Source: gunzip.go in package compress/gzip

Package gzip implements reading and writing of gzip format compressed files, as specified in RFC 1952.

package gzip

import (
	"bufio"
	"compress/flate"
	"encoding/binary"
	"errors"
	"hash/crc32"
	"io"
	"time"
)

const (
	gzipID1     = 0x1f
	gzipID2     = 0x8b
	gzipDeflate = 8
	flagText    = 1 << 0
	flagHdrCrc  = 1 << 1
	flagExtra   = 1 << 2
	flagName    = 1 << 3
	flagComment = 1 << 4
)

ErrChecksum is returned when reading GZIP data that has an invalid checksum.

ErrHeader is returned when reading GZIP data that has an invalid header.

	ErrHeader = errors.New("gzip: invalid header")
)

var le = binary.LittleEndian

noEOF converts io.EOF to io.ErrUnexpectedEOF.

func noEOF(err error) error {
	if err == io.EOF {
		return io.ErrUnexpectedEOF
	}
	return err
}

The gzip file stores a header giving metadata about the compressed file. That header is exposed as the fields of the Writer and Reader structs. Strings must be UTF-8 encoded and may only contain Unicode code points U+0001 through U+00FF, due to limitations of the GZIP file format.

type Header struct {
	Comment string    // comment
	Extra   []byte    // "extra data"
	ModTime time.Time // modification time
	Name    string    // file name
	OS      byte      // operating system type
}

A Reader is an io.Reader that can be read to retrieve uncompressed data from a gzip-format compressed file. In general, a gzip file can be a concatenation of gzip files, each with its own header. Reads from the Reader return the concatenation of the uncompressed data of each. Only the first header is recorded in the Reader fields. Gzip files store a length and checksum of the uncompressed data. The Reader will return an ErrChecksum when Read reaches the end of the uncompressed data if it does not have the expected length or checksum. Clients should treat data returned by Read as tentative until they receive the io.EOF marking the end of the data.

type Reader struct {
	Header       // valid after NewReader or Reader.Reset
	r            flate.Reader
	decompressor io.ReadCloser
	digest       uint32 // CRC-32, IEEE polynomial (section 8)
	size         uint32 // Uncompressed size (section 2.3.1)
	buf          [512]byte
	err          error
	multistream  bool
}

NewReader creates a new Reader reading the given reader. If r does not also implement io.ByteReader, the decompressor may read more data than necessary from r. It is the caller's responsibility to call Close on the Reader when done. The Reader.Header fields will be valid in the Reader returned.

func NewReader(r io.Reader) (*Reader, error) {
	z := new(Reader)
	if err := z.Reset(r); err != nil {
		return nil, err
	}
	return z, nil
}

Reset discards the Reader z's state and makes it equivalent to the result of its original state from NewReader, but reading from r instead. This permits reusing a Reader rather than allocating a new one.

func (z *Reader) Reset(r io.Reader) error {
	*z = Reader{
		decompressor: z.decompressor,
		multistream:  true,
	}
	if rr, ok := r.(flate.Reader); ok {
		z.r = rr
	} else {
		z.r = bufio.NewReader(r)
	}
	z.Header, z.err = z.readHeader()
	return z.err
}

Multistream controls whether the reader supports multistream files. If enabled (the default), the Reader expects the input to be a sequence of individually gzipped data streams, each with its own header and trailer, ending at EOF. The effect is that the concatenation of a sequence of gzipped files is treated as equivalent to the gzip of the concatenation of the sequence. This is standard behavior for gzip readers. Calling Multistream(false) disables this behavior; disabling the behavior can be useful when reading file formats that distinguish individual gzip data streams or mix gzip data streams with other data streams. In this mode, when the Reader reaches the end of the data stream, Read returns io.EOF. The underlying reader must implement io.ByteReader in order to be left positioned just after the gzip stream. To start the next stream, call z.Reset(r) followed by z.Multistream(false). If there is no next stream, z.Reset(r) will return io.EOF.

func (z *Reader) Multistream(ok bool) {
	z.multistream = ok
}

readString reads a NUL-terminated string from z.r. It treats the bytes read as being encoded as ISO 8859-1 (Latin-1) and will output a string encoded using UTF-8. This method always updates z.digest with the data read.

func (z *Reader) readString() (string, error) {
	var err error
	needConv := false
	for i := 0; ; i++ {
		if i >= len(z.buf) {
			return "", ErrHeader
		}
		z.buf[i], err = z.r.ReadByte()
		if err != nil {
			return "", err
		}
		if z.buf[i] > 0x7f {
			needConv = true
		}

Digest covers the NUL terminator.

			z.digest = crc32.Update(z.digest, crc32.IEEETable, z.buf[:i+1])

Strings are ISO 8859-1, Latin-1 (RFC 1952, section 2.3.1).

			if needConv {
				s := make([]rune, 0, i)
				for _, v := range z.buf[:i] {
					s = append(s, rune(v))
				}
				return string(s), nil
			}
			return string(z.buf[:i]), nil
		}
	}
}

readHeader reads the GZIP header according to section 2.3.1. This method does not set z.err.

func (z *Reader) readHeader() (hdr Header, err error) {

RFC 1952, section 2.2, says the following: A gzip file consists of a series of "members" (compressed data sets). Other than this, the specification does not clarify whether a "series" is defined as "one or more" or "zero or more". To err on the side of caution, Go interprets this to mean "zero or more". Thus, it is okay to return io.EOF here.

		return hdr, err
	}
	if z.buf[0] != gzipID1 || z.buf[1] != gzipID2 || z.buf[2] != gzipDeflate {
		return hdr, ErrHeader
	}
	flg := z.buf[3]

Section 2.3.1, the zero value for MTIME means that the modified time is not set.

		hdr.ModTime = time.Unix(t, 0)

z.buf[8] is XFL and is currently ignored.

	hdr.OS = z.buf[9]
	z.digest = crc32.ChecksumIEEE(z.buf[:10])

	if flg&flagExtra != 0 {
		if _, err = io.ReadFull(z.r, z.buf[:2]); err != nil {
			return hdr, noEOF(err)
		}
		z.digest = crc32.Update(z.digest, crc32.IEEETable, z.buf[:2])
		data := make([]byte, le.Uint16(z.buf[:2]))
		if _, err = io.ReadFull(z.r, data); err != nil {
			return hdr, noEOF(err)
		}
		z.digest = crc32.Update(z.digest, crc32.IEEETable, data)
		hdr.Extra = data
	}

	var s string
	if flg&flagName != 0 {
		if s, err = z.readString(); err != nil {
			return hdr, err
		}
		hdr.Name = s
	}

	if flg&flagComment != 0 {
		if s, err = z.readString(); err != nil {
			return hdr, err
		}
		hdr.Comment = s
	}

	if flg&flagHdrCrc != 0 {
		if _, err = io.ReadFull(z.r, z.buf[:2]); err != nil {
			return hdr, noEOF(err)
		}
		digest := le.Uint16(z.buf[:2])
		if digest != uint16(z.digest) {
			return hdr, ErrHeader
		}
	}

	z.digest = 0
	if z.decompressor == nil {
		z.decompressor = flate.NewReader(z.r)
	} else {
		z.decompressor.(flate.Resetter).Reset(z.r, nil)
	}
	return hdr, nil
}

Read implements io.Reader, reading uncompressed bytes from its underlying Reader.

func (z *Reader) Read(p []byte) (n int, err error) {
	if z.err != nil {
		return 0, z.err
	}

	n, z.err = z.decompressor.Read(p)
	z.digest = crc32.Update(z.digest, crc32.IEEETable, p[:n])
	z.size += uint32(n)

In the normal case we return here.

		return n, z.err
	}

Finished file; check checksum and size.

	if _, err := io.ReadFull(z.r, z.buf[:8]); err != nil {
		z.err = noEOF(err)
		return n, z.err
	}
	digest := le.Uint32(z.buf[:4])
	size := le.Uint32(z.buf[4:8])
	if digest != z.digest || size != z.size {
		z.err = ErrChecksum
		return n, z.err
	}
	z.digest, z.size = 0, 0

File is ok; check if there is another.

	if !z.multistream {
		return n, io.EOF
	}
	z.err = nil // Remove io.EOF

	if _, z.err = z.readHeader(); z.err != nil {
		return n, z.err
	}

Read from next file, if necessary.

	if n > 0 {
		return n, nil
	}
	return z.Read(p)
}

Close closes the Reader. It does not close the underlying io.Reader. In order for the GZIP checksum to be verified, the reader must be fully consumed until the io.EOF.