c-family: char8_t and aliasing in C vs C++ [PR111884]

[official-gcc.git] / libgo / go / 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

import "strings"

// Format represents the tar archive format.
//
// The original tar format was introduced in Unix V7.
// Since then, there have been multiple competing formats attempting to
// standardize or extend the V7 format to overcome its limitations.
// The most common formats are the USTAR, PAX, and GNU formats,
// each with their own advantages and limitations.
//
// The following table captures the capabilities of each format:
//
//                        |  USTAR |       PAX |       GNU
//      ------------------+--------+-----------+----------
//      Name              |   256B | unlimited | unlimited
//      Linkname          |   100B | unlimited | unlimited
//      Size              | uint33 | unlimited |    uint89
//      Mode              | uint21 |    uint21 |    uint57
//      Uid/Gid           | uint21 | unlimited |    uint57
//      Uname/Gname       |    32B | unlimited |       32B
//      ModTime           | uint33 | unlimited |     int89
//      AccessTime        |    n/a | unlimited |     int89
//      ChangeTime        |    n/a | unlimited |     int89
//      Devmajor/Devminor | uint21 |    uint21 |    uint57
//      ------------------+--------+-----------+----------
//      string encoding   |  ASCII |     UTF-8 |    binary
//      sub-second times  |     no |       yes |        no
//      sparse files      |     no |       yes |       yes
//
// The table's upper portion shows the Header fields, where each format reports
// the maximum number of bytes allowed for each string field and
// the integer type used to store each numeric field
// (where timestamps are stored as the number of seconds since the Unix epoch).
//
// The table's lower portion shows specialized features of each format,
// such as supported string encodings, support for sub-second timestamps,
// or support for sparse files.
//
// The Writer currently provides no support for sparse files.
type Format int

// Constants to identify various tar formats.
const (
        // Deliberately hide the meaning of constants from public API.
        _ Format = (1 << iota) / 4 // Sequence of 0, 0, 1, 2, 4, 8, etc...

        // FormatUnknown indicates that the format is unknown.
        FormatUnknown

        // The format of the original Unix V7 tar tool prior to standardization.
        formatV7

        // FormatUSTAR represents the USTAR header format defined in POSIX.1-1988.
        //
        // While this format is compatible with most tar readers,
        // the format has several limitations making it unsuitable for some usages.
        // Most notably, it cannot support sparse files, files larger than 8GiB,
        // filenames larger than 256 characters, and non-ASCII filenames.
        //
        // Reference:
        //      http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06
        FormatUSTAR

        // FormatPAX represents the PAX header format defined in POSIX.1-2001.
        //
        // PAX extends USTAR by writing a special file with Typeflag TypeXHeader
        // preceding the original header. This file contains a set of key-value
        // records, which are used to overcome USTAR's shortcomings, in addition to
        // providing the ability to have sub-second resolution for timestamps.
        //
        // Some newer formats add their own extensions to PAX by defining their
        // own keys and assigning certain semantic meaning to the associated values.
        // For example, sparse file support in PAX is implemented using keys
        // defined by the GNU manual (e.g., "GNU.sparse.map").
        //
        // Reference:
        //      http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html
        FormatPAX

        // FormatGNU represents the GNU header format.
        //
        // The GNU header format is older than the USTAR and PAX standards and
        // is not compatible with them. The GNU format supports
        // arbitrary file sizes, filenames of arbitrary encoding and length,
        // sparse files, and other features.
        //
        // It is recommended that PAX be chosen over GNU unless the target
        // application can only parse GNU formatted archives.
        //
        // Reference:
        //      https://www.gnu.org/software/tar/manual/html_node/Standard.html
        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

        formatMax
)

func (f Format) has(f2 Format) bool   { return f&f2 != 0 }
func (f *Format) mayBe(f2 Format)     { *f |= f2 }
func (f *Format) mayOnlyBe(f2 Format) { *f &= f2 }
func (f *Format) mustNotBe(f2 Format) { *f &^= f2 }

var formatNames = map[Format]string{
        formatV7: "V7", FormatUSTAR: "USTAR", FormatPAX: "PAX", FormatGNU: "GNU", formatSTAR: "STAR",
}

func (f Format) String() string {
        var ss []string
        for f2 := Format(1); f2 < formatMax; f2 <<= 1 {
                if f.has(f2) {
                        ss = append(ss, formatNames[f2])
                }
        }
        switch len(ss) {
        case 0:
                return "<unknown>"
        case 1:
                return ss[0]
        default:
                return "(" + strings.Join(ss, " | ") + ")"
        }
}

// 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
)

// blockPadding computes the number of bytes needed to pad offset up to the
// nearest block edge where 0 <= n < blockSize.
func blockPadding(offset int64) (n int64) {
        return -offset & (blockSize - 1)
}

var zeroBlock block

type block [blockSize]byte

// Convert block to any number of formats.
func (b *block) toV7() *headerV7       { return (*headerV7)(b) }
func (b *block) toGNU() *headerGNU     { return (*headerGNU)(b) }
func (b *block) toSTAR() *headerSTAR   { return (*headerSTAR)(b) }
func (b *block) toUSTAR() *headerUSTAR { return (*headerUSTAR)(b) }
func (b *block) toSparse() 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 {
        // Verify checksum.
        var p parser
        value := p.parseOctal(b.toV7().chksum())
        chksum1, chksum2 := b.computeChecksum()
        if p.err != nil || (value != chksum1 && value != chksum2) {
                return FormatUnknown
        }

        // Guess the magic values.
        magic := string(b.toUSTAR().magic())
        version := string(b.toUSTAR().version())
        trailer := string(b.toSTAR().trailer())
        switch {
        case magic == magicUSTAR && trailer == trailerSTAR:
                return formatSTAR
        case magic == magicUSTAR:
                return FormatUSTAR | FormatPAX
        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 Format) {
        // Set the magic values.
        switch {
        case format.has(formatV7):
                // Do nothing.
        case format.has(FormatGNU):
                copy(b.toGNU().magic(), magicGNU)
                copy(b.toGNU().version(), versionGNU)
        case format.has(formatSTAR):
                copy(b.toSTAR().magic(), magicUSTAR)
                copy(b.toSTAR().version(), versionUSTAR)
                copy(b.toSTAR().trailer(), trailerSTAR)
        case format.has(FormatUSTAR | FormatPAX):
                copy(b.toUSTAR().magic(), magicUSTAR)
                copy(b.toUSTAR().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.toV7().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(c)
                signed += int64(int8(c))
        }
        return unsigned, signed
}

// Reset clears the block with all zeros.
func (b *block) reset() {
        *b = block{}
}

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) sparseElem { return sparseElem(s[i*24:]) }
func (s sparseArray) isExtended() []byte     { return s[24*s.maxEntries():][:1] }
func (s sparseArray) maxEntries() int        { return len(s) / 24 }

type sparseElem []byte

func (s sparseElem) offset() []byte { return s[00:][:12] }
func (s sparseElem) length() []byte { return s[12:][:12] }