Tar to squashfs using pseudo file

tar2sfs.go

// $ go run tar2sfs.go distro.tar > INDEX
// $ mksquashfs ./sysroot distro.img -pf INDEX -noappend

package main

import (
	"archive/tar"
	"flag"
	"fmt"
	"io"
	"log"
	"os"
	"strconv"
	"strings"

	"github.com/pkg/errors"
)

func pseudoQuote(name string) string {
	if strings.ContainsAny(name, "\n\" ") {
		return fmt.Sprintf("\"%s\"", strings.ReplaceAll(name, "\"", "\\\""))
	} else {
		return name
	}
}

func pseudoDef(tarpath string, block int64, hdr tar.Header) string {
	var name, uid, gid, data string
	var kind byte

	name = pseudoQuote(hdr.Name)
	mode := hdr.Mode & 07777

	if hdr.Uname != "" {
		uid = hdr.Uname
	} else {
		uid = strconv.Itoa(hdr.Uid)
	}
	if hdr.Gname != "" {
		gid = hdr.Gname
	} else {
		gid = strconv.Itoa(hdr.Gid)
	}

	switch hdr.Typeflag {
	case tar.TypeReg:
		kind = 'f'
		data = fmt.Sprintf(
			" dd status=none if=%q bs=512 skip=%d | tar xOf - --occurrence %q",
			tarpath, block, hdr.Name)
	case tar.TypeDir:
		kind = 'd'
	case tar.TypeBlock:
		kind = 'b'
		data = fmt.Sprintf("%d %d", hdr.Devmajor, hdr.Devminor)
	case tar.TypeChar:
		kind = 'c'
		data = fmt.Sprintf("%d %d", hdr.Devmajor, hdr.Devminor)
	case tar.TypeSymlink:
		kind = 's'
		data = hdr.Linkname
	case tar.TypeLink:
		kind = 's'
		data = hdr.Linkname
	// case tar.TypeFifo:
	// case tar.TypeGNUSparse:
	default:
		log.Fatal("Unsupported tar node type: %c", hdr.Typeflag)
	}

	return fmt.Sprintf("%s %c %o %s %s %s", name, kind, mode, uid, gid, data)
}

func main() {
	flag.Parse()
	if len(flag.Args()) != 1 {
		log.Fatal("Input tar is missing")
	}

	tarpath := flag.Arg(0)

	tarfd, err := os.Open(tarpath)
	if err != nil {
		log.Fatal(err)
	}
	defer tarfd.Close()

	iter := tar.NewReader(tarfd)
	var offset int64 = 0
	for {
		hdr, err := iter.Next()
		if err == io.EOF {
			break
		}
		if err != nil {
			log.Fatal(errors.Wrap(err, tarpath))
		}

		fmt.Println(pseudoDef(tarpath, offset, *hdr))

		offset, err = tarfd.Seek(0, io.SeekCurrent)
		if offset%512 != 0 {
			panic("Tar header is not aligned")
		}
		if hdr.Size > 0 {
			offset = (offset / 512) + (hdr.Size-1)/512 + 1
		} else {
			offset /= 512
		}
	}
}