// +build linux

package overlay

import (
	"bufio"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"strconv"
	"strings"
	"sync"

	"github.com/containers/storage/drivers"
	"github.com/containers/storage/drivers/overlayutils"
	"github.com/containers/storage/drivers/quota"
	"github.com/containers/storage/pkg/archive"
	"github.com/containers/storage/pkg/chrootarchive"
	"github.com/containers/storage/pkg/directory"
	"github.com/containers/storage/pkg/fsutils"
	"github.com/containers/storage/pkg/idtools"
	"github.com/containers/storage/pkg/locker"
	"github.com/containers/storage/pkg/mount"
	"github.com/containers/storage/pkg/parsers"
	"github.com/containers/storage/pkg/parsers/kernel"
	"github.com/containers/storage/pkg/system"
	units "github.com/docker/go-units"
	"github.com/opencontainers/selinux/go-selinux/label"
	"github.com/pkg/errors"
	"github.com/sirupsen/logrus"
	"golang.org/x/sys/unix"
)

var (
	// untar defines the untar method
	untar = chrootarchive.UntarUncompressed
)

// This backend uses the overlay union filesystem for containers
// with diff directories for each layer.

// This version of the overlay driver requires at least kernel
// 4.0.0 in order to support mounting multiple diff directories.

// Each container/image has at least a "diff" directory and "link" file.
// If there is also a "lower" file when there are diff layers
// below as well as "merged" and "work" directories. The "diff" directory
// has the upper layer of the overlay and is used to capture any
// changes to the layer. The "lower" file contains all the lower layer
// mounts separated by ":" and ordered from uppermost to lowermost
// layers. The overlay itself is mounted in the "merged" directory,
// and the "work" dir is needed for overlay to work.

// The "link" file for each layer contains a unique string for the layer.
// Under the "l" directory at the root there will be a symbolic link
// with that unique string pointing the "diff" directory for the layer.
// The symbolic links are used to reference lower layers in the "lower"
// file and on mount. The links are used to shorten the total length
// of a layer reference without requiring changes to the layer identifier
// or root directory. Mounts are always done relative to root and
// referencing the symbolic links in order to ensure the number of
// lower directories can fit in a single page for making the mount
// syscall. A hard upper limit of 128 lower layers is enforced to ensure
// that mounts do not fail due to length.

const (
	linkDir   = "l"
	lowerFile = "lower"
	maxDepth  = 128

	// idLength represents the number of random characters
	// which can be used to create the unique link identifer
	// for every layer. If this value is too long then the
	// page size limit for the mount command may be exceeded.
	// The idLength should be selected such that following equation
	// is true (512 is a buffer for label metadata).
	// ((idLength + len(linkDir) + 1) * maxDepth) <= (pageSize - 512)
	idLength = 26
)

type overlayOptions struct {
	overrideKernelCheck bool
	imageStores         []string
	quota               quota.Quota
}

// Driver contains information about the home directory and the list of active mounts that are created using this driver.
type Driver struct {
	name          string
	home          string
	uidMaps       []idtools.IDMap
	gidMaps       []idtools.IDMap
	ctr           *graphdriver.RefCounter
	quotaCtl      *quota.Control
	options       overlayOptions
	naiveDiff     graphdriver.DiffDriver
	supportsDType bool
	locker        *locker.Locker
}

var (
	backingFs             = "<unknown>"
	projectQuotaSupported = false

	useNaiveDiffLock sync.Once
	useNaiveDiffOnly bool
)

func init() {
	graphdriver.Register("overlay", Init)
	graphdriver.Register("overlay2", Init)
}

// Init returns the a native diff driver for overlay filesystem.
// If overlay filesystem is not supported on the host, graphdriver.ErrNotSupported is returned as error.
// If an overlay filesystem is not supported over an existing filesystem then error graphdriver.ErrIncompatibleFS is returned.
func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {
	opts, err := parseOptions(options)
	if err != nil {
		return nil, err
	}

	if err := supportsOverlay(); err != nil {
		return nil, errors.Wrap(graphdriver.ErrNotSupported, "kernel does not support overlay fs")
	}

	// require kernel 4.0.0 to ensure multiple lower dirs are supported
	v, err := kernel.GetKernelVersion()
	if err != nil {
		return nil, err
	}
	if kernel.CompareKernelVersion(*v, kernel.VersionInfo{Kernel: 4, Major: 0, Minor: 0}) < 0 {
		if !opts.overrideKernelCheck {
			return nil, errors.Wrap(graphdriver.ErrNotSupported, "kernel too old to provide multiple lowers feature for overlay")
		}
		logrus.Warn("Using pre-4.0.0 kernel for overlay, mount failures may require kernel update")
	}

	fsMagic, err := graphdriver.GetFSMagic(home)
	if err != nil {
		return nil, err
	}
	if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
		backingFs = fsName
	}

	// check if they are running over btrfs, aufs, zfs, overlay, or ecryptfs
	switch fsMagic {
	case graphdriver.FsMagicAufs, graphdriver.FsMagicZfs, graphdriver.FsMagicOverlay, graphdriver.FsMagicEcryptfs:
		logrus.Errorf("'overlay' is not supported over %s", backingFs)
		return nil, errors.Wrapf(graphdriver.ErrIncompatibleFS, "'overlay' is not supported over %s", backingFs)
	case graphdriver.FsMagicBtrfs:
		// Support for OverlayFS on BTRFS was added in kernel 4.7
		// See https://btrfs.wiki.kernel.org/index.php/Changelog
		if kernel.CompareKernelVersion(*v, kernel.VersionInfo{Kernel: 4, Major: 7, Minor: 0}) < 0 {
			if !opts.overrideKernelCheck {
				logrus.Errorf("'overlay' requires kernel 4.7 to use on %s", backingFs)
				return nil, errors.Wrapf(graphdriver.ErrIncompatibleFS, "'overlay' requires kernel 4.7 to use on %s", backingFs)
			}
			logrus.Warn("Using pre-4.7.0 kernel for overlay on btrfs, may require kernel update")
		}
	}

	rootUID, rootGID, err := idtools.GetRootUIDGID(uidMaps, gidMaps)
	if err != nil {
		return nil, err
	}
	// Create the driver home dir
	if err := idtools.MkdirAllAs(path.Join(home, linkDir), 0700, rootUID, rootGID); err != nil && !os.IsExist(err) {
		return nil, err
	}

	if err := mount.MakePrivate(home); err != nil {
		return nil, err
	}

	supportsDType, err := fsutils.SupportsDType(home)
	if err != nil {
		return nil, err
	}
	if !supportsDType {
		logrus.Warn(overlayutils.ErrDTypeNotSupported("overlay", backingFs))
		// TODO: Will make fatal when CRI-O Has AMI built on RHEL7.4
		// return nil, overlayutils.ErrDTypeNotSupported("overlay", backingFs)
	}

	d := &Driver{
		name:          "overlay",
		home:          home,
		uidMaps:       uidMaps,
		gidMaps:       gidMaps,
		ctr:           graphdriver.NewRefCounter(graphdriver.NewFsChecker(graphdriver.FsMagicOverlay)),
		supportsDType: supportsDType,
		locker:        locker.New(),
		options:       *opts,
	}

	d.naiveDiff = graphdriver.NewNaiveDiffDriver(d, uidMaps, gidMaps)

	if backingFs == "xfs" {
		// Try to enable project quota support over xfs.
		if d.quotaCtl, err = quota.NewControl(home); err == nil {
			projectQuotaSupported = true
		} else if opts.quota.Size > 0 {
			return nil, fmt.Errorf("Storage option overlay.size not supported. Filesystem does not support Project Quota: %v", err)
		}
	} else if opts.quota.Size > 0 {
		// if xfs is not the backing fs then error out if the storage-opt overlay.size is used.
		return nil, fmt.Errorf("Storage Option overlay.size only supported for backingFS XFS. Found %v", backingFs)
	}

	logrus.Debugf("backingFs=%s,  projectQuotaSupported=%v", backingFs, projectQuotaSupported)

	return d, nil
}

func parseOptions(options []string) (*overlayOptions, error) {
	o := &overlayOptions{}
	for _, option := range options {
		key, val, err := parsers.ParseKeyValueOpt(option)
		if err != nil {
			return nil, err
		}
		key = strings.ToLower(key)
		switch key {
		case "overlay.override_kernel_check", "overlay2.override_kernel_check":
			logrus.Debugf("overlay: override_kernelcheck=%s", val)
			o.overrideKernelCheck, err = strconv.ParseBool(val)
			if err != nil {
				return nil, err
			}
		case "overlay.size", "overlay2.size":
			logrus.Debugf("overlay: size=%s", val)
			size, err := units.RAMInBytes(val)
			if err != nil {
				return nil, err
			}
			o.quota.Size = uint64(size)
		case "overlay.imagestore", "overlay2.imagestore":
			logrus.Debugf("overlay: imagestore=%s", val)
			// Additional read only image stores to use for lower paths
			for _, store := range strings.Split(val, ",") {
				store = filepath.Clean(store)
				if !filepath.IsAbs(store) {
					return nil, fmt.Errorf("overlay: image path %q is not absolute.  Can not be relative", store)
				}
				st, err := os.Stat(store)
				if err != nil {
					return nil, fmt.Errorf("overlay: can't stat imageStore dir %s: %v", store, err)
				}
				if !st.IsDir() {
					return nil, fmt.Errorf("overlay: image path %q must be a directory", store)
				}
				o.imageStores = append(o.imageStores, store)
			}
		default:
			return nil, fmt.Errorf("overlay: Unknown option %s", key)
		}
	}
	return o, nil
}

func supportsOverlay() error {
	// We can try to modprobe overlay first before looking at
	// proc/filesystems for when overlay is supported
	exec.Command("modprobe", "overlay").Run()

	f, err := os.Open("/proc/filesystems")
	if err != nil {
		return err
	}
	defer f.Close()

	s := bufio.NewScanner(f)
	for s.Scan() {
		if s.Text() == "nodev\toverlay" {
			return nil
		}
	}
	logrus.Error("'overlay' not found as a supported filesystem on this host. Please ensure kernel is new enough and has overlay support loaded.")
	return errors.Wrap(graphdriver.ErrNotSupported, "'overlay' not found as a supported filesystem on this host. Please ensure kernel is new enough and has overlay support loaded.")
}

func useNaiveDiff(home string) bool {
	useNaiveDiffLock.Do(func() {
		if err := doesSupportNativeDiff(home); err != nil {
			logrus.Warnf("Not using native diff for overlay, this may cause degraded performance for building images: %v", err)
			useNaiveDiffOnly = true
		}
	})
	return useNaiveDiffOnly
}

func (d *Driver) String() string {
	return d.name
}

// Status returns current driver information in a two dimensional string array.
// Output contains "Backing Filesystem" used in this implementation.
func (d *Driver) Status() [][2]string {
	return [][2]string{
		{"Backing Filesystem", backingFs},
		{"Supports d_type", strconv.FormatBool(d.supportsDType)},
		{"Native Overlay Diff", strconv.FormatBool(!useNaiveDiff(d.home))},
	}
}

// Metadata returns meta data about the overlay driver such as
// LowerDir, UpperDir, WorkDir and MergeDir used to store data.
func (d *Driver) Metadata(id string) (map[string]string, error) {
	dir := d.dir(id)
	if _, err := os.Stat(dir); err != nil {
		return nil, err
	}

	metadata := map[string]string{
		"WorkDir":   path.Join(dir, "work"),
		"MergedDir": path.Join(dir, "merged"),
		"UpperDir":  path.Join(dir, "diff"),
	}

	lowerDirs, err := d.getLowerDirs(id)
	if err != nil {
		return nil, err
	}
	if len(lowerDirs) > 0 {
		metadata["LowerDir"] = strings.Join(lowerDirs, ":")
	}

	return metadata, nil
}

// Cleanup any state created by overlay which should be cleaned when daemon
// is being shutdown. For now, we just have to unmount the bind mounted
// we had created.
func (d *Driver) Cleanup() error {
	return mount.Unmount(d.home)
}

// CreateReadWrite creates a layer that is writable for use as a container
// file system.
func (d *Driver) CreateReadWrite(id, parent string, opts *graphdriver.CreateOpts) error {
	if opts != nil && len(opts.StorageOpt) != 0 && !projectQuotaSupported {
		return fmt.Errorf("--storage-opt is supported only for overlay over xfs with 'pquota' mount option")
	}

	if opts == nil {
		opts = &graphdriver.CreateOpts{
			StorageOpt: map[string]string{},
		}
	}

	if _, ok := opts.StorageOpt["size"]; !ok {
		if opts.StorageOpt == nil {
			opts.StorageOpt = map[string]string{}
		}
		opts.StorageOpt["size"] = strconv.FormatUint(d.options.quota.Size, 10)
	}

	return d.create(id, parent, opts)
}

// Create is used to create the upper, lower, and merge directories required for overlay fs for a given id.
// The parent filesystem is used to configure these directories for the overlay.
func (d *Driver) Create(id, parent string, opts *graphdriver.CreateOpts) (retErr error) {
	if opts != nil && len(opts.StorageOpt) != 0 {
		if _, ok := opts.StorageOpt["size"]; ok {
			return fmt.Errorf("--storage-opt size is only supported for ReadWrite Layers")
		}
	}
	return d.create(id, parent, opts)
}

func (d *Driver) create(id, parent string, opts *graphdriver.CreateOpts) (retErr error) {
	dir := d.dir(id)

	rootUID, rootGID, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
	if err != nil {
		return err
	}
	if err := idtools.MkdirAllAs(path.Dir(dir), 0700, rootUID, rootGID); err != nil {
		return err
	}
	if err := idtools.MkdirAs(dir, 0700, rootUID, rootGID); err != nil {
		return err
	}

	defer func() {
		// Clean up on failure
		if retErr != nil {
			os.RemoveAll(dir)
		}
	}()

	if opts != nil && len(opts.StorageOpt) > 0 {
		driver := &Driver{}
		if err := d.parseStorageOpt(opts.StorageOpt, driver); err != nil {
			return err
		}

		if driver.options.quota.Size > 0 {
			// Set container disk quota limit
			if err := d.quotaCtl.SetQuota(dir, driver.options.quota); err != nil {
				return err
			}
		}
	}

	if err := idtools.MkdirAs(path.Join(dir, "diff"), 0755, rootUID, rootGID); err != nil {
		return err
	}

	lid := generateID(idLength)
	if err := os.Symlink(path.Join("..", id, "diff"), path.Join(d.home, linkDir, lid)); err != nil {
		return err
	}

	// Write link id to link file
	if err := ioutil.WriteFile(path.Join(dir, "link"), []byte(lid), 0644); err != nil {
		return err
	}

	// if no parent directory, done
	if parent == "" {
		return nil
	}

	if err := idtools.MkdirAs(path.Join(dir, "work"), 0700, rootUID, rootGID); err != nil {
		return err
	}
	if err := idtools.MkdirAs(path.Join(dir, "merged"), 0700, rootUID, rootGID); err != nil {
		return err
	}

	lower, err := d.getLower(parent)
	if err != nil {
		return err
	}
	if lower != "" {
		if err := ioutil.WriteFile(path.Join(dir, lowerFile), []byte(lower), 0666); err != nil {
			return err
		}
	}

	return nil
}

// Parse overlay storage options
func (d *Driver) parseStorageOpt(storageOpt map[string]string, driver *Driver) error {
	// Read size to set the disk project quota per container
	for key, val := range storageOpt {
		key := strings.ToLower(key)
		switch key {
		case "size":
			size, err := units.RAMInBytes(val)
			if err != nil {
				return err
			}
			driver.options.quota.Size = uint64(size)
		default:
			return fmt.Errorf("Unknown option %s", key)
		}
	}

	return nil
}

func (d *Driver) getLower(parent string) (string, error) {
	parentDir := d.dir(parent)

	// Ensure parent exists
	if _, err := os.Lstat(parentDir); err != nil {
		return "", err
	}

	// Read Parent link fileA
	parentLink, err := ioutil.ReadFile(path.Join(parentDir, "link"))
	if err != nil {
		return "", err
	}
	lowers := []string{path.Join(linkDir, string(parentLink))}

	parentLower, err := ioutil.ReadFile(path.Join(parentDir, lowerFile))
	if err == nil {
		parentLowers := strings.Split(string(parentLower), ":")
		lowers = append(lowers, parentLowers...)
	}
	if len(lowers) > maxDepth {
		return "", errors.New("max depth exceeded")
	}
	return strings.Join(lowers, ":"), nil
}

func (d *Driver) dir(id string) string {
	newpath := path.Join(d.home, id)
	if _, err := os.Stat(newpath); err != nil {
		for _, p := range d.AdditionalImageStores() {
			l := path.Join(p, d.name, id)
			_, err = os.Stat(l)
			if err == nil {
				return l
			}
		}
	}
	return newpath
}

func (d *Driver) getLowerDirs(id string) ([]string, error) {
	var lowersArray []string
	lowers, err := ioutil.ReadFile(path.Join(d.dir(id), lowerFile))
	if err == nil {
		for _, s := range strings.Split(string(lowers), ":") {
			lower := d.dir(s)
			lp, err := os.Readlink(lower)
			if err != nil {
				return nil, err
			}
			lowersArray = append(lowersArray, path.Clean(d.dir(path.Join("link", lp))))
		}
	} else if !os.IsNotExist(err) {
		return nil, err
	}
	return lowersArray, nil
}

// Remove cleans the directories that are created for this id.
func (d *Driver) Remove(id string) error {
	d.locker.Lock(id)
	defer d.locker.Unlock(id)
	dir := d.dir(id)
	lid, err := ioutil.ReadFile(path.Join(dir, "link"))
	if err == nil {
		if err := os.RemoveAll(path.Join(d.home, linkDir, string(lid))); err != nil {
			logrus.Debugf("Failed to remove link: %v", err)
		}
	}

	if err := system.EnsureRemoveAll(dir); err != nil && !os.IsNotExist(err) {
		return err
	}
	return nil
}

// Get creates and mounts the required file system for the given id and returns the mount path.
func (d *Driver) Get(id, mountLabel string) (_ string, retErr error) {
	d.locker.Lock(id)
	defer d.locker.Unlock(id)
	dir := d.dir(id)
	if _, err := os.Stat(dir); err != nil {
		return "", err
	}

	diffDir := path.Join(dir, "diff")
	lowers, err := ioutil.ReadFile(path.Join(dir, lowerFile))
	if err != nil {
		// If no lower, just return diff directory
		if os.IsNotExist(err) {
			return diffDir, nil
		}
		return "", err
	}

	newlowers := ""
	for _, l := range strings.Split(string(lowers), ":") {
		lower := ""
		newpath := path.Join(d.home, l)
		if _, err := os.Stat(newpath); err != nil {
			for _, p := range d.AdditionalImageStores() {
				lower = path.Join(p, d.name, l)
				if _, err2 := os.Stat(lower); err2 == nil {
					break
				}
				lower = ""
			}
			if lower == "" {
				return "", fmt.Errorf("Can't stat lower layer %q: %v", newpath, err)
			}
		} else {
			lower = newpath
		}
		if newlowers == "" {
			newlowers = lower
		} else {
			newlowers = newlowers + ":" + lower
		}
	}

	mergedDir := path.Join(dir, "merged")
	if count := d.ctr.Increment(mergedDir); count > 1 {
		return mergedDir, nil
	}
	defer func() {
		if retErr != nil {
			if c := d.ctr.Decrement(mergedDir); c <= 0 {
				if mntErr := unix.Unmount(mergedDir, 0); mntErr != nil {
					logrus.Errorf("error unmounting %v: %v", mergedDir, mntErr)
				}
			}
		}
	}()

	workDir := path.Join(dir, "work")
	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", newlowers, diffDir, workDir)
	mountData := label.FormatMountLabel(opts, mountLabel)
	mount := unix.Mount
	mountTarget := mergedDir

	pageSize := unix.Getpagesize()

	// Use relative paths and mountFrom when the mount data has exceeded
	// the page size. The mount syscall fails if the mount data cannot
	// fit within a page and relative links make the mount data much
	// smaller at the expense of requiring a fork exec to chroot.
	if len(mountData) > pageSize {
		//FIXME: We need to figure out to get this to work with additional stores
		opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", string(lowers), path.Join(id, "diff"), path.Join(id, "work"))
		mountData = label.FormatMountLabel(opts, mountLabel)
		if len(mountData) > pageSize {
			return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountData))
		}

		mount = func(source string, target string, mType string, flags uintptr, label string) error {
			return mountFrom(d.home, source, target, mType, flags, label)
		}
		mountTarget = path.Join(id, "merged")
	}
	if err := mount("overlay", mountTarget, "overlay", 0, mountData); err != nil {
		return "", fmt.Errorf("error creating overlay mount to %s: %v", mountTarget, err)
	}

	// chown "workdir/work" to the remapped root UID/GID. Overlay fs inside a
	// user namespace requires this to move a directory from lower to upper.
	rootUID, rootGID, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
	if err != nil {
		return "", err
	}

	if err := os.Chown(path.Join(workDir, "work"), rootUID, rootGID); err != nil {
		return "", err
	}

	return mergedDir, nil
}

// Put unmounts the mount path created for the give id.
func (d *Driver) Put(id string) error {
	d.locker.Lock(id)
	defer d.locker.Unlock(id)
	mountpoint := path.Join(d.dir(id), "merged")
	if count := d.ctr.Decrement(mountpoint); count > 0 {
		return nil
	}
	if err := unix.Unmount(mountpoint, unix.MNT_DETACH); err != nil {
		logrus.Debugf("Failed to unmount %s overlay: %s - %v", id, mountpoint, err)
	}
	return nil
}

// Exists checks to see if the id is already mounted.
func (d *Driver) Exists(id string) bool {
	_, err := os.Stat(d.dir(id))
	return err == nil
}

// isParent returns if the passed in parent is the direct parent of the passed in layer
func (d *Driver) isParent(id, parent string) bool {
	lowers, err := d.getLowerDirs(id)
	if err != nil {
		return false
	}
	if parent == "" && len(lowers) > 0 {
		return false
	}

	parentDir := d.dir(parent)
	var ld string
	if len(lowers) > 0 {
		ld = filepath.Dir(lowers[0])
	}
	if ld == "" && parent == "" {
		return true
	}
	return ld == parentDir
}

// ApplyDiff applies the new layer into a root
func (d *Driver) ApplyDiff(id string, parent string, diff io.Reader) (size int64, err error) {
	if !d.isParent(id, parent) {
		return d.naiveDiff.ApplyDiff(id, parent, diff)
	}

	applyDir := d.getDiffPath(id)

	logrus.Debugf("Applying tar in %s", applyDir)
	// Overlay doesn't need the parent id to apply the diff
	if err := untar(diff, applyDir, &archive.TarOptions{
		UIDMaps:        d.uidMaps,
		GIDMaps:        d.gidMaps,
		WhiteoutFormat: archive.OverlayWhiteoutFormat,
	}); err != nil {
		return 0, err
	}

	return directory.Size(applyDir)
}

func (d *Driver) getDiffPath(id string) string {
	dir := d.dir(id)

	return path.Join(dir, "diff")
}

// DiffSize calculates the changes between the specified id
// and its parent and returns the size in bytes of the changes
// relative to its base filesystem directory.
func (d *Driver) DiffSize(id, parent string) (size int64, err error) {
	if useNaiveDiff(d.home) || !d.isParent(id, parent) {
		return d.naiveDiff.DiffSize(id, parent)
	}
	return directory.Size(d.getDiffPath(id))
}

// Diff produces an archive of the changes between the specified
// layer and its parent layer which may be "".
func (d *Driver) Diff(id, parent string) (io.ReadCloser, error) {
	if useNaiveDiff(d.home) || !d.isParent(id, parent) {
		return d.naiveDiff.Diff(id, parent)
	}

	diffPath := d.getDiffPath(id)
	logrus.Debugf("Tar with options on %s", diffPath)
	return archive.TarWithOptions(diffPath, &archive.TarOptions{
		Compression:    archive.Uncompressed,
		UIDMaps:        d.uidMaps,
		GIDMaps:        d.gidMaps,
		WhiteoutFormat: archive.OverlayWhiteoutFormat,
	})
}

// Changes produces a list of changes between the specified layer
// and its parent layer. If parent is "", then all changes will be ADD changes.
func (d *Driver) Changes(id, parent string) ([]archive.Change, error) {
	if useNaiveDiff(d.home) || !d.isParent(id, parent) {
		return d.naiveDiff.Changes(id, parent)
	}
	// Overlay doesn't have snapshots, so we need to get changes from all parent
	// layers.
	diffPath := d.getDiffPath(id)
	layers, err := d.getLowerDirs(id)
	if err != nil {
		return nil, err
	}

	return archive.OverlayChanges(layers, diffPath)
}

// AdditionalImageStores returns additional image stores supported by the driver
func (d *Driver) AdditionalImageStores() []string {
	return d.options.imageStores
}