Implement overlay fs snapshot driver

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

snapshot/overlayfs.go

@@ -0,0 +1,157 @@
+package snapshot
+
+import (
+	"crypto/md5"
+	"encoding/hex"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/docker/containerd"
+)
+
+func NewOverlayfs(root string) (*Overlayfs, error) {
+	if err := os.MkdirAll(root, 0700); err != nil {
+		return nil, err
+	}
+	for _, p := range []string{
+		"snapshots",
+		"active",
+	} {
+		if err := os.MkdirAll(filepath.Join(root, p), 0700); err != nil {
+			return nil, err
+		}
+	}
+	return &Overlayfs{
+		root: root,
+	}, nil
+}
+
+type Overlayfs struct {
+	root string
+}
+
+func (o *Overlayfs) Prepare(key string, parentName string) ([]containerd.Mount, error) {
+	if err := validKey(key); err != nil {
+		return nil, err
+	}
+	active, err := o.newActiveDir(key)
+	if err != nil {
+		return nil, err
+	}
+	if parentName != "" {
+		if err := active.setParent(parentName); err != nil {
+			return nil, err
+		}
+	}
+	return active.mounts()
+}
+
+func (o *Overlayfs) Commit(key string, name string) error {
+	active := o.getActive(key)
+	return active.commit(name)
+}
+
+func (o *Overlayfs) newActiveDir(key string) (*activeDir, error) {
+	var (
+		hash = hash(key)
+		path = filepath.Join(o.root, "active", hash)
+	)
+	a := &activeDir{
+		path:         path,
+		snapshotsDir: filepath.Join(o.root, "snapshots"),
+	}
+	for _, p := range []string{
+		"work",
+		"fs",
+	} {
+		if err := os.MkdirAll(filepath.Join(path, p), 0700); err != nil {
+			a.delete()
+			return nil, err
+		}
+	}
+	return a, nil
+}
+
+func (o *Overlayfs) getActive(key string) *activeDir {
+	return &activeDir{
+		path:         filepath.Join(o.root, "active", hash(key)),
+		snapshotsDir: filepath.Join(o.root, "snapshots"),
+	}
+}
+
+func validKey(key string) error {
+	_, err := filepath.Abs(key)
+	return err
+}
+
+func hash(k string) string {
+	h := md5.New()
+	h.Write([]byte(k))
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+type activeDir struct {
+	snapshotsDir string
+	path         string
+}
+
+func (a *activeDir) delete() error {
+	return os.RemoveAll(a.path)
+}
+
+func (a *activeDir) setParent(name string) error {
+	return os.Symlink(filepath.Join(a.snapshotsDir, name), filepath.Join(a.path, "parent"))
+}
+
+func (a *activeDir) commit(name string) error {
+	if err := os.RemoveAll(filepath.Join(a.path, "work")); err != nil {
+		return err
+	}
+	return os.Rename(a.path, filepath.Join(a.snapshotsDir, name))
+}
+
+func (a *activeDir) mounts() ([]containerd.Mount, error) {
+	var (
+		parentLink = filepath.Join(a.path, "parent")
+		parents    []string
+	)
+	for {
+		snapshot, err := os.Readlink(parentLink)
+		if err != nil {
+			if os.IsNotExist(err) {
+				break
+			}
+			return nil, err
+		}
+		parents = append(parents, filepath.Join(snapshot, "fs"))
+		parentLink = filepath.Join(snapshot, "parent")
+	}
+	if len(parents) == 0 {
+		// if we only have one layer/no parents then just return a bind mount as overlay
+		// will not work
+		return []containerd.Mount{
+			{
+				Source: filepath.Join(a.path, "fs"),
+				Type:   "bind",
+				Options: []string{
+					"rw",
+					"rbind",
+				},
+			},
+		}, nil
+	}
+	options := []string{
+		fmt.Sprintf("workdir=%s", filepath.Join(a.path, "work")),
+		fmt.Sprintf("upperdir=%s", filepath.Join(a.path, "fs")),
+		fmt.Sprintf("lowerdir=%s", strings.Join(parents, ":")),
+	}
+	return []containerd.Mount{
+		{
+			Type:    "overlay",
+			Source:  "overlay",
+			Options: options,
+		},
+	}, nil
+}