libgo/go/archive/tar/writer.go

// 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):
// - catch more errors (no first header, etc.)

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"os"
	"path"
	"strconv"
	"strings"
	"time"
)

var (
	ErrWriteTooLong    = errors.New("archive/tar: write too long")
	ErrFieldTooLong    = errors.New("archive/tar: header field too long")
	ErrWriteAfterClose = errors.New("archive/tar: write after close")
	errNameTooLong     = errors.New("archive/tar: name too long")
)

// A Writer provides sequential writing of a tar archive in POSIX.1 format.
// A tar archive consists of a sequence of files.
// Call WriteHeader to begin a new file, and then call Write to supply that file's data,
// writing at most hdr.Size bytes in total.
type Writer struct {
	w          io.Writer
	err        error
	nb         int64 // number of unwritten bytes for current file entry
	pad        int64 // amount of padding to write after current file entry
	closed     bool
	usedBinary bool // whether the binary numeric field extension was used
}

// NewWriter creates a new Writer writing to w.
func NewWriter(w io.Writer) *Writer { return &Writer{w: w} }

// Flush finishes writing the current file (optional).
func (tw *Writer) Flush() error {
	if tw.nb > 0 {
		tw.err = fmt.Errorf("archive/tar: missed writing %d bytes", tw.nb)
		return tw.err
	}

	n := tw.nb + tw.pad
	for n > 0 && tw.err == nil {
		nr := n
		if nr > blockSize {
			nr = blockSize
		}
		var nw int
		nw, tw.err = tw.w.Write(zeroBlock[0:nr])
		n -= int64(nw)
	}
	tw.nb = 0
	tw.pad = 0
	return tw.err
}

// Write s into b, terminating it with a NUL if there is room.
func (tw *Writer) cString(b []byte, s string) {
	if len(s) > len(b) {
		if tw.err == nil {
			tw.err = ErrFieldTooLong
		}
		return
	}
	copy(b, s)
	if len(s) < len(b) {
		b[len(s)] = 0
	}
}

// Encode x as an octal ASCII string and write it into b with leading zeros.
func (tw *Writer) octal(b []byte, x int64) {
	s := strconv.FormatInt(x, 8)
	// leading zeros, but leave room for a NUL.
	for len(s)+1 < len(b) {
		s = "0" + s
	}
	tw.cString(b, s)
}

// Write x into b, either as octal or as binary (GNUtar/star extension).
func (tw *Writer) numeric(b []byte, x int64) {
	// Try octal first.
	s := strconv.FormatInt(x, 8)
	if len(s) < len(b) {
		tw.octal(b, x)
		return
	}
	// Too big: use binary (big-endian).
	tw.usedBinary = true
	for i := len(b) - 1; x > 0 && i >= 0; i-- {
		b[i] = byte(x)
		x >>= 8
	}
	b[0] |= 0x80 // highest bit indicates binary format
}

var (
	minTime = time.Unix(0, 0)
	// There is room for 11 octal digits (33 bits) of mtime.
	maxTime = minTime.Add((1<<33 - 1) * time.Second)
)

// WriteHeader writes hdr and prepares to accept the file's contents.
// WriteHeader calls Flush if it is not the first header.
// Calling after a Close will return ErrWriteAfterClose.
func (tw *Writer) WriteHeader(hdr *Header) error {
	if tw.closed {
		return ErrWriteAfterClose
	}
	if tw.err == nil {
		tw.Flush()
	}
	if tw.err != nil {
		return tw.err
	}
	// Decide whether or not to use PAX extensions
	// TODO(shanemhansen): we might want to use PAX headers for
	// subsecond time resolution, but for now let's just capture
	// the long name/long symlink use case.
	suffix := hdr.Name
	prefix := ""
	if len(hdr.Name) > fileNameSize || len(hdr.Linkname) > fileNameSize {
		var err error
		prefix, suffix, err = tw.splitUSTARLongName(hdr.Name)
		// Either we were unable to pack the long name into ustar format
		// or the link name is too long; use PAX headers.
		if err == errNameTooLong || len(hdr.Linkname) > fileNameSize {
			if err := tw.writePAXHeader(hdr); err != nil {
				return err
			}
		} else if err != nil {
			return err
		}
	}
	tw.nb = int64(hdr.Size)
	tw.pad = -tw.nb & (blockSize - 1) // blockSize is a power of two

	header := make([]byte, blockSize)
	s := slicer(header)
	tw.cString(s.next(fileNameSize), suffix)

	// Handle out of range ModTime carefully.
	var modTime int64
	if !hdr.ModTime.Before(minTime) && !hdr.ModTime.After(maxTime) {
		modTime = hdr.ModTime.Unix()
	}

	tw.octal(s.next(8), hdr.Mode)          // 100:108
	tw.numeric(s.next(8), int64(hdr.Uid))  // 108:116
	tw.numeric(s.next(8), int64(hdr.Gid))  // 116:124
	tw.numeric(s.next(12), hdr.Size)       // 124:136
	tw.numeric(s.next(12), modTime)        // 136:148
	s.next(8)                              // chksum (148:156)
	s.next(1)[0] = hdr.Typeflag            // 156:157
	tw.cString(s.next(100), hdr.Linkname)  // linkname (157:257)
	copy(s.next(8), []byte("ustar\x0000")) // 257:265
	tw.cString(s.next(32), hdr.Uname)      // 265:297
	tw.cString(s.next(32), hdr.Gname)      // 297:329
	tw.numeric(s.next(8), hdr.Devmajor)    // 329:337
	tw.numeric(s.next(8), hdr.Devminor)    // 337:345
	tw.cString(s.next(155), prefix)        // 345:500
	// Use the GNU magic instead of POSIX magic if we used any GNU extensions.
	if tw.usedBinary {
		copy(header[257:265], []byte("ustar  \x00"))
	}
	// Use the ustar magic if we used ustar long names.
	if len(prefix) > 0 {
		copy(header[257:265], []byte("ustar\000"))
	}

	// The chksum field is terminated by a NUL and a space.
	// This is different from the other octal fields.
	chksum, _ := checksum(header)
	tw.octal(header[148:155], chksum)
	header[155] = ' '

	if tw.err != nil {
		// problem with header; probably integer too big for a field.
		return tw.err
	}

	_, tw.err = tw.w.Write(header)

	return tw.err
}

// writeUSTARLongName splits a USTAR long name hdr.Name.
// name must be < 256 characters. errNameTooLong is returned
// if hdr.Name can't be split. The splitting heuristic
// is compatible with gnu tar.
func (tw *Writer) splitUSTARLongName(name string) (prefix, suffix string, err error) {
	length := len(name)
	if length > fileNamePrefixSize+1 {
		length = fileNamePrefixSize + 1
	} else if name[length-1] == '/' {
		length--
	}
	i := strings.LastIndex(name[:length], "/")
	nlen := length - i - 1
	if i <= 0 || nlen > fileNameSize || nlen == 0 {
		err = errNameTooLong
		return
	}
	prefix, suffix = name[:i], name[i+1:]
	return
}

// writePaxHeader writes an extended pax header to the
// archive.
func (tw *Writer) writePAXHeader(hdr *Header) error {
	// Prepare extended header
	ext := new(Header)
	ext.Typeflag = TypeXHeader
	// Setting ModTime is required for reader parsing to
	// succeed, and seems harmless enough.
	ext.ModTime = hdr.ModTime
	// The spec asks that we namespace our pseudo files
	// with the current pid.
	pid := os.Getpid()
	dir, file := path.Split(hdr.Name)
	ext.Name = path.Join(dir,
		fmt.Sprintf("PaxHeaders.%d", pid), file)[0:100]
	// Construct the body
	var buf bytes.Buffer
	if len(hdr.Name) > fileNameSize {
		fmt.Fprint(&buf, paxHeader("path="+hdr.Name))
	}
	if len(hdr.Linkname) > fileNameSize {
		fmt.Fprint(&buf, paxHeader("linkpath="+hdr.Linkname))
	}
	ext.Size = int64(len(buf.Bytes()))
	if err := tw.WriteHeader(ext); err != nil {
		return err
	}
	if _, err := tw.Write(buf.Bytes()); err != nil {
		return err
	}
	if err := tw.Flush(); err != nil {
		return err
	}
	return nil
}

// paxHeader formats a single pax record, prefixing it with the appropriate length
func paxHeader(msg string) string {
	const padding = 2 // Extra padding for space and newline
	size := len(msg) + padding
	size += len(strconv.Itoa(size))
	record := fmt.Sprintf("%d %s\n", size, msg)
	if len(record) != size {
		// Final adjustment if adding size increased
		// the number of digits in size
		size = len(record)
		record = fmt.Sprintf("%d %s\n", size, msg)
	}
	return record
}

// Write writes to the current entry in the tar archive.
// Write returns the error ErrWriteTooLong if more than
// hdr.Size bytes are written after WriteHeader.
func (tw *Writer) Write(b []byte) (n int, err error) {
	if tw.closed {
		err = ErrWriteTooLong
		return
	}
	overwrite := false
	if int64(len(b)) > tw.nb {
		b = b[0:tw.nb]
		overwrite = true
	}
	n, err = tw.w.Write(b)
	tw.nb -= int64(n)
	if err == nil && overwrite {
		err = ErrWriteTooLong
		return
	}
	tw.err = err
	return
}

// Close closes the tar archive, flushing any unwritten
// data to the underlying writer.
func (tw *Writer) Close() error {
	if tw.err != nil || tw.closed {
		return tw.err
	}
	tw.Flush()
	tw.closed = true
	if tw.err != nil {
		return tw.err
	}

	// trailer: two zero blocks
	for i := 0; i < 2; i++ {
		_, tw.err = tw.w.Write(zeroBlock)
		if tw.err != nil {
			break
		}
	}
	return tw.err
}