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Run Format

Source file src/archive/tar/format.go

  // Copyright 2016 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
  
  // Constants to identify various tar formats.
  const (
  	// The format is unknown.
  	formatUnknown = (1 << iota) / 2 // Sequence of 0, 1, 2, 4, 8, etc...
  
  	// The format of the original Unix V7 tar tool prior to standardization.
  	formatV7
  
  	// The old and new GNU formats, which are incompatible with USTAR.
  	// This does cover the old GNU sparse extension.
  	// This does not cover the GNU sparse extensions using PAX headers,
  	// versions 0.0, 0.1, and 1.0; these fall under the PAX format.
  	formatGNU
  
  	// Schily's tar format, which is incompatible with USTAR.
  	// This does not cover STAR extensions to the PAX format; these fall under
  	// the PAX format.
  	formatSTAR
  
  	// USTAR is the former standardization of tar defined in POSIX.1-1988.
  	// This is incompatible with the GNU and STAR formats.
  	formatUSTAR
  
  	// PAX is the latest standardization of tar defined in POSIX.1-2001.
  	// This is an extension of USTAR and is "backwards compatible" with it.
  	//
  	// Some newer formats add their own extensions to PAX, such as GNU sparse
  	// files and SCHILY extended attributes. Since they are backwards compatible
  	// with PAX, they will be labelled as "PAX".
  	formatPAX
  )
  
  // Magics used to identify various formats.
  const (
  	magicGNU, versionGNU     = "ustar ", " \x00"
  	magicUSTAR, versionUSTAR = "ustar\x00", "00"
  	trailerSTAR              = "tar\x00"
  )
  
  // Size constants from various tar specifications.
  const (
  	blockSize  = 512 // Size of each block in a tar stream
  	nameSize   = 100 // Max length of the name field in USTAR format
  	prefixSize = 155 // Max length of the prefix field in USTAR format
  )
  
  var zeroBlock block
  
  type block [blockSize]byte
  
  // Convert block to any number of formats.
  func (b *block) V7() *headerV7       { return (*headerV7)(b) }
  func (b *block) GNU() *headerGNU     { return (*headerGNU)(b) }
  func (b *block) STAR() *headerSTAR   { return (*headerSTAR)(b) }
  func (b *block) USTAR() *headerUSTAR { return (*headerUSTAR)(b) }
  func (b *block) Sparse() sparseArray { return (sparseArray)(b[:]) }
  
  // GetFormat checks that the block is a valid tar header based on the checksum.
  // It then attempts to guess the specific format based on magic values.
  // If the checksum fails, then formatUnknown is returned.
  func (b *block) GetFormat() (format int) {
  	// Verify checksum.
  	var p parser
  	value := p.parseOctal(b.V7().Chksum())
  	chksum1, chksum2 := b.ComputeChecksum()
  	if p.err != nil || (value != chksum1 && value != chksum2) {
  		return formatUnknown
  	}
  
  	// Guess the magic values.
  	magic := string(b.USTAR().Magic())
  	version := string(b.USTAR().Version())
  	trailer := string(b.STAR().Trailer())
  	switch {
  	case magic == magicUSTAR && trailer == trailerSTAR:
  		return formatSTAR
  	case magic == magicUSTAR:
  		return formatUSTAR
  	case magic == magicGNU && version == versionGNU:
  		return formatGNU
  	default:
  		return formatV7
  	}
  }
  
  // SetFormat writes the magic values necessary for specified format
  // and then updates the checksum accordingly.
  func (b *block) SetFormat(format int) {
  	// Set the magic values.
  	switch format {
  	case formatV7:
  		// Do nothing.
  	case formatGNU:
  		copy(b.GNU().Magic(), magicGNU)
  		copy(b.GNU().Version(), versionGNU)
  	case formatSTAR:
  		copy(b.STAR().Magic(), magicUSTAR)
  		copy(b.STAR().Version(), versionUSTAR)
  		copy(b.STAR().Trailer(), trailerSTAR)
  	case formatUSTAR, formatPAX:
  		copy(b.USTAR().Magic(), magicUSTAR)
  		copy(b.USTAR().Version(), versionUSTAR)
  	default:
  		panic("invalid format")
  	}
  
  	// Update checksum.
  	// This field is special in that it is terminated by a NULL then space.
  	var f formatter
  	field := b.V7().Chksum()
  	chksum, _ := b.ComputeChecksum() // Possible values are 256..128776
  	f.formatOctal(field[:7], chksum) // Never fails since 128776 < 262143
  	field[7] = ' '
  }
  
  // ComputeChecksum computes the checksum for the header block.
  // POSIX specifies a sum of the unsigned byte values, but the Sun tar used
  // signed byte values.
  // We compute and return both.
  func (b *block) ComputeChecksum() (unsigned, signed int64) {
  	for i, c := range b {
  		if 148 <= i && i < 156 {
  			c = ' ' // Treat the checksum field itself as all spaces.
  		}
  		unsigned += int64(uint8(c))
  		signed += int64(int8(c))
  	}
  	return unsigned, signed
  }
  
  type headerV7 [blockSize]byte
  
  func (h *headerV7) Name() []byte     { return h[000:][:100] }
  func (h *headerV7) Mode() []byte     { return h[100:][:8] }
  func (h *headerV7) UID() []byte      { return h[108:][:8] }
  func (h *headerV7) GID() []byte      { return h[116:][:8] }
  func (h *headerV7) Size() []byte     { return h[124:][:12] }
  func (h *headerV7) ModTime() []byte  { return h[136:][:12] }
  func (h *headerV7) Chksum() []byte   { return h[148:][:8] }
  func (h *headerV7) TypeFlag() []byte { return h[156:][:1] }
  func (h *headerV7) LinkName() []byte { return h[157:][:100] }
  
  type headerGNU [blockSize]byte
  
  func (h *headerGNU) V7() *headerV7       { return (*headerV7)(h) }
  func (h *headerGNU) Magic() []byte       { return h[257:][:6] }
  func (h *headerGNU) Version() []byte     { return h[263:][:2] }
  func (h *headerGNU) UserName() []byte    { return h[265:][:32] }
  func (h *headerGNU) GroupName() []byte   { return h[297:][:32] }
  func (h *headerGNU) DevMajor() []byte    { return h[329:][:8] }
  func (h *headerGNU) DevMinor() []byte    { return h[337:][:8] }
  func (h *headerGNU) AccessTime() []byte  { return h[345:][:12] }
  func (h *headerGNU) ChangeTime() []byte  { return h[357:][:12] }
  func (h *headerGNU) Sparse() sparseArray { return (sparseArray)(h[386:][:24*4+1]) }
  func (h *headerGNU) RealSize() []byte    { return h[483:][:12] }
  
  type headerSTAR [blockSize]byte
  
  func (h *headerSTAR) V7() *headerV7      { return (*headerV7)(h) }
  func (h *headerSTAR) Magic() []byte      { return h[257:][:6] }
  func (h *headerSTAR) Version() []byte    { return h[263:][:2] }
  func (h *headerSTAR) UserName() []byte   { return h[265:][:32] }
  func (h *headerSTAR) GroupName() []byte  { return h[297:][:32] }
  func (h *headerSTAR) DevMajor() []byte   { return h[329:][:8] }
  func (h *headerSTAR) DevMinor() []byte   { return h[337:][:8] }
  func (h *headerSTAR) Prefix() []byte     { return h[345:][:131] }
  func (h *headerSTAR) AccessTime() []byte { return h[476:][:12] }
  func (h *headerSTAR) ChangeTime() []byte { return h[488:][:12] }
  func (h *headerSTAR) Trailer() []byte    { return h[508:][:4] }
  
  type headerUSTAR [blockSize]byte
  
  func (h *headerUSTAR) V7() *headerV7     { return (*headerV7)(h) }
  func (h *headerUSTAR) Magic() []byte     { return h[257:][:6] }
  func (h *headerUSTAR) Version() []byte   { return h[263:][:2] }
  func (h *headerUSTAR) UserName() []byte  { return h[265:][:32] }
  func (h *headerUSTAR) GroupName() []byte { return h[297:][:32] }
  func (h *headerUSTAR) DevMajor() []byte  { return h[329:][:8] }
  func (h *headerUSTAR) DevMinor() []byte  { return h[337:][:8] }
  func (h *headerUSTAR) Prefix() []byte    { return h[345:][:155] }
  
  type sparseArray []byte
  
  func (s sparseArray) Entry(i int) sparseNode { return (sparseNode)(s[i*24:]) }
  func (s sparseArray) IsExtended() []byte     { return s[24*s.MaxEntries():][:1] }
  func (s sparseArray) MaxEntries() int        { return len(s) / 24 }
  
  type sparseNode []byte
  
  func (s sparseNode) Offset() []byte   { return s[00:][:12] }
  func (s sparseNode) NumBytes() []byte { return s[12:][:12] }
  

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