registry-image-resource/cmd/in/unpack.go

package main

import (
	"archive/tar"
	"io"
	"path/filepath"
	"strings"

	"github.com/concourse/go-archive/tarfs"
	"github.com/google/go-containerregistry/pkg/v1"
)

const whiteoutPrefix = ".wh."

func unpackImage(dest string, img v1.Image) error {
	layers, err := img.Layers()
	if err != nil {
		return err
	}

	written := map[string]struct{}{}
	removed := map[string]struct{}{}

	// iterate over layers in reverse order; no need to write things files that
	// are modified by later layers anyway
	for i := len(layers) - 1; i >= 0; i-- {
		layer := layers[i]

		r, err := layer.Uncompressed()
		if err != nil {
			return err
		}

		tr := tar.NewReader(r)

		for {
			hdr, err := tr.Next()
			if err == io.EOF {
				break
			}

			if err != nil {
				return err
			}

			path := filepath.Join(dest, filepath.Clean(hdr.Name))
			base := filepath.Base(path)
			dir := filepath.Dir(path)

			if strings.HasPrefix(base, whiteoutPrefix) {
				// layer has marked a file as deleted
				name := strings.TrimPrefix(base, whiteoutPrefix)
				removed[filepath.Join(dir, name)] = struct{}{}
				continue
			}

			if pathIsRemoved(path, removed) {
				// path has been removed by lower layer
				continue
			}

			if _, ok := written[path]; ok {
				// path has already been written by lower layer
				continue
			}

			written[path] = struct{}{}

			if err := tarfs.ExtractEntry(hdr, dest, tr, true); err != nil {
				return err
			}
		}
	}

	return nil
}

func pathIsRemoved(path string, removed map[string]struct{}) bool {
	if _, ok := removed[path]; ok {
		return true
	}

	// check if parent dir has been removed
	for wd := range removed {
		if strings.HasPrefix(path, wd) {
			return true
		}
	}

	return false
}