Merge pull request #496 from stevvooe/unify-snapshot-keyspace

snapshot: clarify active and committed snapshots

snapshot/btrfs/btrfs.go

@@ -6,47 +6,33 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/docker/containerd"
-	"github.com/docker/containerd/log"
+	"github.com/docker/containerd/snapshot"
 	"github.com/pkg/errors"
 	"github.com/stevvooe/go-btrfs"
 )
 
-type Driver struct {
+type Snapshotter struct {
 	device string // maybe we can resolve it with path?
 	root   string // root provides paths for internal storage.
 }
 
-func NewDriver(device, root string) (*Driver, error) {
+func NewSnapshotter(device, root string) (*Snapshotter, error) {
-	return &Driver{device: device, root: root}, nil
-}
-
-func (b *Driver) Prepare(key, parent string) ([]containerd.Mount, error) {
-	return b.makeActive(key, parent, false)
-}
-
-func (b *Driver) View(key, parent string) ([]containerd.Mount, error) {
-	return b.makeActive(key, parent, true)
-}
-
-func (b *Driver) makeActive(key, parent string, readonly bool) ([]containerd.Mount, error) {
 	var (
-		active     = filepath.Join(b.root, "active")
+		active    = filepath.Join(root, "active")
-		parents    = filepath.Join(b.root, "parents")
+		snapshots = filepath.Join(root, "snapshots")
-		snapshots  = filepath.Join(b.root, "snapshots")
+		parents   = filepath.Join(root, "parents")
-		names      = filepath.Join(b.root, "names")
+		index     = filepath.Join(root, "index")
-		keyh       = hash(key)
+		names     = filepath.Join(root, "names")
-		parenth    = hash(parent)
-		dir        = filepath.Join(active, keyh)
-		namep      = filepath.Join(names, keyh)
-		parentlink = filepath.Join(parents, keyh)
-		parentp    = filepath.Join(snapshots, parenth)
 	)
 
 	for _, path := range []string{
 		active,
+		snapshots,
 		parents,
+		index,
 		names,
 	} {
 		if err := os.MkdirAll(path, 0755); err != nil {
@@ -54,15 +40,118 @@ func (b *Driver) makeActive(key, parent string, readonly bool) ([]containerd.Mou
 		}
 	}
 
+	return &Snapshotter{device: device, root: root}, nil
+}
+
+// Stat returns the info for an active or committed snapshot by name or
+// key.
+//
+// Should be used for parent resolution, existence checks and to discern
+// the kind of snapshot.
+func (b *Snapshotter) Stat(key string) (snapshot.Info, error) {
+	// resolve the snapshot out of the index.
+	target, err := os.Readlink(filepath.Join(b.root, "index", hash(key)))
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return snapshot.Info{}, err
+		}
+
+		return snapshot.Info{}, errors.Errorf("snapshot %v not found", key)
+	}
+
+	return b.stat(target)
+}
+
+func (b *Snapshotter) stat(target string) (snapshot.Info, error) {
+	var (
+		parents    = filepath.Join(b.root, "parents")
+		names      = filepath.Join(b.root, "names")
+		namep      = filepath.Join(names, filepath.Base(target))
+		parentlink = filepath.Join(parents, filepath.Base(target))
+	)
+
+	// grab information about the subvolume
+	info, err := btrfs.SubvolInfo(target)
+	if err != nil {
+		return snapshot.Info{}, err
+	}
+
+	// read the name out of the names!
+	nameraw, err := ioutil.ReadFile(namep)
+	if err != nil {
+		return snapshot.Info{}, err
+	}
+
+	// resolve the parents path.
+	parentp, err := os.Readlink(parentlink)
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return snapshot.Info{}, err
+		}
+
+		// no parent!
+	}
+
+	var parent string
+	if parentp != "" {
+		// okay, grab the basename of the parent and look up its name!
+		parentnamep := filepath.Join(names, filepath.Base(parentp))
+
+		p, err := ioutil.ReadFile(parentnamep)
+		if err != nil {
+			return snapshot.Info{}, err
+		}
+
+		parent = string(p)
+	}
+
+	kind := snapshot.KindCommitted
+	if strings.HasPrefix(target, filepath.Join(b.root, "active")) {
+		kind = snapshot.KindActive
+	}
+
+	return snapshot.Info{
+		Name:     string(nameraw),
+		Parent:   parent,
+		Readonly: info.Readonly,
+		Kind:     kind,
+	}, nil
+
+}
+
+func (b *Snapshotter) Prepare(key, parent string) ([]containerd.Mount, error) {
+	return b.makeActive(key, parent, false)
+}
+
+func (b *Snapshotter) View(key, parent string) ([]containerd.Mount, error) {
+	return b.makeActive(key, parent, true)
+}
+
+func (b *Snapshotter) makeActive(key, parent string, readonly bool) ([]containerd.Mount, error) {
+	var (
+		active     = filepath.Join(b.root, "active")
+		snapshots  = filepath.Join(b.root, "snapshots")
+		parents    = filepath.Join(b.root, "parents")
+		index      = filepath.Join(b.root, "index")
+		names      = filepath.Join(b.root, "names")
+		keyh       = hash(key)
+		parenth    = hash(parent)
+		target     = filepath.Join(active, keyh)
+		namep      = filepath.Join(names, keyh)
+		indexlink  = filepath.Join(index, keyh)
+		parentlink = filepath.Join(parents, keyh)
+		parentp    = filepath.Join(snapshots, parenth) // parent must be restricted to snaps
+	)
+
 	if parent == "" {
 		// create new subvolume
 		// btrfs subvolume create /dir
-		if err := btrfs.SubvolCreate(dir); err != nil {
+		if err := btrfs.SubvolCreate(target); err != nil {
 			return nil, err
 		}
 	} else {
 		// btrfs subvolume snapshot /parent /subvol
-		if err := btrfs.SubvolSnapshot(dir, parentp, readonly); err != nil {
+		if err := btrfs.SubvolSnapshot(target, parentp, readonly); err != nil {
 			return nil, err
 		}
 
@@ -71,14 +160,19 @@ func (b *Driver) makeActive(key, parent string, readonly bool) ([]containerd.Mou
 		}
 	}
 
+	// write in the name
 	if err := ioutil.WriteFile(namep, []byte(key), 0644); err != nil {
 		return nil, err
 	}
 
-	return b.mounts(dir)
+	if err := os.Symlink(target, indexlink); err != nil {
+		return nil, err
+	}
+
+	return b.mounts(target)
 }
 
-func (b *Driver) mounts(dir string) ([]containerd.Mount, error) {
+func (b *Snapshotter) mounts(dir string) ([]containerd.Mount, error) {
 	var options []string
 
 	// get the subvolume id back out for the mount
@@ -104,12 +198,13 @@ func (b *Driver) mounts(dir string) ([]containerd.Mount, error) {
 	}, nil
 }
 
-func (b *Driver) Commit(name, key string) error {
+func (b *Snapshotter) Commit(name, key string) error {
 	var (
 		active        = filepath.Join(b.root, "active")
 		snapshots     = filepath.Join(b.root, "snapshots")
-		names         = filepath.Join(b.root, "names")
+		index         = filepath.Join(b.root, "index")
 		parents       = filepath.Join(b.root, "parents")
+		names         = filepath.Join(b.root, "names")
 		keyh          = hash(key)
 		nameh         = hash(name)
 		dir           = filepath.Join(active, keyh)
@@ -118,6 +213,8 @@ func (b *Driver) Commit(name, key string) error {
 		namep         = filepath.Join(names, nameh)
 		keyparentlink = filepath.Join(parents, keyh)
 		parentlink    = filepath.Join(parents, nameh)
+		keyindexlink  = filepath.Join(index, keyh)
+		indexlink     = filepath.Join(index, nameh)
 	)
 
 	info, err := btrfs.SubvolInfo(dir)
@@ -143,9 +240,7 @@ func (b *Driver) Commit(name, key string) error {
 		return err
 	}
 
-	fmt.Println("commit snapshot", target)
 	if err := btrfs.SubvolSnapshot(target, dir, true); err != nil {
-		fmt.Println("snapshot error")
 		return err
 	}
 
@@ -154,6 +249,10 @@ func (b *Driver) Commit(name, key string) error {
 		return err
 	}
 
+	if err := os.Remove(keyindexlink); err != nil {
+		return err
+	}
+
 	if err := ioutil.WriteFile(namep, []byte(name), 0755); err != nil {
 		return err
 	}
@@ -169,6 +268,10 @@ func (b *Driver) Commit(name, key string) error {
 		// into common storage, so let's not fret over it for now.
 	}
 
+	if err := os.Symlink(target, indexlink); err != nil {
+		return err
+	}
+
 	if err := btrfs.SubvolDelete(dir); err != nil {
 		return errors.Wrapf(err, "delete subvol failed on %v", dir)
 	}
@@ -180,82 +283,54 @@ func (b *Driver) Commit(name, key string) error {
 // called on an read-write or readonly transaction.
 //
 // This can be used to recover mounts after calling View or Prepare.
-func (b *Driver) Mounts(key string) ([]containerd.Mount, error) {
+func (b *Snapshotter) Mounts(key string) ([]containerd.Mount, error) {
 	dir := filepath.Join(b.root, "active", hash(key))
 	return b.mounts(dir)
 }
 
 // Remove abandons the transaction identified by key. All resources
 // associated with the key will be removed.
-func (b *Driver) Remove(key string) error {
+func (b *Snapshotter) Remove(key string) error {
 	panic("not implemented")
 }
 
-// Parent returns the parent of snapshot identified by name.
-func (b *Driver) Parent(name string) (string, error) {
-	var (
-		parents    = filepath.Join(b.root, "parents")
-		names      = filepath.Join(b.root, "names")
-		parentlink = filepath.Join(parents, hash(name))
-	)
-
-	parentp, err := os.Readlink(parentlink)
-	if err != nil {
-		if !os.IsNotExist(err) {
-			return "", err
-		}
-
-		return "", nil // no parent!
-	}
-
-	// okay, grab the basename of the parent and look up its name!
-	parentnamep := filepath.Join(names, filepath.Base(parentp))
-
-	p, err := ioutil.ReadFile(parentnamep)
-	if err != nil {
-		return "", err
-	}
-
-	return string(p), nil
-}
-
-// Exists returns true if the snapshot with name exists.
-func (b *Driver) Exists(name string) bool {
-	target := filepath.Join(b.root, "snapshots", hash(name))
-
-	if _, err := os.Stat(target); err != nil {
-		if !os.IsNotExist(err) {
-			// TODO(stevvooe): Very rare condition when this fails horribly,
-			// such as an access error. Ideally, Exists is simple, but we may
-			// consider returning an error.
-			log.L.WithError(err).Fatal("error encountered checking for snapshot existence")
-		}
-
-		return false
-	}
-
-	return true
-}
-
-// Delete the snapshot idenfitied by name.
-//
-// If name has children, the operation will fail.
-func (b *Driver) Delete(name string) error {
-	panic("not implemented")
-}
-
-// TODO(stevvooe): The methods below are still in flux. We'll need to work
-// out the roles of active and committed snapshots for this to become more
-// clear.
-
 // Walk the committed snapshots.
-func (b *Driver) Walk(fn func(name string) error) error {
+func (b *Snapshotter) Walk(fn func(snapshot.Info) error) error {
-	panic("not implemented")
+	// TODO(stevvooe): Copy-pasted almost verbatim from overlay. Really need to
-}
+	// unify the metadata for snapshot implementations.
+	root := filepath.Join(b.root, "index")
+	return filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
 
-// Active will call fn for each active transaction.
+		if path == root {
-func (b *Driver) Active(fn func(key string) error) error {
+			return nil
-	panic("not implemented")
+		}
+
+		if fi.Mode()&os.ModeSymlink == 0 {
+			// only follow links
+			return filepath.SkipDir
+		}
+
+		target, err := os.Readlink(path)
+		if err != nil {
+			if !os.IsNotExist(err) {
+				return err
+			}
+		}
+
+		si, err := b.stat(target)
+		if err != nil {
+			return err
+		}
+
+		if err := fn(si); err != nil {
+			return err
+		}
+
+		return nil
+	})
 }
 
 func hash(k string) string {

snapshot/btrfs/btrfs_test.go

@@ -20,14 +20,14 @@ const (
 
 func TestBtrfs(t *testing.T) {
 	testutil.RequiresRoot(t)
-	snapshot.DriverSuite(t, "Btrfs", func(root string) (snapshot.Driver, func(), error) {
+	snapshot.SnapshotterSuite(t, "Btrfs", func(root string) (snapshot.Snapshotter, func(), error) {
 		device := setupBtrfsLoopbackDevice(t, root)
-		driver, err := NewDriver(device.deviceName, root)
+		snapshotter, err := NewSnapshotter(device.deviceName, root)
 		if err != nil {
 			t.Fatal(err)
 		}
 
-		return driver, func() {
+		return snapshotter, func() {
 			device.remove(t)
 		}, nil
 	})
@@ -53,7 +53,7 @@ func TestBtrfsMounts(t *testing.T) {
 	defer os.RemoveAll(root)
 
 	target := filepath.Join(root, "test")
-	b, err := NewDriver(device.deviceName, root)
+	b, err := NewSnapshotter(device.deviceName, root)
 	if err != nil {
 		t.Fatal(err)
 	}

snapshot/driver.go

@@ -1,302 +0,0 @@
-package snapshot
-
-import (
-	"io/ioutil"
-	"os"
-	"path/filepath"
-	"testing"
-
-	"github.com/docker/containerd"
-	"github.com/docker/containerd/testutil"
-	"github.com/stretchr/testify/assert"
-)
-
-// Driver defines the methods required to implement a snapshot driver for
-// allocating, snapshotting and mounting abstract filesystems. The model works
-// by building up sets of changes with parent-child relationships.
-//
-// These differ from the concept of the graphdriver in that the Manager has no
-// knowledge of images, layers or containers. Users simply prepare and commit
-// directories. We also avoid the integration between graph driver's and the
-// tar format used to represent the changesets.
-//
-// A snapshot represents a filesystem state. Every snapshot has a parent, where
-// the empty parent is represented by the empty string. A diff can be taken
-// between a parent and its snapshot to generate a classic layer.
-//
-// For convention, we define the following terms to be used throughout this
-// interface for driver implementations:
-//
-// 	`name` - refers to a forkable snapshot, typically read only
-// 	`key` - refers to an active transaction, either a prepare or view
-// 	`parent` - refers to the parent in relation to a name
-//
-// TODO(stevvooe): Update this description when things settle.
-//
-// Importing a Layer
-//
-// To import a layer, we simply have the Manager provide a list of
-// mounts to be applied such that our dst will capture a changeset. We start
-// out by getting a path to the layer tar file and creating a temp location to
-// unpack it to:
-//
-//	layerPath, tmpLocation := getLayerPath(), mkTmpDir() // just a path to layer tar file.
-//
-// We then use a Manager to prepare the temporary location as a
-// snapshot point:
-//
-// 	sm := NewManager()
-//	mounts, err := sm.Prepare(tmpLocation, "")
-// 	if err != nil { ... }
-//
-// Note that we provide "" as the parent, since we are applying the diff to an
-// empty directory. We get back a list of mounts from Manager.Prepare.
-// Before proceeding, we perform all these mounts:
-//
-//	if err := MountAll(mounts); err != nil { ... }
-//
-// Once the mounts are performed, our temporary location is ready to capture
-// a diff. In practice, this works similar to a filesystem transaction. The
-// next step is to unpack the layer. We have a special function unpackLayer
-// that applies the contents of the layer to target location and calculates the
-// DiffID of the unpacked layer (this is a requirement for docker
-// implementation):
-//
-//	layer, err := os.Open(layerPath)
-//	if err != nil { ... }
-// 	digest, err := unpackLayer(tmpLocation, layer) // unpack into layer location
-// 	if err != nil { ... }
-//
-// When the above completes, we should have a filesystem the represents the
-// contents of the layer. Careful implementations should verify that digest
-// matches the expected DiffID. When completed, we unmount the mounts:
-//
-//	unmount(mounts) // optional, for now
-//
-// Now that we've verified and unpacked our layer, we create a location to
-// commit the actual diff. For this example, we are just going to use the layer
-// digest, but in practice, this will probably be the ChainID:
-//
-// 	diffPath := filepath.Join("/layers", digest) // name location for the uncompressed layer digest
-//	if err := sm.Commit(diffPath, tmpLocation); err != nil { ... }
-//
-// Now, we have a layer in the Manager that can be accessed with the
-// opaque diffPath provided during commit.
-//
-// Importing the Next Layer
-//
-// Making a layer depend on the above is identical to the process described
-// above except that the parent is provided as diffPath when calling
-// Manager.Prepare:
-//
-// 	mounts, err := sm.Prepare(tmpLocation, parentDiffPath)
-//
-// The diff will be captured at tmpLocation, as the layer is applied.
-//
-// Running a Container
-//
-// To run a container, we simply provide Manager.Prepare the diffPath
-// of the image we want to start the container from. After mounting, the
-// prepared path can be used directly as the container's filesystem:
-//
-// 	mounts, err := sm.Prepare(containerRootFS, imageDiffPath)
-//
-// The returned mounts can then be passed directly to the container runtime. If
-// one would like to create a new image from the filesystem,
-// Manager.Commit is called:
-//
-// 	if err := sm.Commit(newImageDiff, containerRootFS); err != nil { ... }
-//
-// Alternatively, for most container runs, Manager.Rollback will be
-// called to signal Manager to abandon the changes.
-type Driver interface {
-	// Prepare returns a set of mounts corresponding to an active snapshot
-	// transaction, identified by the provided transaction key.
-	//
-	// If a parent is provided, after performing the mounts, the destination
-	// will start with the content of the parent. Changes to the mounted
-	// destination will be captured in relation to the provided parent. The
-	// default parent, "", is an empty directory.
-	//
-	// The changes may be saved to a new snapshot by calling Commit. When one
-	// is done with the transaction, Remove should be called on the key.
-	//
-	// Multiple calls to Prepare or View with the same key should fail.
-	Prepare(key, parent string) ([]containerd.Mount, error)
-
-	// View behaves identically to Prepare except the result may not be committed
-	// back to the snapshot manager. View returns a readonly view on the
-	// parent, with the transaction tracked by the given key.
-	//
-	// This method operates identically to Prepare, except that Mounts returned
-	// may have the readonly flag set. Any modifications to the underlying
-	// filesystem will be ignored.
-	//
-	// Commit may not be called on the provided key. To collect the resources
-	// associated with key, Remove must be called with key as the argument.
-	View(key, parent string) ([]containerd.Mount, error)
-
-	// Commit captures the changes between key and its parent into a snapshot
-	// identified by name.  The name can then be used with the driver's other
-	// methods to create subsequent snapshots.
-	//
-	// A snapshot will be created under name with the parent that started the
-	// transaction.
-	//
-	// Commit may be called multiple times on the same key. Snapshots created
-	// in this manner will all reference the parent used to start the
-	// transaction.
-	Commit(name, key string) error
-
-	// Mounts returns the mounts for the transaction identified by key. Can be
-	// called on an read-write or readonly transaction.
-	//
-	// This can be used to recover mounts after calling View or Prepare.
-	Mounts(key string) ([]containerd.Mount, error)
-
-	// Remove abandons the transaction identified by key. All resources
-	// associated with the key will be removed.
-	Remove(key string) error
-
-	// Parent returns the parent of snapshot identified by name.
-	Parent(name string) (string, error)
-
-	// Exists returns true if the snapshot with name exists.
-	Exists(name string) bool
-
-	// Delete the snapshot idenfitied by name.
-	//
-	// If name has children, the operation will fail.
-	Delete(name string) error
-
-	// TODO(stevvooe): The methods below are still in flux. We'll need to work
-	// out the roles of active and committed snapshots for this to become more
-	// clear.
-
-	// Walk the committed snapshots.
-	Walk(fn func(name string) error) error
-
-	// Active will call fn for each active transaction.
-	Active(fn func(key string) error) error
-}
-
-// DriverSuite runs a test suite on the driver given a factory function.
-func DriverSuite(t *testing.T, name string, driverFn func(root string) (Driver, func(), error)) {
-	t.Run("Basic", makeTest(t, name, driverFn, checkDriverBasic))
-}
-
-func makeTest(t *testing.T, name string, driverFn func(root string) (Driver, func(), error), fn func(t *testing.T, driver Driver, work string)) func(t *testing.T) {
-	return func(t *testing.T) {
-		// Make two directories: a driver root and a play area for the tests:
-		//
-		// 	/tmp
-		// 		work/ -> passed to test functions
-		// 		root/ -> passed to driver
-		//
-		tmpDir, err := ioutil.TempDir("", "snapshot-suite-"+name+"-")
-		if err != nil {
-			t.Fatal(err)
-		}
-		defer os.RemoveAll(tmpDir)
-
-		root := filepath.Join(tmpDir, "root")
-		if err := os.MkdirAll(root, 0777); err != nil {
-			t.Fatal(err)
-		}
-
-		driver, cleanup, err := driverFn(root)
-		if err != nil {
-			t.Fatal(err)
-		}
-		defer cleanup()
-
-		work := filepath.Join(tmpDir, "work")
-		if err := os.MkdirAll(work, 0777); err != nil {
-			t.Fatal(err)
-		}
-
-		defer testutil.DumpDir(t, tmpDir)
-		fn(t, driver, work)
-	}
-}
-
-// checkDriverBasic tests the basic workflow of a snapshot driver.
-func checkDriverBasic(t *testing.T, driver Driver, work string) {
-	preparing := filepath.Join(work, "preparing")
-	if err := os.MkdirAll(preparing, 0777); err != nil {
-		t.Fatal(err)
-	}
-
-	mounts, err := driver.Prepare(preparing, "")
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	if len(mounts) < 1 {
-		t.Fatal("expected mounts to have entries")
-	}
-
-	if err := containerd.MountAll(mounts, preparing); err != nil {
-		t.Fatal(err)
-	}
-	defer testutil.Unmount(t, preparing)
-
-	if err := ioutil.WriteFile(filepath.Join(preparing, "foo"), []byte("foo\n"), 0777); err != nil {
-		t.Fatal(err)
-	}
-
-	if err := os.MkdirAll(filepath.Join(preparing, "a", "b", "c"), 0755); err != nil {
-		t.Fatal(err)
-	}
-
-	committed := filepath.Join(work, "committed")
-	if err := driver.Commit(committed, preparing); err != nil {
-		t.Fatal(err)
-	}
-
-	parent, err := driver.Parent(committed)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	assert.Equal(t, parent, "")
-
-	next := filepath.Join(work, "nextlayer")
-	if err := os.MkdirAll(next, 0777); err != nil {
-		t.Fatal(err)
-	}
-
-	mounts, err = driver.Prepare(next, committed)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := containerd.MountAll(mounts, next); err != nil {
-		t.Fatal(err)
-	}
-	defer testutil.Unmount(t, next)
-
-	if err := ioutil.WriteFile(filepath.Join(next, "bar"), []byte("bar\n"), 0777); err != nil {
-		t.Fatal(err)
-	}
-
-	// also, change content of foo to bar
-	if err := ioutil.WriteFile(filepath.Join(next, "foo"), []byte("bar\n"), 0777); err != nil {
-		t.Fatal(err)
-	}
-
-	if err := os.RemoveAll(filepath.Join(next, "a", "b")); err != nil {
-		t.Log(err)
-	}
-
-	nextCommitted := filepath.Join(work, "committed-next")
-	if err := driver.Commit(nextCommitted, next); err != nil {
-		t.Fatal(err)
-	}
-
-	parent, err = driver.Parent(nextCommitted)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	assert.Equal(t, parent, committed)
-}

snapshot/overlay/overlay.go

@@ -9,33 +9,98 @@ import (
 	"sync"
 
 	"github.com/docker/containerd"
+	"github.com/docker/containerd/snapshot"
 	digest "github.com/opencontainers/go-digest"
+	"github.com/pkg/errors"
 )
 
-type Driver struct {
+type Snapshotter struct {
 	root  string
-	cache *cache
+	links *cache
 }
 
-func NewDriver(root string) (*Driver, error) {
+func NewSnapshotter(root string) (*Snapshotter, error) {
 	if err := os.MkdirAll(root, 0700); err != nil {
 		return nil, err
 	}
 	for _, p := range []string{
-		"snapshots",
+		"committed", // committed snapshots
-		"active",
+		"active",    // active snapshots
+		"index",     // snapshots by hashed name
 	} {
 		if err := os.MkdirAll(filepath.Join(root, p), 0700); err != nil {
 			return nil, err
 		}
 	}
-	return &Driver{
+	return &Snapshotter{
 		root:  root,
-		cache: newCache(),
+		links: newCache(),
 	}, nil
 }
 
-func (o *Driver) Prepare(key, parent string) ([]containerd.Mount, error) {
+// Stat returns the info for an active or committed snapshot by name or
+// key.
+//
+// Should be used for parent resolution, existence checks and to discern
+// the kind of snapshot.
+func (o *Snapshotter) Stat(key string) (snapshot.Info, error) {
+	path, err := o.links.get(filepath.Join(o.root, "index", hash(key)))
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return snapshot.Info{}, err
+		}
+
+		return snapshot.Info{}, errors.Errorf("snapshot %v not found", key)
+	}
+
+	// TODO(stevvooe): We don't confirm the name to avoid the lookup cost.
+	return o.stat(path)
+}
+
+func (o *Snapshotter) stat(path string) (snapshot.Info, error) {
+	ppath, err := o.links.get(filepath.Join(path, "parent"))
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return snapshot.Info{}, err
+		}
+
+		// no parent
+	}
+
+	kp, err := ioutil.ReadFile(filepath.Join(path, "name"))
+	if err != nil {
+		return snapshot.Info{}, err
+	}
+
+	var parent string
+	if ppath != "" {
+		p, err := ioutil.ReadFile(filepath.Join(ppath, "name"))
+		if err != nil {
+			return snapshot.Info{}, err
+		}
+		parent = string(p)
+	}
+
+	ro := true
+	kind := snapshot.KindCommitted
+	if strings.HasPrefix(path, filepath.Join(o.root, "active")) {
+		// TODO(stevvooe): Maybe there is a better way?
+		kind = snapshot.KindActive
+
+		// TODO(stevvooe): We haven't introduced this to overlay yet.
+		// We'll add it when we add tests for it.
+		ro = false
+	}
+
+	return snapshot.Info{
+		Name:     string(kp),
+		Parent:   parent,
+		Kind:     kind,
+		Readonly: ro,
+	}, nil
+}
+
+func (o *Snapshotter) Prepare(key, parent string) ([]containerd.Mount, error) {
 	active, err := o.newActiveDir(key)
 	if err != nil {
 		return nil, err
@@ -45,10 +110,10 @@ func (o *Driver) Prepare(key, parent string) ([]containerd.Mount, error) {
 			return nil, err
 		}
 	}
-	return o.Mounts(key)
+	return active.mounts(o.links)
 }
 
-func (o *Driver) View(key, parent string) ([]containerd.Mount, error) {
+func (o *Snapshotter) View(key, parent string) ([]containerd.Mount, error) {
 	panic("not implemented")
 }
 
@@ -56,70 +121,69 @@ func (o *Driver) View(key, parent string) ([]containerd.Mount, error) {
 // called on an read-write or readonly transaction.
 //
 // This can be used to recover mounts after calling View or Prepare.
-func (o *Driver) Mounts(key string) ([]containerd.Mount, error) {
+func (o *Snapshotter) Mounts(key string) ([]containerd.Mount, error) {
 	active := o.getActive(key)
-	return active.mounts(o.cache)
+	return active.mounts(o.links)
 }
 
-func (o *Driver) Commit(name, key string) error {
+func (o *Snapshotter) Commit(name, key string) error {
 	active := o.getActive(key)
-	return active.commit(name, o.cache)
+	return active.commit(name, o.links)
 }
 
 // Remove abandons the transaction identified by key. All resources
 // associated with the key will be removed.
-func (o *Driver) Remove(key string) error {
+func (o *Snapshotter) Remove(key string) error {
-	panic("not implemented")
-}
-
-// Parent returns the parent of snapshot identified by name.
-func (o *Driver) Parent(name string) (string, error) {
-	ppath, err := o.cache.get(filepath.Join(o.root, "snapshots", hash(name)))
-	if err != nil {
-		if os.IsNotExist(err) {
-			return "", nil // no parent
-		}
-
-		return "", err
-	}
-
-	p, err := ioutil.ReadFile(filepath.Join(ppath, "name"))
-	if err != nil {
-		return "", err
-	}
-
-	return string(p), nil
-}
-
-// Exists returns true if the snapshot with name exists.
-func (o *Driver) Exists(name string) bool {
-	panic("not implemented")
-}
-
-// Delete the snapshot idenfitied by name.
-//
-// If name has children, the operation will fail.
-func (o *Driver) Delete(name string) error {
 	panic("not implemented")
 }
 
 // Walk the committed snapshots.
-func (o *Driver) Walk(fn func(name string) error) error {
+func (o *Snapshotter) Walk(fn func(snapshot.Info) error) error {
-	panic("not implemented")
+	root := filepath.Join(o.root, "index")
+	return filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if path == root {
+			return nil
+		}
+
+		if fi.Mode()&os.ModeSymlink == 0 {
+			// only follow links
+			return filepath.SkipDir
+		}
+
+		target, err := o.links.get(path)
+		if err != nil {
+			if !os.IsNotExist(err) {
+				return err
+			}
+		}
+
+		si, err := o.stat(target)
+		if err != nil {
+			return err
+		}
+
+		if err := fn(si); err != nil {
+			return err
+		}
+
+		return nil
+	})
 }
 
-// Active will call fn for each active transaction.
+func (o *Snapshotter) newActiveDir(key string) (*activeDir, error) {
-func (o *Driver) Active(fn func(key string) error) error {
-	panic("not implemented")
-}
-
-func (o *Driver) newActiveDir(key string) (*activeDir, error) {
 	var (
 		path      = filepath.Join(o.root, "active", hash(key))
+		name      = filepath.Join(path, "name")
+		indexlink = filepath.Join(o.root, "index", hash(key))
 	)
 	a := &activeDir{
 		path:         path,
-		snapshotsDir: filepath.Join(o.root, "snapshots"),
+		committedDir: filepath.Join(o.root, "committed"),
+		indexlink:    indexlink,
 	}
 	for _, p := range []string{
 		"work",
@@ -130,13 +194,26 @@ func (o *Driver) newActiveDir(key string) (*activeDir, error) {
 			return nil, err
 		}
 	}
+
+	if err := ioutil.WriteFile(name, []byte(key), 0644); err != nil {
+		a.delete()
+		return nil, err
+	}
+
+	// link from namespace
+	if err := os.Symlink(path, indexlink); err != nil {
+		a.delete()
+		return nil, err
+	}
+
 	return a, nil
 }
 
-func (o *Driver) getActive(key string) *activeDir {
+func (o *Snapshotter) getActive(key string) *activeDir {
 	return &activeDir{
 		path:         filepath.Join(o.root, "active", hash(key)),
-		snapshotsDir: filepath.Join(o.root, "snapshots"),
+		committedDir: filepath.Join(o.root, "committed"),
+		indexlink:    filepath.Join(o.root, "index", hash(key)),
 	}
 }
 
@@ -145,8 +222,9 @@ func hash(k string) string {
 }
 
 type activeDir struct {
-	snapshotsDir string
+	committedDir string
 	path         string
+	indexlink    string
 }
 
 func (a *activeDir) delete() error {
@@ -154,7 +232,7 @@ func (a *activeDir) delete() error {
 }
 
 func (a *activeDir) setParent(name string) error {
-	return os.Symlink(filepath.Join(a.snapshotsDir, hash(name)), filepath.Join(a.path, "parent"))
+	return os.Symlink(filepath.Join(a.committedDir, hash(name)), filepath.Join(a.path, "parent"))
 }
 
 func (a *activeDir) commit(name string, c *cache) error {
@@ -173,7 +251,20 @@ func (a *activeDir) commit(name string, c *cache) error {
 	}
 
 	c.invalidate(a.path) // clears parent cache, since we end up moving.
-	return os.Rename(a.path, filepath.Join(a.snapshotsDir, hash(name)))
+	c.invalidate(filepath.Join(a.path, "parent"))
+	c.invalidate(a.indexlink)
+
+	committed := filepath.Join(a.committedDir, hash(name))
+	if err := os.Rename(a.path, committed); err != nil {
+		return err
+	}
+
+	if err := os.Remove(a.indexlink); err != nil {
+		return err
+	}
+
+	indexlink := filepath.Join(filepath.Dir(a.indexlink), hash(name))
+	return os.Symlink(committed, indexlink)
 }
 
 func (a *activeDir) mounts(c *cache) ([]containerd.Mount, error) {
@@ -183,10 +274,11 @@ func (a *activeDir) mounts(c *cache) ([]containerd.Mount, error) {
 		current = a.path
 	)
 	for {
-		if current, err = c.get(current); err != nil {
+		if current, err = c.get(filepath.Join(current, "parent")); err != nil {
 			if os.IsNotExist(err) {
 				break
 			}
+
 			return nil, err
 		}
 		parents = append(parents, filepath.Join(current, "fs"))
@@ -221,32 +313,32 @@ func (a *activeDir) mounts(c *cache) ([]containerd.Mount, error) {
 
 func newCache() *cache {
 	return &cache{
-		parents: make(map[string]string),
+		links: make(map[string]string),
 	}
 }
 
 type cache struct {
 	mu    sync.Mutex
-	parents map[string]string
+	links map[string]string
 }
 
 func (c *cache) get(path string) (string, error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	parentRoot, ok := c.parents[path]
+	target, ok := c.links[path]
 	if !ok {
-		link, err := os.Readlink(filepath.Join(path, "parent"))
+		link, err := os.Readlink(path)
 		if err != nil {
 			return "", err
 		}
-		c.parents[path], parentRoot = link, link
+		c.links[path], target = link, link
 	}
-	return parentRoot, nil
+	return target, nil
 }
 
 func (c *cache) invalidate(path string) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	delete(c.parents, path)
+	delete(c.links, path)
 }

snapshot/overlay/overlay_test.go

@@ -13,12 +13,24 @@ import (
 )
 
 func TestOverlay(t *testing.T) {
+	testutil.RequiresRoot(t)
+	snapshot.SnapshotterSuite(t, "Overlay", func(root string) (snapshot.Snapshotter, func(), error) {
+		snapshotter, err := NewSnapshotter(root)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		return snapshotter, func() {}, nil
+	})
+}
+
+func TestOverlayMounts(t *testing.T) {
 	root, err := ioutil.TempDir("", "overlay")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(root)
-	o, err := NewDriver(root)
+	o, err := NewSnapshotter(root)
 	if err != nil {
 		t.Error(err)
 		return
@@ -53,7 +65,7 @@ func TestOverlayCommit(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(root)
-	o, err := NewDriver(root)
+	o, err := NewSnapshotter(root)
 	if err != nil {
 		t.Error(err)
 		return
@@ -81,7 +93,7 @@ func TestOverlayOverlayMount(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(root)
-	o, err := NewDriver(root)
+	o, err := NewSnapshotter(root)
 	if err != nil {
 		t.Error(err)
 		return
@@ -115,7 +127,7 @@ func TestOverlayOverlayMount(t *testing.T) {
 		sh    = hash("base")
 		work  = "workdir=" + filepath.Join(root, "active", ah, "work")
 		upper = "upperdir=" + filepath.Join(root, "active", ah, "fs")
-		lower = "lowerdir=" + filepath.Join(root, "snapshots", sh, "fs")
+		lower = "lowerdir=" + filepath.Join(root, "committed", sh, "fs")
 	)
 	for i, v := range []string{
 		work,
@@ -135,7 +147,7 @@ func TestOverlayOverlayRead(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(root)
-	o, err := NewDriver(root)
+	o, err := NewSnapshotter(root)
 	if err != nil {
 		t.Error(err)
 		return
@@ -179,15 +191,3 @@ func TestOverlayOverlayRead(t *testing.T) {
 		return
 	}
 }
-
-func TestOverlayDriverSuite(t *testing.T) {
-	testutil.RequiresRoot(t)
-	snapshot.DriverSuite(t, "Overlay", func(root string) (snapshot.Driver, func(), error) {
-		driver, err := NewDriver(root)
-		if err != nil {
-			t.Fatal(err)
-		}
-
-		return driver, func() {}, nil
-	})
-}

snapshot/snapshotter.go

@@ -0,0 +1,347 @@
+package snapshot
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/docker/containerd"
+	"github.com/docker/containerd/testutil"
+	"github.com/stretchr/testify/assert"
+)
+
+// Kind identifies the kind of snapshot.
+type Kind int
+
+// definitions of snapshot kinds
+const (
+	KindActive Kind = iota
+	KindCommitted
+)
+
+// Info provides information about a particular snapshot.
+type Info struct {
+	Name     string // name or key of snapshot
+	Parent   string // name of parent snapshot
+	Kind     Kind   // active or committed snapshot
+	Readonly bool   // true if readonly, only valid for active
+}
+
+// Snapshotter defines the methods required to implement a snapshot snapshotter for
+// allocating, snapshotting and mounting filesystem changesets. The model works
+// by building up sets of changes with parent-child relationships.
+//
+// A snapshot represents a filesystem state. Every snapshot has a parent, where
+// the empty parent is represented by the empty string. A diff can be taken
+// between a parent and its snapshot to generate a classic layer.
+//
+// An active snapshot is created by calling `Prepare`. After mounting, changes
+// can be made to the snapshot. The act of commiting creates a committed
+// snapshot. The committed snapshot will get the parent of active snapshot. The
+// committed snapshot can then be used as a parent. Active snapshots can never
+// act as a parent.
+//
+// Snapshots are best understood by their lifecycle. Active snapshots are
+// always created with Prepare or View. Committed snapshots are always created
+// with Commit.  Active snapshots never become committed snapshots and vice
+// versa. All snapshots may be removed.
+//
+// For consistency, we define the following terms to be used throughout this
+// interface for snapshotter implementations:
+//
+// 	`key` - refers to an active snapshot
+// 	`name` - refers to a committed snapshot
+// 	`parent` - refers to the parent in relation
+//
+// Most methods take various combinations of these identifiers. Typically,
+// `name` and `parent` will be used in cases where a method *only* takes
+// committed snapshots. `key` will be used to refer to active snapshots in most
+// cases, except where noted. All variables used to access snapshots use the
+// same key space. For example, an active snapshot may not share the same key
+// with a committed snapshot.
+//
+// We cover several examples below to demonstrate the utility of a snapshot
+// snapshotter.
+//
+// Importing a Layer
+//
+// To import a layer, we simply have the Manager provide a list of
+// mounts to be applied such that our dst will capture a changeset. We start
+// out by getting a path to the layer tar file and creating a temp location to
+// unpack it to:
+//
+//	layerPath, tmpDir := getLayerPath(), mkTmpDir() // just a path to layer tar file.
+//
+// We start by using a Snapshotter to Prepare a new snapshot transaction, using a
+// key and descending from the empty parent "":
+//
+//	mounts, err := snapshotter.Prepare(key, "")
+// 	if err != nil { ... }
+//
+// We get back a list of mounts from Snapshotter.Prepare, with the key identifying
+// the active snapshot. Mount this to the temporary location with the
+// following:
+//
+//	if err := MountAll(mounts, tmpDir); err != nil { ... }
+//
+// Once the mounts are performed, our temporary location is ready to capture
+// a diff. In practice, this works similar to a filesystem transaction. The
+// next step is to unpack the layer. We have a special function unpackLayer
+// that applies the contents of the layer to target location and calculates the
+// DiffID of the unpacked layer (this is a requirement for docker
+// implementation):
+//
+//	layer, err := os.Open(layerPath)
+//	if err != nil { ... }
+// 	digest, err := unpackLayer(tmpLocation, layer) // unpack into layer location
+// 	if err != nil { ... }
+//
+// When the above completes, we should have a filesystem the represents the
+// contents of the layer. Careful implementations should verify that digest
+// matches the expected DiffID. When completed, we unmount the mounts:
+//
+//	unmount(mounts) // optional, for now
+//
+// Now that we've verified and unpacked our layer, we commit the active
+// snapshot to a name. For this example, we are just going to use the layer
+// digest, but in practice, this will probably be the ChainID:
+//
+//	if err := snapshotter.Commit(digest.String(), key); err != nil { ... }
+//
+// Now, we have a layer in the Snapshotter that can be accessed with the digest
+// provided during commit. Once you have committed the snapshot, the active
+// snapshot can be removed with the following:
+//
+// 	snapshotter.Remove(key)
+//
+// Importing the Next Layer
+//
+// Making a layer depend on the above is identical to the process described
+// above except that the parent is provided as parent when calling
+// Manager.Prepare, assuming a clean tmpLocation:
+//
+// 	mounts, err := sm.Prepare(tmpLocation, parentDigest)
+//
+// We then mount, apply and commit, as we did above. The new snapshot will be
+// based on the content of the previous one.
+//
+// Running a Container
+//
+// To run a container, we simply provide Snapshotter.Prepare the committed image
+// snapshot as the parent. After mounting, the prepared path can
+// be used directly as the container's filesystem:
+//
+// 	mounts, err := sm.Prepare(containerKey, imageRootFSChainID)
+//
+// The returned mounts can then be passed directly to the container runtime. If
+// one would like to create a new image from the filesystem, Manager.Commit is
+// called:
+//
+// 	if err := sm.Commit(newImageSnapshot, containerKey); err != nil { ... }
+//
+// Alternatively, for most container runs, Manager.Remove will be called to
+// signal the Snapshotter to abandon the changes.
+type Snapshotter interface {
+	// Stat returns the info for an active or committed snapshot by name or
+	// key.
+	//
+	// Should be used for parent resolution, existence checks and to discern
+	// the kind of snapshot.
+	Stat(key string) (Info, error)
+
+	// Mounts returns the mounts for the active snapshot transaction identified
+	// by key. Can be called on an read-write or readonly transaction. This is
+	// available only for active snapshots.
+	//
+	// This can be used to recover mounts after calling View or Prepare.
+	Mounts(key string) ([]containerd.Mount, error)
+
+	// Prepare creates an active snapshot identified by key descending from the
+	// provided parent.  The returned mounts can be used to mount the snapshot
+	// to capture changes.
+	//
+	// If a parent is provided, after performing the mounts, the destination
+	// will start with the content of the parent. The parent must be a
+	// committed snapshot. Changes to the mounted destination will be captured
+	// in relation to the parent. The default parent, "", is an empty
+	// directory.
+	//
+	// The changes may be saved to a committed snapshot by calling Commit. When
+	// one is done with the transaction, Remove should be called on the key.
+	//
+	// Multiple calls to Prepare or View with the same key should fail.
+	Prepare(key, parent string) ([]containerd.Mount, error)
+
+	// View behaves identically to Prepare except the result may not be
+	// committed back to the snapshot snapshotter. View returns a readonly view on
+	// the parent, with the active snapshot being tracked by the given key.
+	//
+	// This method operates identically to Prepare, except that Mounts returned
+	// may have the readonly flag set. Any modifications to the underlying
+	// filesystem will be ignored. Implementations may perform this in a more
+	// efficient manner that differs from what would be attempted with
+	// `Prepare`.
+	//
+	// Commit may not be called on the provided key and will return an error.
+	// To collect the resources associated with key, Remove must be called with
+	// key as the argument.
+	View(key, parent string) ([]containerd.Mount, error)
+
+	// Commit captures the changes between key and its parent into a snapshot
+	// identified by name.  The name can then be used with the snapshotter's other
+	// methods to create subsequent snapshots.
+	//
+	// A committed snapshot will be created under name with the parent of the
+	// active snapshot.
+	//
+	// Commit may be called multiple times on the same key. Snapshots created
+	// in this manner will all reference the parent used to start the
+	// transaction.
+	Commit(name, key string) error
+
+	// Remove the committed or active snapshot by the provided key.
+	//
+	// All resources associated with the key will be removed.
+	//
+	// If the snapshot is a parent of another snapshot, its children must be
+	// removed before proceeding.
+	Remove(key string) error
+
+	// Walk the committed snapshots. For each snapshot in the snapshotter, the
+	// function will be called.
+	Walk(fn func(Info) error) error
+}
+
+// SnapshotterSuite runs a test suite on the snapshotter given a factory function.
+func SnapshotterSuite(t *testing.T, name string, snapshotterFn func(root string) (Snapshotter, func(), error)) {
+	t.Run("Basic", makeTest(t, name, snapshotterFn, checkSnapshotterBasic))
+}
+
+func makeTest(t *testing.T, name string, snapshotterFn func(root string) (Snapshotter, func(), error), fn func(t *testing.T, snapshotter Snapshotter, work string)) func(t *testing.T) {
+	return func(t *testing.T) {
+		// Make two directories: a snapshotter root and a play area for the tests:
+		//
+		// 	/tmp
+		// 		work/ -> passed to test functions
+		// 		root/ -> passed to snapshotter
+		//
+		tmpDir, err := ioutil.TempDir("", "snapshot-suite-"+name+"-")
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer os.RemoveAll(tmpDir)
+
+		root := filepath.Join(tmpDir, "root")
+		if err := os.MkdirAll(root, 0777); err != nil {
+			t.Fatal(err)
+		}
+
+		snapshotter, cleanup, err := snapshotterFn(root)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer cleanup()
+
+		work := filepath.Join(tmpDir, "work")
+		if err := os.MkdirAll(work, 0777); err != nil {
+			t.Fatal(err)
+		}
+
+		defer testutil.DumpDir(t, tmpDir)
+		fn(t, snapshotter, work)
+	}
+}
+
+// checkSnapshotterBasic tests the basic workflow of a snapshot snapshotter.
+func checkSnapshotterBasic(t *testing.T, snapshotter Snapshotter, work string) {
+	preparing := filepath.Join(work, "preparing")
+	if err := os.MkdirAll(preparing, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	mounts, err := snapshotter.Prepare(preparing, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(mounts) < 1 {
+		t.Fatal("expected mounts to have entries")
+	}
+
+	if err := containerd.MountAll(mounts, preparing); err != nil {
+		t.Fatal(err)
+	}
+	defer testutil.Unmount(t, preparing)
+
+	if err := ioutil.WriteFile(filepath.Join(preparing, "foo"), []byte("foo\n"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.MkdirAll(filepath.Join(preparing, "a", "b", "c"), 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	committed := filepath.Join(work, "committed")
+	if err := snapshotter.Commit(committed, preparing); err != nil {
+		t.Fatal(err)
+	}
+
+	si, err := snapshotter.Stat(committed)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assert.Equal(t, si.Parent, "")
+
+	next := filepath.Join(work, "nextlayer")
+	if err := os.MkdirAll(next, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	mounts, err = snapshotter.Prepare(next, committed)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := containerd.MountAll(mounts, next); err != nil {
+		t.Fatal(err)
+	}
+	defer testutil.Unmount(t, next)
+
+	if err := ioutil.WriteFile(filepath.Join(next, "bar"), []byte("bar\n"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	// also, change content of foo to bar
+	if err := ioutil.WriteFile(filepath.Join(next, "foo"), []byte("bar\n"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.RemoveAll(filepath.Join(next, "a", "b")); err != nil {
+		t.Log(err)
+	}
+
+	nextCommitted := filepath.Join(work, "committed-next")
+	if err := snapshotter.Commit(nextCommitted, next); err != nil {
+		t.Fatal(err)
+	}
+
+	si2, err := snapshotter.Stat(nextCommitted)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	assert.Equal(t, si2.Parent, committed)
+
+	expected := map[string]Info{
+		si.Name:  si,
+		si2.Name: si2,
+	}
+	walked := map[string]Info{} // walk is not ordered
+	assert.NoError(t, snapshotter.Walk(func(si Info) error {
+		walked[si.Name] = si
+		return nil
+	}))
+
+	assert.Equal(t, expected, walked)
+}