205  	}
   206  
   207  	return w.cw.w.(*bufio.Writer).Flush()
   208  }
   209  
   210  // Create adds a file to the zip file using the provided name.
   211  // It returns a [Writer] to which the file contents should be written.
   212  // The file contents will be compressed using the [Deflate] method.
   213  // The name must be a relative path: it must not start with a drive
   214  // letter (e.g. C:) or leading slash, and only forward slashes are
   215  // allowed. To create a directory instead of a file, add a trailing
   216  // slash to the name. Duplicate names will not overwrite previous entries
   217  // and are appended to the zip file.
   218  // The file's contents must be written to the [io.Writer] before the next
   219  // call to [Writer.Create], [Writer.CreateHeader], or [Writer.Close].
   220  func (w *Writer) Create(name string) (io.Writer, error) {
   221  	header := &FileHeader{
   222  		Name:   name,
   223  		Method: Deflate,
   224  	}
   225  	return w.CreateHeader(header)
   226  }
   227  
   228  // detectUTF8 reports whether s is a valid UTF-8 string, and whether the string
   229  // must be considered UTF-8 encoding (i.e., not compatible with CP-437, ASCII,
   230  // or any other common encoding).
   231  func detectUTF8(s string) (valid, require bool) {
   232  	for i := 0; i < len(s); {
   233  		r, size := utf8.DecodeRuneInString(s[i:])
   234  		i += size
   235  		// Officially, ZIP uses CP-437, but many readers use the system's
   236  		// local character encoding. Most encoding are compatible with a large
   237  		// subset of CP-437, which itself is ASCII-like.
   238  		//
   239  		// Forbid 0x7e and 0x5c since EUC-KR and Shift-JIS replace those
   240  		// characters with localized currency and overline characters.
   241  		if r < 0x20 || r > 0x7d || r == 0x5c {
   242  			if !utf8.ValidRune(r) || (r == utf8.RuneError && size == 1) {
   243  				return false, false
   244  			}
   245  			require = true
   246  		}
   247  	}
   248  	return true, require
   249  }
   250  
   251  // prepare performs the bookkeeping operations required at the start of
   252  // CreateHeader and CreateRaw.
   253  func (w *Writer) prepare(fh *FileHeader) error {
   254  	if w.last != nil && !w.last.closed {
   255  		if err := w.last.close(); err != nil {
   256  			return err
   257  		}
   258  	}
   259  	if len(w.dir) > 0 && w.dir[len(w.dir)-1].FileHeader == fh {
   260  		// See https://golang.org/issue/11144 confusion.
   261  		return errors.New("archive/zip: invalid duplicate FileHeader")
   262  	}
   263  	return nil
   264  }
   265  
   266  // CreateHeader adds a file to the zip archive using the provided [FileHeader]
   267  // for the file metadata. [Writer] takes ownership of fh and may mutate
   268  // its fields. The caller must not modify fh after calling [Writer.CreateHeader].
   269  //
   270  // This returns a [Writer] to which the file contents should be written.
   271  // The file's contents must be written to the io.Writer before the next
   272  // call to [Writer.Create], [Writer.CreateHeader], [Writer.CreateRaw], or [Writer.Close].
   273  func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
   274  	if err := w.prepare(fh); err != nil {
   275  		return nil, err
   276  	}
   277  
   278  	// The ZIP format has a sad state of affairs regarding character encoding.
   279  	// Officially, the name and comment fields are supposed to be encoded
   280  	// in CP-437 (which is mostly compatible with ASCII), unless the UTF-8
   281  	// flag bit is set. However, there are several problems:
   282  	//
   283  	//	* Many ZIP readers still do not support UTF-8.
   284  	//	* If the UTF-8 flag is cleared, several readers simply interpret the
   285  	//	name and comment fields as whatever the local system encoding is.
   286  	//
   287  	// In order to avoid breaking readers without UTF-8 support,
   288  	// we avoid setting the UTF-8 flag if the strings are CP-437 compatible.
   289  	// However, if the strings require multibyte UTF-8 encoding and is a
   290  	// valid UTF-8 string, then we set the UTF-8 bit.
   291  	//
   292  	// For the case, where the user explicitly wants to specify the encoding
   293  	// as UTF-8, they will need to set the flag bit themselves.
   294  	utf8Valid1, utf8Require1 := detectUTF8(fh.Name)
   295  	utf8Valid2, utf8Require2 := detectUTF8(fh.Comment)
   296  	switch {
   297  	case fh.NonUTF8:
   298  		fh.Flags &^= 0x800
   299  	case (utf8Require1 || utf8Require2) && (utf8Valid1 && utf8Valid2):
   300  		fh.Flags |= 0x800
   301  	}
   302  
   303  	fh.CreatorVersion = fh.CreatorVersion&0xff00 | zipVersion20 // preserve compatibility byte
   304  	fh.ReaderVersion = zipVersion20
   305  
   306  	// If Modified is set, this takes precedence over MS-DOS timestamp fields.
   307  	if !fh.Modified.IsZero() {
   308  		// Contrary to the FileHeader.SetModTime method, we intentionally
   309  		// do not convert to UTC, because we assume the user intends to encode
   310  		// the date using the specified timezone. A user may want this control
   311  		// because many legacy ZIP readers interpret the timestamp according
   312  		// to the local timezone.
   313  		//
   314  		// The timezone is only non-UTC if a user directly sets the Modified
   315  		// field directly themselves. All other approaches sets UTC.
   316  		fh.ModifiedDate, fh.ModifiedTime = timeToMsDosTime(fh.Modified)
   317  
   318  		// Use "extended timestamp" format since this is what Info-ZIP uses.
   319  		// Nearly every major ZIP implementation uses a different format,
   320  		// but at least most seem to be able to understand the other formats.
   321  		//
   322  		// This format happens to be identical for both local and central header
   323  		// if modification time is the only timestamp being encoded.
   324  		var mbuf [9]byte // 2*SizeOf(uint16) + SizeOf(uint8) + SizeOf(uint32)
   325  		mt := uint32(fh.Modified.Unix())
   326  		eb := writeBuf(mbuf[:])
   327  		eb.uint16(extTimeExtraID)
   328  		eb.uint16(5)  // Size: SizeOf(uint8) + SizeOf(uint32)
   329  		eb.uint8(1)   // Flags: ModTime
   330  		eb.uint32(mt) // ModTime
   331  		fh.Extra = append(fh.Extra, mbuf[:]...)
   332  	}
   333  
   334  	var (
   335  		ow io.Writer
   336  		fw *fileWriter
   337  	)
   338  	h := &header{
   339  		FileHeader: fh,
   340  		offset:     uint64(w.cw.count),
   341  	}
   342  
   343  	if strings.HasSuffix(fh.Name, "/") {
   344  		// Set the compression method to Store to ensure data length is truly zero,
   345  		// which the writeHeader method always encodes for the size fields.
   346  		// This is necessary as most compression formats have non-zero lengths
   347  		// even when compressing an empty string.
   348  		fh.Method = Store
   349  		fh.Flags &^= 0x8 // we will not write a data descriptor
   350  
   351  		// Explicitly clear sizes as they have no meaning for directories.
   352  		fh.CompressedSize = 0
   353  		fh.CompressedSize64 = 0
   354  		fh.UncompressedSize = 0
   355  		fh.UncompressedSize64 = 0
   356  
   357  		ow = dirWriter{}
   358  	} else {
   359  		fh.Flags |= 0x8 // we will write a data descriptor
   360  
   361  		fw = &fileWriter{
   362  			zipw:      w.cw,
   363  			compCount: &countWriter{w: w.cw},
   364  			crc32:     crc32.NewIEEE(),
   365  		}
   366  		comp := w.compressor(fh.Method)
   367  		if comp == nil {
   368  			return nil, ErrAlgorithm
   369  		}
   370  		var err error
   371  		fw.comp, err = comp(fw.compCount)
   372  		if err != nil {
   373  			return nil, err
   374  		}
   375  		fw.rawCount = &countWriter{w: fw.comp}
   376  		fw.header = h
   377  		ow = fw
   378  	}
   379  	w.dir = append(w.dir, h)
   380  	if err := writeHeader(w.cw, h); err != nil {
   381  		return nil, err
   382  	}
   383  	// If we're creating a directory, fw is nil.
   384  	w.last = fw
   385  	return ow, nil
   386  }
   387  
   388  func writeHeader(w io.Writer, h *header) error {
   389  	const maxUint16 = 1<<16 - 1
   390  	if len(h.Name) > maxUint16 {
   391  		return errLongName
   392  	}
   393  	if len(h.Extra) > maxUint16 {
   394  		return errLongExtra
   395  	}
   396  
   397  	var buf [fileHeaderLen]byte
   398  	b := writeBuf(buf[:])
   399  	b.uint32(uint32(fileHeaderSignature))
   400  	b.uint16(h.ReaderVersion)
   401  	b.uint16(h.Flags)
   402  	b.uint16(h.Method)
   403  	b.uint16(h.ModifiedTime)
   404  	b.uint16(h.ModifiedDate)
   405  	// In raw mode (caller does the compression), the values are either
   406  	// written here or in the trailing data descriptor based on the header
   407  	// flags.
   408  	if h.raw && !h.hasDataDescriptor() {
   409  		b.uint32(h.CRC32)
   410  		b.uint32(uint32(min(h.CompressedSize64, uint32max)))
   411  		b.uint32(uint32(min(h.UncompressedSize64, uint32max)))
   412  	} else {
   413  		// When this package handle the compression, these values are
   414  		// always written to the trailing data descriptor.
   415  		b.uint32(0) // crc32
   416  		b.uint32(0) // compressed size
   417  		b.uint32(0) // uncompressed size
   418  	}
   419  	b.uint16(uint16(len(h.Name)))
   420  	b.uint16(uint16(len(h.Extra)))
   421  	if _, err := w.Write(buf[:]); err != nil {
   422  		return err
   423  	}
   424  	if _, err := io.WriteString(w, h.Name); err != nil {
   425  		return err
   426  	}
   427  	_, err := w.Write(h.Extra)
   428  	return err
   429  }
   430  
   431  // CreateRaw adds a file to the zip archive using the provided [FileHeader] and
   432  // returns a [Writer] to which the file contents should be written. The file's
   433  // contents must be written to the io.Writer before the next call to [Writer.Create],
   434  // [Writer.CreateHeader], [Writer.CreateRaw], or [Writer.Close].
   435  //
   436  // In contrast to [Writer.CreateHeader], the bytes passed to Writer are not compressed.
   437  //
   438  // CreateRaw's argument is stored in w. If the argument is a pointer to the embedded
   439  // [FileHeader] in a [File] obtained from a [Reader] created from in-memory data,
   440  // then w will refer to all of that memory.
   441  func (w *Writer) CreateRaw(fh *FileHeader) (io.Writer, error) {
   442  	if err := w.prepare(fh); err != nil {
   443  		return nil, err
   444  	}
   445  
   446  	fh.CompressedSize = uint32(min(fh.CompressedSize64, uint32max))
   447  	fh.UncompressedSize = uint32(min(fh.UncompressedSize64, uint32max))
   448  
   449  	h := &header{
   450  		FileHeader: fh,
   451  		offset:     uint64(w.cw.count),
   452  		raw:        true,
   453  	}
   454  	w.dir = append(w.dir, h)
   455  	if err := writeHeader(w.cw, h); err != nil {
   456  		return nil, err
   457  	}
   458  
   459  	if strings.HasSuffix(fh.Name, "/") {
   460  		w.last = nil
   461  		return dirWriter{}, nil
   462  	}
   463  
   464  	fw := &fileWriter{
   465  		header: h,
   466  		zipw:   w.cw,
   467  	}
   468  	w.last = fw
   469  	return fw, nil
   470  }
   471  
   472  // Copy copies the file f (obtained from a [Reader]) into w. It copies the raw
   473  // form directly bypassing decompression, compression, and validation.
   474  func (w *Writer) Copy(f *File) error {
   475  	r, err := f.OpenRaw()
   476  	if err != nil {
   477  		return err
   478  	}
   479  	// Copy the FileHeader so w doesn't store a pointer to the data
   480  	// of f's entire archive. See #65499.
   481  	fh := f.FileHeader
   482  	fw, err := w.CreateRaw(&fh)
   483  	if err != nil {
   484  		return err
   485  	}
   486  	_, err = io.Copy(fw, r)
   487  	return err
   488  }
   489  
   490  // RegisterCompressor registers or overrides a custom compressor for a specific
   491  // method ID. If a compressor for a given method is not found, [Writer] will
   492  // default to looking up the compressor at the package level.
   493  func (w *Writer) RegisterCompressor(method uint16, comp Compressor) {
   494  	if w.compressors == nil {
   495  		w.compressors = make(map[uint16]Compressor)
   496  	}
   497  	w.compressors[method] = comp
   498  }
   499  
   500  // AddFS adds the files from fs.FS to the archive.
   501  // It walks the directory tree starting at the root of the filesystem
   502  // adding each file to the zip using deflate while maintaining the directory structure.
   503  func (w *Writer) AddFS(fsys fs.FS) error {
   504  	return fs.WalkDir(fsys, ".", func(name string, d fs.DirEntry, err error) error {
   505  		if err != nil {
   506  			return err
   507  		}
   508  		if name == "." {
   509  			return nil
   510  		}
   511  		info, err := d.Info()
   512  		if err != nil {
   513  			return err
   514  		}
   515  		if !d.IsDir() && !info.Mode().IsRegular() {
   516  			return errors.New("zip: cannot add non-regular file")
   517  		}
   518  		h, err := FileInfoHeader(info)
   519  		if err != nil {
   520  			return err
   521  		}
   522  		h.Name = name
   523  		h.Method = Deflate
   524  		fw, err := w.CreateHeader(h)
   525  		if err != nil {
   526  			return err
   527  		}
   528  		if d.IsDir() {
   529  			return nil
   530  		}
   531  		f, err := fsys.Open(name)
   532  		if err != nil {
   533  			return err
   534  		}
   535  		defer f.Close()
   536  		_, err = io.Copy(fw, f)
   537  		return err
   538  	})
   539  }
   540  
   541  func (w *Writer) compressor(method uint16) Compressor {
   542  	comp := w.compressors[method]
   543  	if comp == nil {
   544  		comp = compressor(method)
   545  	}
   546  	return comp
   547  }
   548  
   549  type dirWriter struct{}
   550  
   551  func (dirWriter) Write(b []byte) (int, error) {
   552  	if len(b) == 0 {
   553  		return 0, nil
   554  	}
   555  	return 0, errors.New("zip: write to directory")
   556  }
   557  
   558  type fileWriter struct {
   559  	*header
   560  	zipw      io.Writer
   561  	rawCount  *countWriter
   562  	comp      io.WriteCloser
   563  	compCount *countWriter
   564  	crc32     hash.Hash32
   565  	closed    bool
   566  }
   567  
   568  func (w *fileWriter) Write(p []byte) (int, error) {
   569  	if w.closed {
   570  		return 0, errors.New("zip: write to closed file")
   571  	}
   572  	if w.raw {
   573  		return w.zipw.Write(p)
   574  	}
   575  	w.crc32.Write(p)
   576  	return w.rawCount.Write(p)
   577  }
   578  
   579  func (w *fileWriter) close() error {
   580  	if w.closed {
   581  		return errors.New("zip: file closed twice")
   582  	}
   583  	w.closed = true
   584  	if w.raw {
   585  		return w.writeDataDescriptor()
   586  	}
   587  	if err := w.comp.Close(); err != nil {
   588  		return err
   589  	}
   590  
   591  	// update FileHeader
   592  	fh := w.header.FileHeader
   593  	fh.CRC32 = w.crc32.Sum32()
   594  	fh.CompressedSize64 = uint64(w.compCount.count)
   595  	fh.UncompressedSize64 = uint64(w.rawCount.count)
   596  
   597  	if fh.isZip64() {
   598  		fh.CompressedSize = uint32max
   599  		fh.UncompressedSize = uint32max
   600  		fh.ReaderVersion = zipVersion45 // requires 4.5 - File uses ZIP64 format extensions
   601  	} else {
   602  		fh.CompressedSize = uint32(fh.CompressedSize64)
   603  		fh.UncompressedSize = uint32(fh.UncompressedSize64)
   604  	}
   605  
   606  	return w.writeDataDescriptor()
   607  }
   608  
   609  func (w *fileWriter) writeDataDescriptor() error {
   610  	if !w.hasDataDescriptor() {
   611  		return nil
   612  	}
   613  	// Write data descriptor. This is more complicated than one would
   614  	// think, see e.g. comments in zipfile.c:putextended() and
   615  	// https://bugs.openjdk.org/browse/JDK-7073588.
   616  	// The approach here is to write 8 byte sizes if needed without
   617  	// adding a zip64 extra in the local header (too late anyway).
   618  	var buf []byte
   619  	if w.isZip64() {
   620  		buf = make([]byte, dataDescriptor64Len)
   621  	} else {
   622  		buf = make([]byte, dataDescriptorLen)
   623  	}
   624  	b := writeBuf(buf)
   625  	b.uint32(dataDescriptorSignature) // de-facto standard, required by OS X
   626  	b.uint32(w.CRC32)
   627  	if w.isZip64() {
   628  		b.uint64(w.CompressedSize64)
   629  		b.uint64(w.UncompressedSize64)
   630  	} else {
   631  		b.uint32(w.CompressedSize)
   632  		b.uint32(w.UncompressedSize)
   633  	}
   634  	_, err := w.zipw.Write(buf)
   635  	return err
   636  }
   637  
   638  type countWriter struct {
   639  	w     io.Writer
   640  	count int64
   641  }
   642  
   643  func (w *countWriter) Write(p []byte) (int, error) {
   644  	n, err := w.w.Write(p)
   645  	w.count += int64(n)
   646  	return n, err
   647  }
   648  
   649  type nopCloser struct {
   650  	io.Writer
   651  }
   652  
   653  func (w nopCloser) Close() error {
   654  	return nil
   655  }
   656  
   657  type writeBuf []byte
   658  
   659  func (b *writeBuf) uint8(v uint8) {
   660  	(*b)[0] = v
   661  	*b = (*b)[1:]
   662  }
   663  
   664  func (b *writeBuf) uint16(v uint16) {
   665  	binary.LittleEndian.PutUint16(*b, v)
   666  	*b = (*b)[2:]
   667  }
   668  
   669  func (b *writeBuf) uint32(v uint32) {
   670  	binary.LittleEndian.PutUint32(*b, v)
   671  	*b = (*b)[4:]
   672  }
   673  
   674  func (b *writeBuf) uint64(v uint64) {
   675  	binary.LittleEndian.PutUint64(*b, v)
   676  	*b = (*b)[8:]
   677  }
   678
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