Add separate overlay2 driver

Adds a new overlay driver which uses multiple lower directories to create the union fs.
Additionally it uses symlinks and relative mount paths to allow a depth of 128 and stay within the mount page size limit.
Diffs and done directly over a single directory allowing diffs to be done efficiently and without the need fo the naive diff driver.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

daemon/graphdriver/overlay2/mount.go

@@ -0,0 +1,91 @@
+// +build linux
+
+package overlay2
+
+import (
+	"bytes"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"syscall"
+
+	"github.com/docker/docker/pkg/reexec"
+)
+
+func init() {
+	reexec.Register("docker-mountfrom", mountFromMain)
+}
+
+func fatal(err error) {
+	fmt.Fprint(os.Stderr, err)
+	os.Exit(1)
+}
+
+type mountOptions struct {
+	Device string
+	Target string
+	Type   string
+	Label  string
+	Flag   uint32
+}
+
+func mountFrom(dir, device, target, mType, label string) error {
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		return fmt.Errorf("mountfrom pipe failure: %v", err)
+	}
+
+	options := &mountOptions{
+		Device: device,
+		Target: target,
+		Type:   mType,
+		Flag:   0,
+		Label:  label,
+	}
+
+	cmd := reexec.Command("docker-mountfrom", dir)
+	cmd.Stdin = r
+
+	output := bytes.NewBuffer(nil)
+	cmd.Stdout = output
+	cmd.Stderr = output
+
+	if err := cmd.Start(); err != nil {
+		return fmt.Errorf("mountfrom error on re-exec cmd: %v", err)
+	}
+	//write the options to the pipe for the untar exec to read
+	if err := json.NewEncoder(w).Encode(options); err != nil {
+		return fmt.Errorf("mountfrom json encode to pipe failed: %v", err)
+	}
+	w.Close()
+
+	if err := cmd.Wait(); err != nil {
+		return fmt.Errorf("mountfrom re-exec error: %v: output: %s", err, output)
+	}
+	return nil
+}
+
+// mountfromMain is the entry-point for docker-mountfrom on re-exec.
+func mountFromMain() {
+	runtime.LockOSThread()
+	flag.Parse()
+
+	var options *mountOptions
+
+	if err := json.NewDecoder(os.Stdin).Decode(&options); err != nil {
+		fatal(err)
+	}
+
+	if err := os.Chdir(flag.Arg(0)); err != nil {
+		fatal(err)
+	}
+
+	if err := syscall.Mount(options.Device, options.Target, options.Type, uintptr(options.Flag), options.Label); err != nil {
+		fatal(err)
+	}
+
+	os.Exit(0)
+}

daemon/graphdriver/overlay2/overlay.go

@@ -0,0 +1,476 @@
+// +build linux
+
+package overlay2
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path"
+	"strings"
+	"syscall"
+
+	"github.com/Sirupsen/logrus"
+
+	"github.com/docker/docker/daemon/graphdriver"
+	"github.com/docker/docker/pkg/archive"
+	"github.com/docker/docker/pkg/chrootarchive"
+	"github.com/docker/docker/pkg/directory"
+	"github.com/docker/docker/pkg/idtools"
+	"github.com/docker/docker/pkg/mount"
+	"github.com/docker/docker/pkg/parsers/kernel"
+
+	"github.com/opencontainers/runc/libcontainer/label"
+)
+
+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 (
+	driverName = "overlay2"
+	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
+)
+
+// Driver contains information about the home directory and the list of active mounts that are created using this driver.
+type Driver struct {
+	home    string
+	uidMaps []idtools.IDMap
+	gidMaps []idtools.IDMap
+	ctr     *graphdriver.RefCounter
+}
+
+var backingFs = "<unknown>"
+
+func init() {
+	graphdriver.Register(driverName, 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 a overlay filesystem is not supported over a existing filesystem then error graphdriver.ErrIncompatibleFS is returned.
+func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {
+
+	if err := supportsOverlay(); err != nil {
+		return nil, graphdriver.ErrNotSupported
+	}
+
+	// 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 {
+		return nil, graphdriver.ErrNotSupported
+	}
+
+	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 or overlay
+	switch fsMagic {
+	case graphdriver.FsMagicBtrfs:
+		logrus.Error("'overlay' is not supported over btrfs.")
+		return nil, graphdriver.ErrIncompatibleFS
+	case graphdriver.FsMagicAufs:
+		logrus.Error("'overlay' is not supported over aufs.")
+		return nil, graphdriver.ErrIncompatibleFS
+	case graphdriver.FsMagicZfs:
+		logrus.Error("'overlay' is not supported over zfs.")
+		return nil, graphdriver.ErrIncompatibleFS
+	case graphdriver.FsMagicOverlay:
+		logrus.Error("'overlay' is not supported over overlay.")
+		return nil, graphdriver.ErrIncompatibleFS
+	}
+
+	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
+	}
+
+	d := &Driver{
+		home:    home,
+		uidMaps: uidMaps,
+		gidMaps: gidMaps,
+		ctr:     graphdriver.NewRefCounter(graphdriver.NewFsChecker(graphdriver.FsMagicOverlay)),
+	}
+
+	return d, 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 graphdriver.ErrNotSupported
+}
+
+func (d *Driver) String() string {
+	return driverName
+}
+
+// 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},
+	}
+}
+
+// GetMetadata returns meta data about the overlay driver such as
+// LowerDir, UpperDir, WorkDir and MergeDir used to store data.
+func (d *Driver) GetMetadata(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, mountLabel string, storageOpt map[string]string) error {
+	return d.Create(id, parent, mountLabel, storageOpt)
+}
+
+// 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, mountLabel string, storageOpt map[string]string) (retErr error) {
+
+	if len(storageOpt) != 0 {
+		return fmt.Errorf("--storage-opt is not supported for overlay")
+	}
+
+	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 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
+}
+
+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 {
+	return path.Join(d.home, id)
+}
+
+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), ":") {
+			lp, err := os.Readlink(path.Join(d.home, s))
+			if err != nil {
+				return nil, err
+			}
+			lowersArray = append(lowersArray, path.Clean(path.Join(d.home, "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 {
+	if err := os.RemoveAll(d.dir(id)); 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 string, mountLabel string) (s string, err error) {
+	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
+	}
+
+	mergedDir := path.Join(dir, "merged")
+	if count := d.ctr.Increment(mergedDir); count > 1 {
+		return mergedDir, nil
+	}
+	defer func() {
+		if err != nil {
+			if c := d.ctr.Decrement(mergedDir); c <= 0 {
+				syscall.Unmount(mergedDir, 0)
+			}
+		}
+	}()
+
+	workDir := path.Join(dir, "work")
+	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", string(lowers), path.Join(id, "diff"), path.Join(id, "work"))
+	mountLabel = label.FormatMountLabel(opts, mountLabel)
+	if len(mountLabel) > syscall.Getpagesize() {
+		return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountLabel))
+	}
+
+	if err := mountFrom(d.home, "overlay", path.Join(id, "merged"), "overlay", mountLabel); err != nil {
+		return "", fmt.Errorf("error creating overlay mount to %s: %v", mergedDir, 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 {
+	mountpoint := path.Join(d.dir(id), "merged")
+	if count := d.ctr.Decrement(mountpoint); count > 0 {
+		return nil
+	}
+	if err := syscall.Unmount(mountpoint, 0); err != nil {
+		logrus.Debugf("Failed to unmount %s overlay: %v", id, 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
+}
+
+// ApplyDiff applies the new layer into a root
+func (d *Driver) ApplyDiff(id string, parent string, diff archive.Reader) (size int64, err error) {
+	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 d.DiffSize(id, parent)
+}
+
+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) {
+	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) (archive.Archive, error) {
+	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) {
+	// 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)
+}

daemon/graphdriver/overlay2/overlay_test.go

@@ -0,0 +1,106 @@
+// +build linux
+
+package overlay2
+
+import (
+	"os"
+	"syscall"
+	"testing"
+
+	"github.com/docker/docker/daemon/graphdriver"
+	"github.com/docker/docker/daemon/graphdriver/graphtest"
+	"github.com/docker/docker/pkg/archive"
+	"github.com/docker/docker/pkg/reexec"
+)
+
+func init() {
+	// Do not sure chroot to speed run time and allow archive
+	// errors or hangs to be debugged directly from the test process.
+	untar = archive.UntarUncompressed
+	graphdriver.ApplyUncompressedLayer = archive.ApplyUncompressedLayer
+
+	reexec.Init()
+}
+
+func cdMountFrom(dir, device, target, mType, label string) error {
+	wd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+	os.Chdir(dir)
+	defer os.Chdir(wd)
+
+	return syscall.Mount(device, target, mType, 0, label)
+}
+
+// This avoids creating a new driver for each test if all tests are run
+// Make sure to put new tests between TestOverlaySetup and TestOverlayTeardown
+func TestOverlaySetup(t *testing.T) {
+	graphtest.GetDriver(t, driverName)
+}
+
+func TestOverlayCreateEmpty(t *testing.T) {
+	graphtest.DriverTestCreateEmpty(t, driverName)
+}
+
+func TestOverlayCreateBase(t *testing.T) {
+	graphtest.DriverTestCreateBase(t, driverName)
+}
+
+func TestOverlayCreateSnap(t *testing.T) {
+	graphtest.DriverTestCreateSnap(t, driverName)
+}
+
+func TestOverlay128LayerRead(t *testing.T) {
+	graphtest.DriverTestDeepLayerRead(t, 128, driverName)
+}
+
+func TestOverlayDiffApply10Files(t *testing.T) {
+	graphtest.DriverTestDiffApply(t, 10, driverName)
+}
+
+func TestOverlayChanges(t *testing.T) {
+	graphtest.DriverTestChanges(t, driverName)
+}
+
+func TestOverlayTeardown(t *testing.T) {
+	graphtest.PutDriver(t)
+}
+
+// Benchmarks should always setup new driver
+
+func BenchmarkExists(b *testing.B) {
+	graphtest.DriverBenchExists(b, driverName)
+}
+
+func BenchmarkGetEmpty(b *testing.B) {
+	graphtest.DriverBenchGetEmpty(b, driverName)
+}
+
+func BenchmarkDiffBase(b *testing.B) {
+	graphtest.DriverBenchDiffBase(b, driverName)
+}
+
+func BenchmarkDiffSmallUpper(b *testing.B) {
+	graphtest.DriverBenchDiffN(b, 10, 10, driverName)
+}
+
+func BenchmarkDiff10KFileUpper(b *testing.B) {
+	graphtest.DriverBenchDiffN(b, 10, 10000, driverName)
+}
+
+func BenchmarkDiff10KFilesBottom(b *testing.B) {
+	graphtest.DriverBenchDiffN(b, 10000, 10, driverName)
+}
+
+func BenchmarkDiffApply100(b *testing.B) {
+	graphtest.DriverBenchDiffApplyN(b, 100, driverName)
+}
+
+func BenchmarkDiff20Layers(b *testing.B) {
+	graphtest.DriverBenchDeepLayerDiff(b, 20, driverName)
+}
+
+func BenchmarkRead20Layers(b *testing.B) {
+	graphtest.DriverBenchDeepLayerRead(b, 20, driverName)
+}

daemon/graphdriver/overlay2/overlay_unsupported.go

@@ -0,0 +1,3 @@
+// +build !linux
+
+package overlay2

daemon/graphdriver/overlay2/randomid.go

@@ -0,0 +1,80 @@
+// +build linux
+
+package overlay2
+
+import (
+	"crypto/rand"
+	"encoding/base32"
+	"fmt"
+	"io"
+	"os"
+	"syscall"
+	"time"
+
+	"github.com/Sirupsen/logrus"
+)
+
+// generateID creates a new random string identifier with the given length
+func generateID(l int) string {
+	const (
+		// ensures we backoff for less than 450ms total. Use the following to
+		// select new value, in units of 10ms:
+		// 	n*(n+1)/2 = d -> n^2 + n - 2d -> n = (sqrt(8d + 1) - 1)/2
+		maxretries = 9
+		backoff    = time.Millisecond * 10
+	)
+
+	var (
+		totalBackoff time.Duration
+		count        int
+		retries      int
+		size         = (l*5 + 7) / 8
+		u            = make([]byte, size)
+	)
+	// TODO: Include time component, counter component, random component
+
+	for {
+		// This should never block but the read may fail. Because of this,
+		// we just try to read the random number generator until we get
+		// something. This is a very rare condition but may happen.
+		b := time.Duration(retries) * backoff
+		time.Sleep(b)
+		totalBackoff += b
+
+		n, err := io.ReadFull(rand.Reader, u[count:])
+		if err != nil {
+			if retryOnError(err) && retries < maxretries {
+				count += n
+				retries++
+				logrus.Errorf("error generating version 4 uuid, retrying: %v", err)
+				continue
+			}
+
+			// Any other errors represent a system problem. What did someone
+			// do to /dev/urandom?
+			panic(fmt.Errorf("error reading random number generator, retried for %v: %v", totalBackoff.String(), err))
+		}
+
+		break
+	}
+
+	s := base32.StdEncoding.EncodeToString(u)
+
+	return s[:l]
+}
+
+// retryOnError tries to detect whether or not retrying would be fruitful.
+func retryOnError(err error) bool {
+	switch err := err.(type) {
+	case *os.PathError:
+		return retryOnError(err.Err) // unpack the target error
+	case syscall.Errno:
+		if err == syscall.EPERM {
+			// EPERM represents an entropy pool exhaustion, a condition under
+			// which we backoff and retry.
+			return true
+		}
+	}
+
+	return false
+}

daemon/graphdriver/register/register_overlay.go

@@ -5,4 +5,5 @@ package register
 import (
 	// register the overlay graphdriver
 	_ "github.com/docker/docker/daemon/graphdriver/overlay"
+	_ "github.com/docker/docker/daemon/graphdriver/overlay2"
 )