Source file src/pkg/compress/gzip/gunzip.go

     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		"errors"
    13		"hash"
    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	func makeReader(r io.Reader) flate.Reader {
    31		if rr, ok := r.(flate.Reader); ok {
    32			return rr
    33		}
    34		return bufio.NewReader(r)
    35	}
    36	
    37	var (
    38		// ErrChecksum is returned when reading GZIP data that has an invalid checksum.
    39		ErrChecksum = errors.New("gzip: invalid checksum")
    40		// ErrHeader is returned when reading GZIP data that has an invalid header.
    41		ErrHeader = errors.New("gzip: invalid header")
    42	)
    43	
    44	// The gzip file stores a header giving metadata about the compressed file.
    45	// That header is exposed as the fields of the Writer and Reader structs.
    46	type Header struct {
    47		Comment string    // comment
    48		Extra   []byte    // "extra data"
    49		ModTime time.Time // modification time
    50		Name    string    // file name
    51		OS      byte      // operating system type
    52	}
    53	
    54	// A Reader is an io.Reader that can be read to retrieve
    55	// uncompressed data from a gzip-format compressed file.
    56	//
    57	// In general, a gzip file can be a concatenation of gzip files,
    58	// each with its own header.  Reads from the Reader
    59	// return the concatenation of the uncompressed data of each.
    60	// Only the first header is recorded in the Reader fields.
    61	//
    62	// Gzip files store a length and checksum of the uncompressed data.
    63	// The Reader will return a ErrChecksum when Read
    64	// reaches the end of the uncompressed data if it does not
    65	// have the expected length or checksum.  Clients should treat data
    66	// returned by Read as tentative until they receive the io.EOF
    67	// marking the end of the data.
    68	type Reader struct {
    69		Header
    70		r            flate.Reader
    71		decompressor io.ReadCloser
    72		digest       hash.Hash32
    73		size         uint32
    74		flg          byte
    75		buf          [512]byte
    76		err          error
    77	}
    78	
    79	// NewReader creates a new Reader reading the given reader.
    80	// The implementation buffers input and may read more data than necessary from r.
    81	// It is the caller's responsibility to call Close on the Reader when done.
    82	func NewReader(r io.Reader) (*Reader, error) {
    83		z := new(Reader)
    84		z.r = makeReader(r)
    85		z.digest = crc32.NewIEEE()
    86		if err := z.readHeader(true); err != nil {
    87			return nil, err
    88		}
    89		return z, nil
    90	}
    91	
    92	// GZIP (RFC 1952) is little-endian, unlike ZLIB (RFC 1950).
    93	func get4(p []byte) uint32 {
    94		return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
    95	}
    96	
    97	func (z *Reader) readString() (string, error) {
    98		var err error
    99		needconv := false
   100		for i := 0; ; i++ {
   101			if i >= len(z.buf) {
   102				return "", ErrHeader
   103			}
   104			z.buf[i], err = z.r.ReadByte()
   105			if err != nil {
   106				return "", err
   107			}
   108			if z.buf[i] > 0x7f {
   109				needconv = true
   110			}
   111			if z.buf[i] == 0 {
   112				// GZIP (RFC 1952) specifies that strings are NUL-terminated ISO 8859-1 (Latin-1).
   113				if needconv {
   114					s := make([]rune, 0, i)
   115					for _, v := range z.buf[0:i] {
   116						s = append(s, rune(v))
   117					}
   118					return string(s), nil
   119				}
   120				return string(z.buf[0:i]), nil
   121			}
   122		}
   123		panic("not reached")
   124	}
   125	
   126	func (z *Reader) read2() (uint32, error) {
   127		_, err := io.ReadFull(z.r, z.buf[0:2])
   128		if err != nil {
   129			return 0, err
   130		}
   131		return uint32(z.buf[0]) | uint32(z.buf[1])<<8, nil
   132	}
   133	
   134	func (z *Reader) readHeader(save bool) error {
   135		_, err := io.ReadFull(z.r, z.buf[0:10])
   136		if err != nil {
   137			return err
   138		}
   139		if z.buf[0] != gzipID1 || z.buf[1] != gzipID2 || z.buf[2] != gzipDeflate {
   140			return ErrHeader
   141		}
   142		z.flg = z.buf[3]
   143		if save {
   144			z.ModTime = time.Unix(int64(get4(z.buf[4:8])), 0)
   145			// z.buf[8] is xfl, ignored
   146			z.OS = z.buf[9]
   147		}
   148		z.digest.Reset()
   149		z.digest.Write(z.buf[0:10])
   150	
   151		if z.flg&flagExtra != 0 {
   152			n, err := z.read2()
   153			if err != nil {
   154				return err
   155			}
   156			data := make([]byte, n)
   157			if _, err = io.ReadFull(z.r, data); err != nil {
   158				return err
   159			}
   160			if save {
   161				z.Extra = data
   162			}
   163		}
   164	
   165		var s string
   166		if z.flg&flagName != 0 {
   167			if s, err = z.readString(); err != nil {
   168				return err
   169			}
   170			if save {
   171				z.Name = s
   172			}
   173		}
   174	
   175		if z.flg&flagComment != 0 {
   176			if s, err = z.readString(); err != nil {
   177				return err
   178			}
   179			if save {
   180				z.Comment = s
   181			}
   182		}
   183	
   184		if z.flg&flagHdrCrc != 0 {
   185			n, err := z.read2()
   186			if err != nil {
   187				return err
   188			}
   189			sum := z.digest.Sum32() & 0xFFFF
   190			if n != sum {
   191				return ErrHeader
   192			}
   193		}
   194	
   195		z.digest.Reset()
   196		z.decompressor = flate.NewReader(z.r)
   197		return nil
   198	}
   199	
   200	func (z *Reader) Read(p []byte) (n int, err error) {
   201		if z.err != nil {
   202			return 0, z.err
   203		}
   204		if len(p) == 0 {
   205			return 0, nil
   206		}
   207	
   208		n, err = z.decompressor.Read(p)
   209		z.digest.Write(p[0:n])
   210		z.size += uint32(n)
   211		if n != 0 || err != io.EOF {
   212			z.err = err
   213			return
   214		}
   215	
   216		// Finished file; check checksum + size.
   217		if _, err := io.ReadFull(z.r, z.buf[0:8]); err != nil {
   218			z.err = err
   219			return 0, err
   220		}
   221		crc32, isize := get4(z.buf[0:4]), get4(z.buf[4:8])
   222		sum := z.digest.Sum32()
   223		if sum != crc32 || isize != z.size {
   224			z.err = ErrChecksum
   225			return 0, z.err
   226		}
   227	
   228		// File is ok; is there another?
   229		if err = z.readHeader(false); err != nil {
   230			z.err = err
   231			return
   232		}
   233	
   234		// Yes.  Reset and read from it.
   235		z.digest.Reset()
   236		z.size = 0
   237		return z.Read(p)
   238	}
   239	
   240	// Close closes the Reader. It does not close the underlying io.Reader.
   241	func (z *Reader) Close() error { return z.decompressor.Close() }