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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		"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 a 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. If the underlying reader implements io.ByteReader,
   130	// it will 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		n, z.err = z.decompressor.Read(p)
   252		z.digest = crc32.Update(z.digest, crc32.IEEETable, p[:n])
   253		z.size += uint32(n)
   254		if z.err != io.EOF {
   255			// In the normal case we return here.
   256			return n, z.err
   257		}
   258	
   259		// Finished file; check checksum and size.
   260		if _, err := io.ReadFull(z.r, z.buf[:8]); err != nil {
   261			z.err = noEOF(err)
   262			return n, z.err
   263		}
   264		digest := le.Uint32(z.buf[:4])
   265		size := le.Uint32(z.buf[4:8])
   266		if digest != z.digest || size != z.size {
   267			z.err = ErrChecksum
   268			return n, z.err
   269		}
   270		z.digest, z.size = 0, 0
   271	
   272		// File is ok; check if there is another.
   273		if !z.multistream {
   274			return n, io.EOF
   275		}
   276		z.err = nil // Remove io.EOF
   277	
   278		if _, z.err = z.readHeader(); z.err != nil {
   279			return n, z.err
   280		}
   281	
   282		// Read from next file, if necessary.
   283		if n > 0 {
   284			return n, nil
   285		}
   286		return z.Read(p)
   287	}
   288	
   289	// Close closes the Reader. It does not close the underlying io.Reader.
   290	// In order for the GZIP checksum to be verified, the reader must be
   291	// fully consumed until the io.EOF.
   292	func (z *Reader) Close() error { return z.decompressor.Close() }
   293	

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