func getParent(file *os.File, imgID string, debug log.Logger) (string, error) {
var parent string
_, err := file.Seek(0, 0)
if err != nil {
return "", fmt.Errorf("error seeking file: %v", err)
}
jsonPath := filepath.Join(imgID, "json")
parentWalker := func(t *tarball.TarFile) error {
if filepath.Clean(t.Name()) == jsonPath {
jsonb, err := ioutil.ReadAll(t.TarStream)
if err != nil {
return fmt.Errorf("error reading layer json: %v", err)
}
var dockerData types.DockerImageData
if err := json.Unmarshal(jsonb, &dockerData); err != nil {
return fmt.Errorf("error unmarshaling layer data: %v", err)
}
parent = dockerData.Parent
}
return nil
}
tr := tar.NewReader(file)
if err := tarball.Walk(*tr, parentWalker); err != nil {
return "", err
}
debug.Printf("Layer %q depends on layer %q", imgID, parent)
return parent, nil
}
// getAncestry computes an image ancestry, returning an ordered list
// of dependencies starting from the topmost image to the base.
// It checks for dependency loops via duplicate detection in the image
// chain and errors out in such cases.
func getAncestry(file *os.File, imgID string, debug log.Logger) ([]string, error) {
var ancestry []string
deps := make(map[string]bool)
curImgID := imgID
var err error
for curImgID != "" {
if deps[curImgID] {
return nil, fmt.Errorf("dependency loop detected at image %q", curImgID)
}
deps[curImgID] = true
ancestry = append(ancestry, curImgID)
debug.Printf("Getting ancestry for layer %q", curImgID)
curImgID, err = getParent(file, curImgID, debug)
if err != nil {
return nil, err
}
}
return ancestry, nil
}
