undocker/rootfs/rootfs.go

212 lines
4.8 KiB
Go

package rootfs
import (
"archive/tar"
"encoding/json"
"errors"
"fmt"
"io"
"path/filepath"
"strings"
"go.uber.org/multierr"
)
const (
_manifestJSON = "manifest.json"
_layerSuffix = "/layer.tar"
_whReaddir = ".wh..wh..opq"
_whPrefix = ".wh."
)
var (
errBadManifest = errors.New("bad or missing manifest.json")
)
type dockerManifestJSON []struct {
Layers []string `json:"Layers"`
}
// RootFS accepts a docker layer tarball and writes it to outfile.
// 1. create map[string]io.ReadSeeker for each layer.
// 2. parse manifest.json and get the layer order.
// 3. go through each layer in order and write:
// a) to an ordered slice: the file name.
// b) to an FS map: where does the file come from?
// I) layer name
// II) offset (0 being the first file in the layer)
// 4. go through
func RootFS(in io.ReadSeeker, out io.Writer) (err error) {
tr := tar.NewReader(in)
tw := tar.NewWriter(out)
defer func() { err = multierr.Append(err, tw.Close()) }()
// layerOffsets maps a layer name (a9b123c0daa/layer.tar) to it's offset
layerOffsets := map[string]int64{}
// manifest is the docker manifest in the image
var manifest dockerManifestJSON
// get layer offsets and manifest.json
for {
hdr, err := tr.Next()
if err == io.EOF {
break
}
if hdr.Typeflag != tar.TypeReg {
continue
}
switch {
case filepath.Clean(hdr.Name) == _manifestJSON:
dec := json.NewDecoder(tr)
if err := dec.Decode(&manifest); err != nil {
return fmt.Errorf("decode %s: %w", _manifestJSON, err)
}
case strings.HasSuffix(hdr.Name, _layerSuffix):
here, err := in.Seek(0, io.SeekCurrent)
if err != nil {
return err
}
layerOffsets[hdr.Name] = here
}
}
if len(manifest) == 0 || len(layerOffsets) != len(manifest[0].Layers) {
return errBadManifest
}
// enumerate layers the way they would be laid down in the image
layers := make([]int64, len(layerOffsets))
for i, name := range manifest[0].Layers {
layers[i] = layerOffsets[name]
}
// file2layer maps a filename to layer number (index in "layers")
file2layer := map[string]int{}
// whreaddir maps `wh..wh..opq` file to a layer; see doc.go
whreaddir := map[string]int{}
// wh maps a filename to a layer until which it should be ignored,
// inclusively; see doc.go
wh := map[string]int{}
// iterate over all files, construct `file2layer`, `whreaddir`, `wh`
for i, offset := range layers {
if _, err := in.Seek(offset, io.SeekStart); err != nil {
return err
}
tr = tar.NewReader(in)
for {
hdr, err := tr.Next()
if err == io.EOF {
break
}
if err != nil {
return err
}
if hdr.Typeflag == tar.TypeDir {
continue
}
// according to aufs documentation, whiteout files should be
// hardlinks. I saw at least one docker container using regular
// files for whiteouts.
if hdr.Typeflag == tar.TypeLink || hdr.Typeflag == tar.TypeReg {
basename := filepath.Base(hdr.Name)
basedir := filepath.Dir(hdr.Name)
if basename == _whReaddir {
whreaddir[basedir] = i
continue
} else if strings.HasPrefix(basename, _whPrefix) {
fname := strings.TrimPrefix(basename, _whPrefix)
wh[filepath.Join(basedir, fname)] = i
continue
}
}
file2layer[hdr.Name] = i
}
}
// construct directories to whiteout, for each layer.
whIgnore := whiteoutDirs(whreaddir, len(layers))
// iterate through all layers, all files, and write files.
for i, offset := range layers {
if _, err := in.Seek(offset, io.SeekStart); err != nil {
return err
}
tr = tar.NewReader(in)
for {
hdr, err := tr.Next()
if err == io.EOF {
break
}
if err != nil {
return err
}
if layer, ok := wh[hdr.Name]; ok && layer >= i {
continue
}
if whIgnore[i].HasPrefix(hdr.Name) {
continue
}
if hdr.Typeflag != tar.TypeDir && file2layer[hdr.Name] != i {
continue
}
if err := writeFile(tr, tw, hdr); err != nil {
return err
}
}
}
return nil
}
func writeFile(tr *tar.Reader, tw *tar.Writer, hdr *tar.Header) error {
hdrOut := &tar.Header{
Typeflag: hdr.Typeflag,
Name: hdr.Name,
Linkname: hdr.Linkname,
Size: hdr.Size,
Mode: int64(hdr.Mode & 0777),
Uid: hdr.Uid,
Gid: hdr.Gid,
Uname: hdr.Uname,
Gname: hdr.Gname,
ModTime: hdr.ModTime,
Devmajor: hdr.Devmajor,
Devminor: hdr.Devminor,
Format: tar.FormatGNU,
}
if err := tw.WriteHeader(hdrOut); err != nil {
return err
}
if hdr.Typeflag == tar.TypeReg {
if _, err := io.Copy(tw, tr); err != nil {
return err
}
}
return nil
}
func whiteoutDirs(whreaddir map[string]int, nlayers int) []*tree {
ret := make([]*tree, nlayers)
for i := range ret {
ret[i] = newTree()
}
for fname, layer := range whreaddir {
if layer == 0 {
continue
}
ret[layer-1].Add(fname)
}
for i := nlayers - 1; i > 0; i-- {
ret[i-1].Merge(ret[i])
}
return ret
}