141 lines
2.9 KiB
Go
141 lines
2.9 KiB
Go
package rootfs
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import (
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"archive/tar"
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"encoding/json"
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"io"
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"strings"
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)
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const (
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_manifestJSON = "manifest.json"
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)
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type dockerManifestJSON []struct {
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Config string `json:"Config"`
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Layers []string `json:"Layers"`
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}
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// Rootfs accepts a docker layer tarball and writes it to outfile.
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// 1. create map[string]io.ReadSeeker for each layer.
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// 2. parse manifest.json and get the layer order.
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// 3. go through each layer in order and write:
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// a) to an ordered slice: the file name.
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// b) to an FS map: where does the file come from?
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// I) layer name
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// II) offset (0 being the first file in the layer)
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// 4. go through
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func RootFS(in io.ReadSeeker, out io.Writer) error {
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tr := tar.NewReader(in)
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tw := tar.NewWriter(out)
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// layerOffsets maps a layer name (a9b123c0daa/layer.tar) to it's offset
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layerOffsets := map[string]int64{}
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// manifest is the docker manifest in the image
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var manifest dockerManifestJSON
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// phase 1: get layer offsets and manifest.json
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for {
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hdr, err := tr.Next()
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if err == io.EOF {
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break
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}
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if hdr.Typeflag != tar.TypeReg {
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continue
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}
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switch {
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case hdr.Name == _manifestJSON:
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dec := json.NewDecoder(tr)
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if err := dec.Decode(&manifest); err != nil {
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return err
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}
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case strings.HasSuffix(hdr.Name, "/layer.tar"):
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here, err := in.Seek(0, io.SeekCurrent)
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if err != nil {
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return err
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}
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layerOffsets[hdr.Name] = here
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}
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}
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// phase 1.5: enumerate layers
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layers := make([]int64, len(layerOffsets))
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for i, name := range manifest[0].Layers {
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layers[i] = layerOffsets[name]
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}
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// file2layer maps a filename to layer number (index in "layers")
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file2layer := map[string]int{}
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// phase 2: iterate through all layers and save filenames
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// for all kinds of files.
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for i, offset := range layers {
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if _, err := in.Seek(offset, io.SeekStart); err != nil {
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return err
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}
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tr = tar.NewReader(in)
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for {
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hdr, err := tr.Next()
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if err == io.EOF {
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break
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}
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if err != nil {
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return err
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}
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file2layer[hdr.Name] = i
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}
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}
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// phase 3: iterate through all layers and write files.
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for i, offset := range layers {
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if _, err := in.Seek(offset, io.SeekStart); err != nil {
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return err
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}
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tr = tar.NewReader(in)
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for {
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hdr, err := tr.Next()
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if err == io.EOF {
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break
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}
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if err != nil {
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return err
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}
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if file2layer[hdr.Name] != i {
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continue
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}
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hdrOut := &tar.Header{
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Typeflag: hdr.Typeflag,
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Name: hdr.Name,
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Linkname: hdr.Linkname,
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Size: hdr.Size,
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Mode: int64(hdr.Mode & 0777),
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Uid: hdr.Uid,
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Gid: hdr.Gid,
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Uname: hdr.Uname,
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Gname: hdr.Gname,
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ModTime: hdr.ModTime,
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Devmajor: hdr.Devmajor,
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Devminor: hdr.Devminor,
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Format: tar.FormatGNU,
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}
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if err := tw.WriteHeader(hdrOut); err != nil {
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return err
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}
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if hdr.Typeflag == tar.TypeReg {
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if _, err := io.Copy(tw, tr); err != nil {
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return err
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}
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}
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}
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}
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return tw.Close()
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}
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