summaryrefslogtreecommitdiff
path: root/libgo/go/archive/tar/reader.go
blob: 1b40af812ae627a267b9331bda7e7a00970ae21c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package tar

// TODO(dsymonds):
//   - pax extensions

import (
	"bytes"
	"errors"
	"io"
	"io/ioutil"
	"os"
	"strconv"
	"time"
)

var (
	ErrHeader = errors.New("archive/tar: invalid tar header")
)

// A Reader provides sequential access to the contents of a tar archive.
// A tar archive consists of a sequence of files.
// The Next method advances to the next file in the archive (including the first),
// and then it can be treated as an io.Reader to access the file's data.
//
// Example:
//	tr := tar.NewReader(r)
//	for {
//		hdr, err := tr.Next()
//		if err == io.EOF {
//			// end of tar archive
//			break
//		}
//		if err != nil {
//			// handle error
//		}
//		io.Copy(data, tr)
//	}
type Reader struct {
	r   io.Reader
	err error
	nb  int64 // number of unread bytes for current file entry
	pad int64 // amount of padding (ignored) after current file entry
}

// NewReader creates a new Reader reading from r.
func NewReader(r io.Reader) *Reader { return &Reader{r: r} }

// Next advances to the next entry in the tar archive.
func (tr *Reader) Next() (*Header, error) {
	var hdr *Header
	if tr.err == nil {
		tr.skipUnread()
	}
	if tr.err == nil {
		hdr = tr.readHeader()
	}
	return hdr, tr.err
}

// Parse bytes as a NUL-terminated C-style string.
// If a NUL byte is not found then the whole slice is returned as a string.
func cString(b []byte) string {
	n := 0
	for n < len(b) && b[n] != 0 {
		n++
	}
	return string(b[0:n])
}

func (tr *Reader) octal(b []byte) int64 {
	// Removing leading spaces.
	for len(b) > 0 && b[0] == ' ' {
		b = b[1:]
	}
	// Removing trailing NULs and spaces.
	for len(b) > 0 && (b[len(b)-1] == ' ' || b[len(b)-1] == '\x00') {
		b = b[0 : len(b)-1]
	}
	x, err := strconv.ParseUint(cString(b), 8, 64)
	if err != nil {
		tr.err = err
	}
	return int64(x)
}

// Skip any unread bytes in the existing file entry, as well as any alignment padding.
func (tr *Reader) skipUnread() {
	nr := tr.nb + tr.pad // number of bytes to skip
	tr.nb, tr.pad = 0, 0
	if sr, ok := tr.r.(io.Seeker); ok {
		if _, err := sr.Seek(nr, os.SEEK_CUR); err == nil {
			return
		}
	}
	_, tr.err = io.CopyN(ioutil.Discard, tr.r, nr)
}

func (tr *Reader) verifyChecksum(header []byte) bool {
	if tr.err != nil {
		return false
	}

	given := tr.octal(header[148:156])
	unsigned, signed := checksum(header)
	return given == unsigned || given == signed
}

func (tr *Reader) readHeader() *Header {
	header := make([]byte, blockSize)
	if _, tr.err = io.ReadFull(tr.r, header); tr.err != nil {
		return nil
	}

	// Two blocks of zero bytes marks the end of the archive.
	if bytes.Equal(header, zeroBlock[0:blockSize]) {
		if _, tr.err = io.ReadFull(tr.r, header); tr.err != nil {
			return nil
		}
		if bytes.Equal(header, zeroBlock[0:blockSize]) {
			tr.err = io.EOF
		} else {
			tr.err = ErrHeader // zero block and then non-zero block
		}
		return nil
	}

	if !tr.verifyChecksum(header) {
		tr.err = ErrHeader
		return nil
	}

	// Unpack
	hdr := new(Header)
	s := slicer(header)

	hdr.Name = cString(s.next(100))
	hdr.Mode = tr.octal(s.next(8))
	hdr.Uid = int(tr.octal(s.next(8)))
	hdr.Gid = int(tr.octal(s.next(8)))
	hdr.Size = tr.octal(s.next(12))
	hdr.ModTime = time.Unix(tr.octal(s.next(12)), 0)
	s.next(8) // chksum
	hdr.Typeflag = s.next(1)[0]
	hdr.Linkname = cString(s.next(100))

	// The remainder of the header depends on the value of magic.
	// The original (v7) version of tar had no explicit magic field,
	// so its magic bytes, like the rest of the block, are NULs.
	magic := string(s.next(8)) // contains version field as well.
	var format string
	switch magic {
	case "ustar\x0000": // POSIX tar (1003.1-1988)
		if string(header[508:512]) == "tar\x00" {
			format = "star"
		} else {
			format = "posix"
		}
	case "ustar  \x00": // old GNU tar
		format = "gnu"
	}

	switch format {
	case "posix", "gnu", "star":
		hdr.Uname = cString(s.next(32))
		hdr.Gname = cString(s.next(32))
		devmajor := s.next(8)
		devminor := s.next(8)
		if hdr.Typeflag == TypeChar || hdr.Typeflag == TypeBlock {
			hdr.Devmajor = tr.octal(devmajor)
			hdr.Devminor = tr.octal(devminor)
		}
		var prefix string
		switch format {
		case "posix", "gnu":
			prefix = cString(s.next(155))
		case "star":
			prefix = cString(s.next(131))
			hdr.AccessTime = time.Unix(tr.octal(s.next(12)), 0)
			hdr.ChangeTime = time.Unix(tr.octal(s.next(12)), 0)
		}
		if len(prefix) > 0 {
			hdr.Name = prefix + "/" + hdr.Name
		}
	}

	if tr.err != nil {
		tr.err = ErrHeader
		return nil
	}

	// Maximum value of hdr.Size is 64 GB (12 octal digits),
	// so there's no risk of int64 overflowing.
	tr.nb = int64(hdr.Size)
	tr.pad = -tr.nb & (blockSize - 1) // blockSize is a power of two

	return hdr
}

// Read reads from the current entry in the tar archive.
// It returns 0, io.EOF when it reaches the end of that entry,
// until Next is called to advance to the next entry.
func (tr *Reader) Read(b []byte) (n int, err error) {
	if tr.nb == 0 {
		// file consumed
		return 0, io.EOF
	}

	if int64(len(b)) > tr.nb {
		b = b[0:tr.nb]
	}
	n, err = tr.r.Read(b)
	tr.nb -= int64(n)

	if err == io.EOF && tr.nb > 0 {
		err = io.ErrUnexpectedEOF
	}
	tr.err = err
	return
}