gunzip.go

Documentation: compress/gzip

		 1  // Copyright 2009 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 gzip implements reading and writing of gzip format compressed files,
		 6  // as specified in RFC 1952.
		 7  package gzip
		 8  
		 9  import (
		10  	"bufio"
		11  	"compress/flate"
		12  	"encoding/binary"
		13  	"errors"
		14  	"hash/crc32"
		15  	"io"
		16  	"time"
		17  )
		18  
		19  const (
		20  	gzipID1		 = 0x1f
		21  	gzipID2		 = 0x8b
		22  	gzipDeflate = 8
		23  	flagText		= 1 << 0
		24  	flagHdrCrc	= 1 << 1
		25  	flagExtra	 = 1 << 2
		26  	flagName		= 1 << 3
		27  	flagComment = 1 << 4
		28  )
		29  
		30  var (
		31  	// ErrChecksum is returned when reading GZIP data that has an invalid checksum.
		32  	ErrChecksum = errors.New("gzip: invalid checksum")
		33  	// ErrHeader is returned when reading GZIP data that has an invalid header.
		34  	ErrHeader = errors.New("gzip: invalid header")
		35  )
		36  
		37  var le = binary.LittleEndian
		38  
		39  // noEOF converts io.EOF to io.ErrUnexpectedEOF.
		40  func noEOF(err error) error {
		41  	if err == io.EOF {
		42  		return io.ErrUnexpectedEOF
		43  	}
		44  	return err
		45  }
		46  
		47  // The gzip file stores a header giving metadata about the compressed file.
		48  // That header is exposed as the fields of the Writer and Reader structs.
		49  //
		50  // Strings must be UTF-8 encoded and may only contain Unicode code points
		51  // U+0001 through U+00FF, due to limitations of the GZIP file format.
		52  type Header struct {
		53  	Comment string		// comment
		54  	Extra	 []byte		// "extra data"
		55  	ModTime time.Time // modification time
		56  	Name		string		// file name
		57  	OS			byte			// operating system type
		58  }
		59  
		60  // A Reader is an io.Reader that can be read to retrieve
		61  // uncompressed data from a gzip-format compressed file.
		62  //
		63  // In general, a gzip file can be a concatenation of gzip files,
		64  // each with its own header. Reads from the Reader
		65  // return the concatenation of the uncompressed data of each.
		66  // Only the first header is recorded in the Reader fields.
		67  //
		68  // Gzip files store a length and checksum of the uncompressed data.
		69  // The Reader will return an ErrChecksum when Read
		70  // reaches the end of the uncompressed data if it does not
		71  // have the expected length or checksum. Clients should treat data
		72  // returned by Read as tentative until they receive the io.EOF
		73  // marking the end of the data.
		74  type Reader struct {
		75  	Header			 // valid after NewReader or Reader.Reset
		76  	r						flate.Reader
		77  	decompressor io.ReadCloser
		78  	digest			 uint32 // CRC-32, IEEE polynomial (section 8)
		79  	size				 uint32 // Uncompressed size (section 2.3.1)
		80  	buf					[512]byte
		81  	err					error
		82  	multistream	bool
		83  }
		84  
		85  // NewReader creates a new Reader reading the given reader.
		86  // If r does not also implement io.ByteReader,
		87  // the decompressor may read more data than necessary from r.
		88  //
		89  // It is the caller's responsibility to call Close on the Reader when done.
		90  //
		91  // The Reader.Header fields will be valid in the Reader returned.
		92  func NewReader(r io.Reader) (*Reader, error) {
		93  	z := new(Reader)
		94  	if err := z.Reset(r); err != nil {
		95  		return nil, err
		96  	}
		97  	return z, nil
		98  }
		99  
	 100  // Reset discards the Reader z's state and makes it equivalent to the
	 101  // result of its original state from NewReader, but reading from r instead.
	 102  // This permits reusing a Reader rather than allocating a new one.
	 103  func (z *Reader) Reset(r io.Reader) error {
	 104  	*z = Reader{
	 105  		decompressor: z.decompressor,
	 106  		multistream:	true,
	 107  	}
	 108  	if rr, ok := r.(flate.Reader); ok {
	 109  		z.r = rr
	 110  	} else {
	 111  		z.r = bufio.NewReader(r)
	 112  	}
	 113  	z.Header, z.err = z.readHeader()
	 114  	return z.err
	 115  }
	 116  
	 117  // Multistream controls whether the reader supports multistream files.
	 118  //
	 119  // If enabled (the default), the Reader expects the input to be a sequence
	 120  // of individually gzipped data streams, each with its own header and
	 121  // trailer, ending at EOF. The effect is that the concatenation of a sequence
	 122  // of gzipped files is treated as equivalent to the gzip of the concatenation
	 123  // of the sequence. This is standard behavior for gzip readers.
	 124  //
	 125  // Calling Multistream(false) disables this behavior; disabling the behavior
	 126  // can be useful when reading file formats that distinguish individual gzip
	 127  // data streams or mix gzip data streams with other data streams.
	 128  // In this mode, when the Reader reaches the end of the data stream,
	 129  // Read returns io.EOF. The underlying reader must implement io.ByteReader
	 130  // in order to be left positioned just after the gzip stream.
	 131  // To start the next stream, call z.Reset(r) followed by z.Multistream(false).
	 132  // If there is no next stream, z.Reset(r) will return io.EOF.
	 133  func (z *Reader) Multistream(ok bool) {
	 134  	z.multistream = ok
	 135  }
	 136  
	 137  // readString reads a NUL-terminated string from z.r.
	 138  // It treats the bytes read as being encoded as ISO 8859-1 (Latin-1) and
	 139  // will output a string encoded using UTF-8.
	 140  // This method always updates z.digest with the data read.
	 141  func (z *Reader) readString() (string, error) {
	 142  	var err error
	 143  	needConv := false
	 144  	for i := 0; ; i++ {
	 145  		if i >= len(z.buf) {
	 146  			return "", ErrHeader
	 147  		}
	 148  		z.buf[i], err = z.r.ReadByte()
	 149  		if err != nil {
	 150  			return "", err
	 151  		}
	 152  		if z.buf[i] > 0x7f {
	 153  			needConv = true
	 154  		}
	 155  		if z.buf[i] == 0 {
	 156  			// Digest covers the NUL terminator.
	 157  			z.digest = crc32.Update(z.digest, crc32.IEEETable, z.buf[:i+1])
	 158  
	 159  			// Strings are ISO 8859-1, Latin-1 (RFC 1952, section 2.3.1).
	 160  			if needConv {
	 161  				s := make([]rune, 0, i)
	 162  				for _, v := range z.buf[:i] {
	 163  					s = append(s, rune(v))
	 164  				}
	 165  				return string(s), nil
	 166  			}
	 167  			return string(z.buf[:i]), nil
	 168  		}
	 169  	}
	 170  }
	 171  
	 172  // readHeader reads the GZIP header according to section 2.3.1.
	 173  // This method does not set z.err.
	 174  func (z *Reader) readHeader() (hdr Header, err error) {
	 175  	if _, err = io.ReadFull(z.r, z.buf[:10]); err != nil {
	 176  		// RFC 1952, section 2.2, says the following:
	 177  		//	A gzip file consists of a series of "members" (compressed data sets).
	 178  		//
	 179  		// Other than this, the specification does not clarify whether a
	 180  		// "series" is defined as "one or more" or "zero or more". To err on the
	 181  		// side of caution, Go interprets this to mean "zero or more".
	 182  		// Thus, it is okay to return io.EOF here.
	 183  		return hdr, err
	 184  	}
	 185  	if z.buf[0] != gzipID1 || z.buf[1] != gzipID2 || z.buf[2] != gzipDeflate {
	 186  		return hdr, ErrHeader
	 187  	}
	 188  	flg := z.buf[3]
	 189  	if t := int64(le.Uint32(z.buf[4:8])); t > 0 {
	 190  		// Section 2.3.1, the zero value for MTIME means that the
	 191  		// modified time is not set.
	 192  		hdr.ModTime = time.Unix(t, 0)
	 193  	}
	 194  	// z.buf[8] is XFL and is currently ignored.
	 195  	hdr.OS = z.buf[9]
	 196  	z.digest = crc32.ChecksumIEEE(z.buf[:10])
	 197  
	 198  	if flg&flagExtra != 0 {
	 199  		if _, err = io.ReadFull(z.r, z.buf[:2]); err != nil {
	 200  			return hdr, noEOF(err)
	 201  		}
	 202  		z.digest = crc32.Update(z.digest, crc32.IEEETable, z.buf[:2])
	 203  		data := make([]byte, le.Uint16(z.buf[:2]))
	 204  		if _, err = io.ReadFull(z.r, data); err != nil {
	 205  			return hdr, noEOF(err)
	 206  		}
	 207  		z.digest = crc32.Update(z.digest, crc32.IEEETable, data)
	 208  		hdr.Extra = data
	 209  	}
	 210  
	 211  	var s string
	 212  	if flg&flagName != 0 {
	 213  		if s, err = z.readString(); err != nil {
	 214  			return hdr, err
	 215  		}
	 216  		hdr.Name = s
	 217  	}
	 218  
	 219  	if flg&flagComment != 0 {
	 220  		if s, err = z.readString(); err != nil {
	 221  			return hdr, err
	 222  		}
	 223  		hdr.Comment = s
	 224  	}
	 225  
	 226  	if flg&flagHdrCrc != 0 {
	 227  		if _, err = io.ReadFull(z.r, z.buf[:2]); err != nil {
	 228  			return hdr, noEOF(err)
	 229  		}
	 230  		digest := le.Uint16(z.buf[:2])
	 231  		if digest != uint16(z.digest) {
	 232  			return hdr, ErrHeader
	 233  		}
	 234  	}
	 235  
	 236  	z.digest = 0
	 237  	if z.decompressor == nil {
	 238  		z.decompressor = flate.NewReader(z.r)
	 239  	} else {
	 240  		z.decompressor.(flate.Resetter).Reset(z.r, nil)
	 241  	}
	 242  	return hdr, nil
	 243  }
	 244  
	 245  // Read implements io.Reader, reading uncompressed bytes from its underlying Reader.
	 246  func (z *Reader) Read(p []byte) (n int, err error) {
	 247  	if z.err != nil {
	 248  		return 0, z.err
	 249  	}
	 250  
	 251  	for n == 0 {
	 252  		n, z.err = z.decompressor.Read(p)
	 253  		z.digest = crc32.Update(z.digest, crc32.IEEETable, p[:n])
	 254  		z.size += uint32(n)
	 255  		if z.err != io.EOF {
	 256  			// In the normal case we return here.
	 257  			return n, z.err
	 258  		}
	 259  
	 260  		// Finished file; check checksum and size.
	 261  		if _, err := io.ReadFull(z.r, z.buf[:8]); err != nil {
	 262  			z.err = noEOF(err)
	 263  			return n, z.err
	 264  		}
	 265  		digest := le.Uint32(z.buf[:4])
	 266  		size := le.Uint32(z.buf[4:8])
	 267  		if digest != z.digest || size != z.size {
	 268  			z.err = ErrChecksum
	 269  			return n, z.err
	 270  		}
	 271  		z.digest, z.size = 0, 0
	 272  
	 273  		// File is ok; check if there is another.
	 274  		if !z.multistream {
	 275  			return n, io.EOF
	 276  		}
	 277  		z.err = nil // Remove io.EOF
	 278  
	 279  		if _, z.err = z.readHeader(); z.err != nil {
	 280  			return n, z.err
	 281  		}
	 282  	}
	 283  
	 284  	return n, nil
	 285  }
	 286  
	 287  // Close closes the Reader. It does not close the underlying io.Reader.
	 288  // In order for the GZIP checksum to be verified, the reader must be
	 289  // fully consumed until the io.EOF.
	 290  func (z *Reader) Close() error { return z.decompressor.Close() }
	 291
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