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Source file src/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		multistream  bool
    78	}
    79	
    80	// NewReader creates a new Reader reading the given reader.
    81	// If r does not also implement io.ByteReader,
    82	// the decompressor may read more data than necessary from r.
    83	// It is the caller's responsibility to call Close on the Reader when done.
    84	func NewReader(r io.Reader) (*Reader, error) {
    85		z := new(Reader)
    86		z.r = makeReader(r)
    87		z.multistream = true
    88		z.digest = crc32.NewIEEE()
    89		if err := z.readHeader(true); err != nil {
    90			return nil, err
    91		}
    92		return z, nil
    93	}
    94	
    95	// Reset discards the Reader z's state and makes it equivalent to the
    96	// result of its original state from NewReader, but reading from r instead.
    97	// This permits reusing a Reader rather than allocating a new one.
    98	func (z *Reader) Reset(r io.Reader) error {
    99		z.r = makeReader(r)
   100		if z.digest == nil {
   101			z.digest = crc32.NewIEEE()
   102		} else {
   103			z.digest.Reset()
   104		}
   105		z.size = 0
   106		z.err = nil
   107		z.multistream = true
   108		return z.readHeader(true)
   109	}
   110	
   111	// Multistream controls whether the reader supports multistream files.
   112	//
   113	// If enabled (the default), the Reader expects the input to be a sequence
   114	// of individually gzipped data streams, each with its own header and
   115	// trailer, ending at EOF. The effect is that the concatenation of a sequence
   116	// of gzipped files is treated as equivalent to the gzip of the concatenation
   117	// of the sequence. This is standard behavior for gzip readers.
   118	//
   119	// Calling Multistream(false) disables this behavior; disabling the behavior
   120	// can be useful when reading file formats that distinguish individual gzip
   121	// data streams or mix gzip data streams with other data streams.
   122	// In this mode, when the Reader reaches the end of the data stream,
   123	// Read returns io.EOF. If the underlying reader implements io.ByteReader,
   124	// it will be left positioned just after the gzip stream.
   125	// To start the next stream, call z.Reset(r) followed by z.Multistream(false).
   126	// If there is no next stream, z.Reset(r) will return io.EOF.
   127	func (z *Reader) Multistream(ok bool) {
   128		z.multistream = ok
   129	}
   130	
   131	// GZIP (RFC 1952) is little-endian, unlike ZLIB (RFC 1950).
   132	func get4(p []byte) uint32 {
   133		return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
   134	}
   135	
   136	func (z *Reader) readString() (string, error) {
   137		var err error
   138		needconv := false
   139		for i := 0; ; i++ {
   140			if i >= len(z.buf) {
   141				return "", ErrHeader
   142			}
   143			z.buf[i], err = z.r.ReadByte()
   144			if err != nil {
   145				return "", err
   146			}
   147			if z.buf[i] > 0x7f {
   148				needconv = true
   149			}
   150			if z.buf[i] == 0 {
   151				// GZIP (RFC 1952) specifies that strings are NUL-terminated ISO 8859-1 (Latin-1).
   152				if needconv {
   153					s := make([]rune, 0, i)
   154					for _, v := range z.buf[0:i] {
   155						s = append(s, rune(v))
   156					}
   157					return string(s), nil
   158				}
   159				return string(z.buf[0:i]), nil
   160			}
   161		}
   162	}
   163	
   164	func (z *Reader) read2() (uint32, error) {
   165		_, err := io.ReadFull(z.r, z.buf[0:2])
   166		if err != nil {
   167			return 0, err
   168		}
   169		return uint32(z.buf[0]) | uint32(z.buf[1])<<8, nil
   170	}
   171	
   172	func (z *Reader) readHeader(save bool) error {
   173		_, err := io.ReadFull(z.r, z.buf[0:10])
   174		if err != nil {
   175			return err
   176		}
   177		if z.buf[0] != gzipID1 || z.buf[1] != gzipID2 || z.buf[2] != gzipDeflate {
   178			return ErrHeader
   179		}
   180		z.flg = z.buf[3]
   181		if save {
   182			z.ModTime = time.Unix(int64(get4(z.buf[4:8])), 0)
   183			// z.buf[8] is xfl, ignored
   184			z.OS = z.buf[9]
   185		}
   186		z.digest.Reset()
   187		z.digest.Write(z.buf[0:10])
   188	
   189		if z.flg&flagExtra != 0 {
   190			n, err := z.read2()
   191			if err != nil {
   192				return err
   193			}
   194			data := make([]byte, n)
   195			if _, err = io.ReadFull(z.r, data); err != nil {
   196				return err
   197			}
   198			if save {
   199				z.Extra = data
   200			}
   201		}
   202	
   203		var s string
   204		if z.flg&flagName != 0 {
   205			if s, err = z.readString(); err != nil {
   206				return err
   207			}
   208			if save {
   209				z.Name = s
   210			}
   211		}
   212	
   213		if z.flg&flagComment != 0 {
   214			if s, err = z.readString(); err != nil {
   215				return err
   216			}
   217			if save {
   218				z.Comment = s
   219			}
   220		}
   221	
   222		if z.flg&flagHdrCrc != 0 {
   223			n, err := z.read2()
   224			if err != nil {
   225				return err
   226			}
   227			sum := z.digest.Sum32() & 0xFFFF
   228			if n != sum {
   229				return ErrHeader
   230			}
   231		}
   232	
   233		z.digest.Reset()
   234		if z.decompressor == nil {
   235			z.decompressor = flate.NewReader(z.r)
   236		} else {
   237			z.decompressor.(flate.Resetter).Reset(z.r, nil)
   238		}
   239		return nil
   240	}
   241	
   242	func (z *Reader) Read(p []byte) (n int, err error) {
   243		if z.err != nil {
   244			return 0, z.err
   245		}
   246		if len(p) == 0 {
   247			return 0, nil
   248		}
   249	
   250		n, err = z.decompressor.Read(p)
   251		z.digest.Write(p[0:n])
   252		z.size += uint32(n)
   253		if n != 0 || err != io.EOF {
   254			z.err = err
   255			return
   256		}
   257	
   258		// Finished file; check checksum + size.
   259		if _, err := io.ReadFull(z.r, z.buf[0:8]); err != nil {
   260			z.err = err
   261			return 0, err
   262		}
   263		crc32, isize := get4(z.buf[0:4]), get4(z.buf[4:8])
   264		sum := z.digest.Sum32()
   265		if sum != crc32 || isize != z.size {
   266			z.err = ErrChecksum
   267			return 0, z.err
   268		}
   269	
   270		// File is ok; is there another?
   271		if !z.multistream {
   272			return 0, io.EOF
   273		}
   274	
   275		if err = z.readHeader(false); err != nil {
   276			z.err = err
   277			return
   278		}
   279	
   280		// Yes.  Reset and read from it.
   281		z.digest.Reset()
   282		z.size = 0
   283		return z.Read(p)
   284	}
   285	
   286	// Close closes the Reader. It does not close the underlying io.Reader.
   287	func (z *Reader) Close() error { return z.decompressor.Close() }
   288	

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