moby/pkg/chrootarchive/diff_unix.go

//+build !windows

package chrootarchive

import (
	"bytes"
	"encoding/json"
	"flag"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"runtime"

	"github.com/docker/docker/pkg/archive"
	"github.com/docker/docker/pkg/reexec"
	"github.com/docker/docker/pkg/system"
)

type applyLayerResponse struct {
	LayerSize int64 `json:"layerSize"`
}

// applyLayer is the entry-point for docker-applylayer on re-exec. This is not
// used on Windows as it does not support chroot, hence no point sandboxing
// through chroot and rexec.
func applyLayer() {

	var (
		tmpDir = ""
		err    error
	)
	runtime.LockOSThread()
	flag.Parse()

	if err := chroot(flag.Arg(0)); err != nil {
		fatal(err)
	}

	// We need to be able to set any perms
	oldmask, err := system.Umask(0)
	defer system.Umask(oldmask)
	if err != nil {
		fatal(err)
	}

	if tmpDir, err = ioutil.TempDir("/", "temp-docker-extract"); err != nil {
		fatal(err)
	}

	os.Setenv("TMPDIR", tmpDir)
	size, err := archive.UnpackLayer("/", os.Stdin)
	os.RemoveAll(tmpDir)
	if err != nil {
		fatal(err)
	}

	encoder := json.NewEncoder(os.Stdout)
	if err := encoder.Encode(applyLayerResponse{size}); err != nil {
		fatal(fmt.Errorf("unable to encode layerSize JSON: %s", err))
	}

	flush(os.Stdout)
	flush(os.Stdin)
	os.Exit(0)
}

// ApplyLayer parses a diff in the standard layer format from `layer`, and
// applies it to the directory `dest`. Returns the size in bytes of the
// contents of the layer.
func ApplyLayer(dest string, layer archive.ArchiveReader) (size int64, err error) {
	dest = filepath.Clean(dest)
	decompressed, err := archive.DecompressStream(layer)
	if err != nil {
		return 0, err
	}

	defer decompressed.Close()

	cmd := reexec.Command("docker-applyLayer", dest)
	cmd.Stdin = decompressed

	outBuf, errBuf := new(bytes.Buffer), new(bytes.Buffer)
	cmd.Stdout, cmd.Stderr = outBuf, errBuf

	if err = cmd.Run(); err != nil {
		return 0, fmt.Errorf("ApplyLayer %s stdout: %s stderr: %s", err, outBuf, errBuf)
	}

	// Stdout should be a valid JSON struct representing an applyLayerResponse.
	response := applyLayerResponse{}
	decoder := json.NewDecoder(outBuf)
	if err = decoder.Decode(&response); err != nil {
		return 0, fmt.Errorf("unable to decode ApplyLayer JSON response: %s", err)
	}

	return response.LayerSize, nil
}