overlayfs.go

// +build linux

package overlayfs

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

	log "github.com/Sirupsen/logrus"
	"github.com/docker/docker/daemon/graphdriver"
	"github.com/docker/docker/pkg/archive"
	"github.com/docker/libcontainer/label"
)

// This is a small wrapper over the NaiveDiffWriter that lets us have a custom
// implementation of ApplyDiff()

var (
	ErrApplyDiffFallback = fmt.Errorf("Fall back to normal ApplyDiff")
)

type ApplyDiffProtoDriver interface {
	graphdriver.ProtoDriver
	ApplyDiff(id, parent string, diff archive.ArchiveReader) (bytes int64, err error)
}

type naiveDiffDriverWithApply struct {
	graphdriver.Driver
	applyDiff ApplyDiffProtoDriver
}

func NaiveDiffDriverWithApply(driver ApplyDiffProtoDriver) graphdriver.Driver {
	return &naiveDiffDriverWithApply{
		Driver:    graphdriver.NaiveDiffDriver(driver),
		applyDiff: driver,
	}
}

func (d *naiveDiffDriverWithApply) ApplyDiff(id, parent string, diff archive.ArchiveReader) (int64, error) {
	b, err := d.applyDiff.ApplyDiff(id, parent, diff)
	if err == ErrApplyDiffFallback {
		return d.Driver.ApplyDiff(id, parent, diff)
	}
	return b, err
}

// This backend uses the overlayfs union filesystem for containers
// plus hard link file sharing for images.

// Each container/image can have a "root" subdirectory which is a plain
// filesystem hierarchy, or they can use overlayfs.

// If they use overlayfs there is a "upper" directory and a "lower-id"
// file, as well as "merged" and "work" directories. The "upper"
// directory has the upper layer of the overlay, and "lower-id" contains
// the id of the parent whose "root" directory shall be used as the lower
// layer in the overlay. The overlay itself is mounted in the "merged"
// directory, and the "work" dir is needed for overlayfs to work.

// When a overlay layer is created there are two cases, either the
// parent has a "root" dir, then we start out with a empty "upper"
// directory overlaid on the parents root. This is typically the
// case with the init layer of a container which is based on an image.
// If there is no "root" in the parent, we inherit the lower-id from
// the parent and start by making a copy if the parents "upper" dir.
// This is typically the case for a container layer which copies
// its parent -init upper layer.

// Additionally we also have a custom implementation of ApplyLayer
// which makes a recursive copy of the parent "root" layer using
// hardlinks to share file data, and then applies the layer on top
// of that. This means all child images share file (but not directory)
// data with the parent.

type ActiveMount struct {
	count   int
	path    string
	mounted bool
}
type Driver struct {
	home       string
	sync.Mutex // Protects concurrent modification to active
	active     map[string]*ActiveMount
}

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

func Init(home string, options []string) (graphdriver.Driver, error) {
	if err := supportsOverlayfs(); err != nil {
		return nil, graphdriver.ErrNotSupported
	}

	// Create the driver home dir
	if err := os.MkdirAll(home, 0755); err != nil && !os.IsExist(err) {
		return nil, err
	}

	d := &Driver{
		home:   home,
		active: make(map[string]*ActiveMount),
	}

	return NaiveDiffDriverWithApply(d), nil
}

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

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

	s := bufio.NewScanner(f)
	for s.Scan() {
		if strings.Contains(s.Text(), "overlayfs") {
			return nil
		}
	}
	log.Error("'overlayfs' not found as a supported filesystem on this host. Please ensure kernel is new enough and has overlayfs support loaded.")
	return graphdriver.ErrNotSupported
}

func (d *Driver) String() string {
	return "overlayfs"
}

func (d *Driver) Status() [][2]string {
	return nil
}

func (d *Driver) Cleanup() error {
	return nil
}

func (d *Driver) Create(id string, parent string) (retErr error) {
	dir := d.dir(id)
	if err := os.MkdirAll(path.Dir(dir), 0700); err != nil {
		return err
	}
	if err := os.Mkdir(dir, 0700); err != nil {
		return err
	}

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

	// Toplevel images are just a "root" dir
	if parent == "" {
		if err := os.Mkdir(path.Join(dir, "root"), 0755); err != nil {
			return err
		}
		return nil
	}

	parentDir := d.dir(parent)

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

	// If parent has a root, just do a overlayfs to it
	parentRoot := path.Join(parentDir, "root")

	if s, err := os.Lstat(parentRoot); err == nil {
		if err := os.Mkdir(path.Join(dir, "upper"), s.Mode()); err != nil {
			return err
		}
		if err := os.Mkdir(path.Join(dir, "work"), 0700); err != nil {
			return err
		}
		if err := os.Mkdir(path.Join(dir, "merged"), 0700); err != nil {
			return err
		}
		if err := ioutil.WriteFile(path.Join(dir, "lower-id"), []byte(parent), 0666); err != nil {
			return err
		}
		return nil
	}

	// Otherwise, copy the upper and the lower-id from the parent

	lowerId, err := ioutil.ReadFile(path.Join(parentDir, "lower-id"))
	if err != nil {
		return err
	}

	if err := ioutil.WriteFile(path.Join(dir, "lower-id"), lowerId, 0666); err != nil {
		return err
	}

	parentUpperDir := path.Join(parentDir, "upper")
	s, err := os.Lstat(parentUpperDir)
	if err != nil {
		return err
	}

	upperDir := path.Join(dir, "upper")
	if err := os.Mkdir(upperDir, s.Mode()); err != nil {
		return err
	}
	if err := os.Mkdir(path.Join(dir, "work"), 0700); err != nil {
		return err
	}
	if err := os.Mkdir(path.Join(dir, "merged"), 0700); err != nil {
		return err
	}

	return copyDir(parentUpperDir, upperDir, 0)
}

func (d *Driver) dir(id string) string {
	return path.Join(d.home, id)
}

func (d *Driver) Remove(id string) error {
	dir := d.dir(id)
	if _, err := os.Stat(dir); err != nil {
		return err
	}
	return os.RemoveAll(dir)
}

func (d *Driver) Get(id string, mountLabel string) (string, error) {
	// Protect the d.active from concurrent access
	d.Lock()
	defer d.Unlock()

	mount := d.active[id]
	if mount != nil {
		mount.count++
		return mount.path, nil
	} else {
		mount = &ActiveMount{count: 1}
	}

	dir := d.dir(id)
	if _, err := os.Stat(dir); err != nil {
		return "", err
	}

	// If id has a root, just return it
	rootDir := path.Join(dir, "root")
	if _, err := os.Stat(rootDir); err == nil {
		mount.path = rootDir
		d.active[id] = mount
		return mount.path, nil
	}

	lowerId, err := ioutil.ReadFile(path.Join(dir, "lower-id"))
	if err != nil {
		return "", err
	}
	lowerDir := path.Join(d.dir(string(lowerId)), "root")
	upperDir := path.Join(dir, "upper")
	workDir := path.Join(dir, "work")
	mergedDir := path.Join(dir, "merged")

	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", lowerDir, upperDir, workDir)
	if err := syscall.Mount("overlayfs", mergedDir, "overlayfs", 0, label.FormatMountLabel(opts, mountLabel)); err != nil {
		return "", err
	}
	mount.path = mergedDir
	mount.mounted = true
	d.active[id] = mount

	return mount.path, nil
}

func (d *Driver) Put(id string) {
	// Protect the d.active from concurrent access
	d.Lock()
	defer d.Unlock()

	mount := d.active[id]
	if mount == nil {
		log.Debugf("Put on a non-mounted device %s", id)
		return
	}

	mount.count--
	if mount.count > 0 {
		return
	}

	if mount.mounted {
		if err := syscall.Unmount(mount.path, 0); err != nil {
			log.Debugf("Failed to unmount %s overlayfs: %v", id, err)
		}
	}

	delete(d.active, id)
}

func (d *Driver) ApplyDiff(id string, parent string, diff archive.ArchiveReader) (bytes int64, err error) {
	dir := d.dir(id)

	if parent == "" {
		return 0, ErrApplyDiffFallback
	}

	parentRootDir := path.Join(d.dir(parent), "root")
	if _, err := os.Stat(parentRootDir); err != nil {
		return 0, ErrApplyDiffFallback
	}

	// We now know there is a parent, and it has a "root" directory containing
	// the full root filesystem. We can just hardlink it and apply the
	// layer. This relies on two things:
	// 1) ApplyDiff is only run once on a clean (no writes to upper layer) container
	// 2) ApplyDiff doesn't do any in-place writes to files (would break hardlinks)
	// These are all currently true and are not expected to break

	tmpRootDir, err := ioutil.TempDir(dir, "tmproot")
	if err != nil {
		return 0, err
	}
	defer func() {
		if err != nil {
			os.RemoveAll(tmpRootDir)
		} else {
			os.RemoveAll(path.Join(dir, "upper"))
			os.RemoveAll(path.Join(dir, "work"))
			os.RemoveAll(path.Join(dir, "merged"))
			os.RemoveAll(path.Join(dir, "lower-id"))
		}
	}()

	if err = copyDir(parentRootDir, tmpRootDir, CopyHardlink); err != nil {
		return 0, err
	}

	if err := archive.ApplyLayer(tmpRootDir, diff); err != nil {
		return 0, err
	}

	rootDir := path.Join(dir, "root")
	if err := os.Rename(tmpRootDir, rootDir); err != nil {
		return 0, err
	}

	changes, err := archive.ChangesDirs(rootDir, parentRootDir)
	if err != nil {
		return 0, err
	}

	return archive.ChangesSize(rootDir, changes), nil
}

func (d *Driver) Exists(id string) bool {
	_, err := os.Stat(d.dir(id))
	return err == nil
}