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archive/tar: centralize all information about tar header format

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The Reader and Writer have hard-coded constants regarding the
offsets and lengths of certain fields in the tar format sprinkled
all over. This makes it harder to verify that the offsets are
correct since a reviewer would need to search for them throughout
the code. Instead, all information about the layout of header
fields should be centralized in one single file. This has the
advantage of being both centralized, and also acting as a form
of documentation about the header struct format.

This method was chosen over using "encoding/binary" since that
method would cause an allocation of a header struct every time
binary.Read was called. This method causes zero allocations and
its logic is no longer than if structs were declared.

Updates #12594

Change-Id: Ic7a0565d2a2cd95d955547ace3b6dea2b57fab34
Reviewed-on: https://go-review.googlesource.com/14669
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>
This commit is contained in:
Joe Tsai 2015-09-17 00:22:56 -07:00 • committed by Vincent Batts
parent 5753de3a3b
commit 2c3c708698
5 changed files with 317 additions and 212 deletions
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37
archive/tar/common.go
View file

@ -21,10 +21,8 @@ import (
"time"
)
// Header type flags.
const (
blockSize = 512
// Types
TypeReg = '0' // regular file
TypeRegA = '\x00' // regular file
TypeLink = '1' // hard link
@ -61,12 +59,6 @@ type Header struct {
Xattrs map[string]string
}
// File name constants from the tar spec.
const (
fileNameSize = 100 // Maximum number of bytes in a standard tar name.
fileNamePrefixSize = 155 // Maximum number of ustar extension bytes.
)
// FileInfo returns an os.FileInfo for the Header.
func (h *Header) FileInfo() os.FileInfo {
return headerFileInfo{h}
@ -279,33 +271,6 @@ func FileInfoHeader(fi os.FileInfo, link string) (*Header, error) {
return h, nil
}
var zeroBlock = make([]byte, blockSize)
// POSIX specifies a sum of the unsigned byte values, but the Sun tar uses signed byte values.
// We compute and return both.
func checksum(header []byte) (unsigned int64, signed int64) {
for i := 0; i < len(header); i++ {
if i == 148 {
// The chksum field (header[148:156]) is special: it should be treated as space bytes.
unsigned += ' ' * 8
signed += ' ' * 8
i += 7
continue
}
unsigned += int64(header[i])
signed += int64(int8(header[i]))
}
return
}
type slicer []byte
func (sp *slicer) next(n int) (b []byte) {
s := *sp
b, *sp = s[0:n], s[n:]
return
}
func isASCII(s string) bool {
for _, c := range s {
if c >= 0x80 {