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jeremybenn |
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package tar
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// TODO(dsymonds):
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// - pax extensions
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import (
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"bytes"
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"errors"
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"io"
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"io/ioutil"
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"os"
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"strconv"
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"time"
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)
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var (
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ErrHeader = errors.New("invalid tar header")
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)
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// A Reader provides sequential access to the contents of a tar archive.
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// A tar archive consists of a sequence of files.
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// The Next method advances to the next file in the archive (including the first),
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// and then it can be treated as an io.Reader to access the file's data.
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//
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// Example:
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// tr := tar.NewReader(r)
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// for {
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// hdr, err := tr.Next()
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// if err == io.EOF {
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// // end of tar archive
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// break
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// }
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// if err != nil {
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// // handle error
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// }
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// io.Copy(data, tr)
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// }
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type Reader struct {
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r io.Reader
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err error
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nb int64 // number of unread bytes for current file entry
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pad int64 // amount of padding (ignored) after current file entry
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}
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// NewReader creates a new Reader reading from r.
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func NewReader(r io.Reader) *Reader { return &Reader{r: r} }
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// Next advances to the next entry in the tar archive.
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func (tr *Reader) Next() (*Header, error) {
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var hdr *Header
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if tr.err == nil {
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tr.skipUnread()
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}
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if tr.err == nil {
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hdr = tr.readHeader()
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}
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return hdr, tr.err
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}
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// Parse bytes as a NUL-terminated C-style string.
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// If a NUL byte is not found then the whole slice is returned as a string.
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func cString(b []byte) string {
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n := 0
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for n < len(b) && b[n] != 0 {
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n++
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}
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return string(b[0:n])
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}
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func (tr *Reader) octal(b []byte) int64 {
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// Removing leading spaces.
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for len(b) > 0 && b[0] == ' ' {
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b = b[1:]
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}
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// Removing trailing NULs and spaces.
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for len(b) > 0 && (b[len(b)-1] == ' ' || b[len(b)-1] == '\x00') {
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b = b[0 : len(b)-1]
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}
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x, err := strconv.ParseUint(cString(b), 8, 64)
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if err != nil {
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tr.err = err
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}
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return int64(x)
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}
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// Skip any unread bytes in the existing file entry, as well as any alignment padding.
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func (tr *Reader) skipUnread() {
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nr := tr.nb + tr.pad // number of bytes to skip
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tr.nb, tr.pad = 0, 0
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if sr, ok := tr.r.(io.Seeker); ok {
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if _, err := sr.Seek(nr, os.SEEK_CUR); err == nil {
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return
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}
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}
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_, tr.err = io.CopyN(ioutil.Discard, tr.r, nr)
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}
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func (tr *Reader) verifyChecksum(header []byte) bool {
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if tr.err != nil {
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return false
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}
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given := tr.octal(header[148:156])
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unsigned, signed := checksum(header)
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return given == unsigned || given == signed
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}
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func (tr *Reader) readHeader() *Header {
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header := make([]byte, blockSize)
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if _, tr.err = io.ReadFull(tr.r, header); tr.err != nil {
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return nil
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}
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// Two blocks of zero bytes marks the end of the archive.
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if bytes.Equal(header, zeroBlock[0:blockSize]) {
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if _, tr.err = io.ReadFull(tr.r, header); tr.err != nil {
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return nil
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}
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if bytes.Equal(header, zeroBlock[0:blockSize]) {
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tr.err = io.EOF
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} else {
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tr.err = ErrHeader // zero block and then non-zero block
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}
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return nil
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}
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if !tr.verifyChecksum(header) {
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tr.err = ErrHeader
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return nil
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}
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// Unpack
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hdr := new(Header)
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s := slicer(header)
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hdr.Name = cString(s.next(100))
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hdr.Mode = tr.octal(s.next(8))
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hdr.Uid = int(tr.octal(s.next(8)))
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hdr.Gid = int(tr.octal(s.next(8)))
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hdr.Size = tr.octal(s.next(12))
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hdr.ModTime = time.Unix(tr.octal(s.next(12)), 0)
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s.next(8) // chksum
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hdr.Typeflag = s.next(1)[0]
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hdr.Linkname = cString(s.next(100))
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// The remainder of the header depends on the value of magic.
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// The original (v7) version of tar had no explicit magic field,
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// so its magic bytes, like the rest of the block, are NULs.
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magic := string(s.next(8)) // contains version field as well.
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var format string
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switch magic {
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case "ustar\x0000": // POSIX tar (1003.1-1988)
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if string(header[508:512]) == "tar\x00" {
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format = "star"
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} else {
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format = "posix"
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}
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case "ustar \x00": // old GNU tar
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format = "gnu"
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}
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switch format {
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case "posix", "gnu", "star":
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hdr.Uname = cString(s.next(32))
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hdr.Gname = cString(s.next(32))
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devmajor := s.next(8)
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devminor := s.next(8)
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if hdr.Typeflag == TypeChar || hdr.Typeflag == TypeBlock {
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hdr.Devmajor = tr.octal(devmajor)
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hdr.Devminor = tr.octal(devminor)
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}
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var prefix string
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switch format {
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case "posix", "gnu":
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prefix = cString(s.next(155))
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case "star":
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prefix = cString(s.next(131))
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hdr.AccessTime = time.Unix(tr.octal(s.next(12)), 0)
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hdr.ChangeTime = time.Unix(tr.octal(s.next(12)), 0)
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}
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if len(prefix) > 0 {
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hdr.Name = prefix + "/" + hdr.Name
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}
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}
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if tr.err != nil {
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tr.err = ErrHeader
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return nil
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}
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// Maximum value of hdr.Size is 64 GB (12 octal digits),
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// so there's no risk of int64 overflowing.
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tr.nb = int64(hdr.Size)
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tr.pad = -tr.nb & (blockSize - 1) // blockSize is a power of two
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return hdr
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}
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// Read reads from the current entry in the tar archive.
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// It returns 0, io.EOF when it reaches the end of that entry,
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// until Next is called to advance to the next entry.
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func (tr *Reader) Read(b []byte) (n int, err error) {
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if tr.nb == 0 {
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// file consumed
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return 0, io.EOF
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}
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if int64(len(b)) > tr.nb {
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b = b[0:tr.nb]
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}
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n, err = tr.r.Read(b)
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tr.nb -= int64(n)
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if err == io.EOF && tr.nb > 0 {
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err = io.ErrUnexpectedEOF
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}
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tr.err = err
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return
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}
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