diff: resplit Applier from Differ

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

client.go

@@ -458,7 +458,7 @@ func (c *Client) ImageService() images.Store {
 }
 
 // DiffService returns the underlying Differ
-func (c *Client) DiffService() diff.Differ {
+func (c *Client) DiffService() diff.DiffApplier {
 	return NewDiffServiceFromClient(diffapi.NewDiffClient(c.conn))
 }
 

diff.go

@@ -11,7 +11,7 @@ import (
 
 // NewDiffServiceFromClient returns a new diff service which communicates
 // over a GRPC connection.
-func NewDiffServiceFromClient(client diffapi.DiffClient) diff.Differ {
+func NewDiffServiceFromClient(client diffapi.DiffClient) diff.DiffApplier {
 	return &diffRemote{
 		client: client,
 	}
@@ -33,7 +33,7 @@ func (r *diffRemote) Apply(ctx context.Context, diff ocispec.Descriptor, mounts
 	return toDescriptor(resp.Applied), nil
 }
 
-func (r *diffRemote) DiffMounts(ctx context.Context, a, b []mount.Mount, opts ...diff.Opt) (ocispec.Descriptor, error) {
+func (r *diffRemote) Diff(ctx context.Context, a, b []mount.Mount, opts ...diff.Opt) (ocispec.Descriptor, error) {
 	var config diff.Config
 	for _, opt := range opts {
 		if err := opt(&config); err != nil {

diff/diff.go

@@ -24,21 +24,33 @@ type Config struct {
 // Opt is used to configure a diff operation
 type Opt func(*Config) error
 
-// Differ allows the apply and creation of filesystem diffs between mounts
+// Differ allows creation of filesystem diffs between mounts
 type Differ interface {
+	// Diff computes the difference between two mounts and returns a
+	// descriptor for the computed diff. The options can provide
+	// a ref which can be used to track the content creation of the diff.
+	// The media type which is used to determine the format of the created
+	// content can also be provided as an option.
+	Diff(ctx context.Context, lower, upper []mount.Mount, opts ...Opt) (ocispec.Descriptor, error)
+}
+
+// Applier allows applying diffs between mounts
+type Applier interface {
 	// Apply applies the content referred to by the given descriptor to
 	// the provided mount. The method of applying is based on the
 	// implementation and content descriptor. For example, in the common
 	// case the descriptor is a file system difference in tar format,
 	// that tar would be applied on top of the mounts.
 	Apply(ctx context.Context, desc ocispec.Descriptor, mount []mount.Mount) (ocispec.Descriptor, error)
+}
 
-	// DiffMounts computes the difference between two mounts and returns a
-	// descriptor for the computed diff. The options can provide
-	// a ref which can be used to track the content creation of the diff.
-	// The media type which is used to determine the format of the created
-	// content can also be provided as an option.
-	DiffMounts(ctx context.Context, lower, upper []mount.Mount, opts ...Opt) (ocispec.Descriptor, error)
+// DiffApplier is the interface that groups the basic Apply and Diff methods.
+//
+// golint says `type name will be used as diff.DiffApplier by other packages, and that stutters`,
+// but that can be ignored now.
+type DiffApplier interface { // nolint: golint
+	Applier
+	Differ
 }
 
 // WithMediaType sets the media type to use for creating the diff, without

diff/walking/differ.go

@@ -51,9 +51,9 @@ type walkingDiff struct {
 
 var emptyDesc = ocispec.Descriptor{}
 
-// NewWalkingDiff is a generic implementation of diff.Differ.
+// NewWalkingDiff is a generic implementation of diff.DiffApplier.
 // NewWalkingDiff is expected to work with any filesystem.
-func NewWalkingDiff(store content.Store) (diff.Differ, error) {
+func NewWalkingDiff(store content.Store) (diff.DiffApplier, error) {
 	return &walkingDiff{
 		store: store,
 	}, nil
@@ -125,9 +125,9 @@ func (s *walkingDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mounts
 	return ocidesc, nil
 }
 
-// DiffMounts creates a diff between the given mounts and uploads the result
+// Diff creates a diff between the given mounts and uploads the result
 // to the content store.
-func (s *walkingDiff) DiffMounts(ctx context.Context, lower, upper []mount.Mount, opts ...diff.Opt) (d ocispec.Descriptor, err error) {
+func (s *walkingDiff) Diff(ctx context.Context, lower, upper []mount.Mount, opts ...diff.Opt) (d ocispec.Descriptor, err error) {
 	var config diff.Config
 	for _, opt := range opts {
 		if err := opt(&config); err != nil {

rootfs/apply.go

@@ -30,7 +30,7 @@ type Layer struct {
 // The returned result is a chain id digest representing all the applied layers.
 // Layers are applied in order they are given, making the first layer the
 // bottom-most layer in the layer chain.
-func ApplyLayers(ctx context.Context, layers []Layer, sn snapshots.Snapshotter, a diff.Differ) (digest.Digest, error) {
+func ApplyLayers(ctx context.Context, layers []Layer, sn snapshots.Snapshotter, a diff.Applier) (digest.Digest, error) {
 	var chain []digest.Digest
 	for _, layer := range layers {
 		if _, err := ApplyLayer(ctx, layer, chain, sn, a); err != nil {
@@ -46,7 +46,7 @@ func ApplyLayers(ctx context.Context, layers []Layer, sn snapshots.Snapshotter,
 // ApplyLayer applies a single layer on top of the given provided layer chain,
 // using the provided snapshotter and applier. If the layer was unpacked true
 // is returned, if the layer already exists false is returned.
-func ApplyLayer(ctx context.Context, layer Layer, chain []digest.Digest, sn snapshots.Snapshotter, a diff.Differ, opts ...snapshots.Opt) (bool, error) {
+func ApplyLayer(ctx context.Context, layer Layer, chain []digest.Digest, sn snapshots.Snapshotter, a diff.Applier, opts ...snapshots.Opt) (bool, error) {
 	var (
 		parent  = identity.ChainID(chain)
 		chainID = identity.ChainID(append(chain, layer.Diff.Digest))

rootfs/diff.go

@@ -42,5 +42,5 @@ func Diff(ctx context.Context, snapshotID string, sn snapshots.Snapshotter, d di
 		defer sn.Remove(ctx, upperKey)
 	}
 
-	return d.DiffMounts(ctx, lower, upper, opts...)
+	return d.Diff(ctx, lower, upper, opts...)
 }

services/diff/service.go

@@ -38,7 +38,7 @@ func init() {
 			}
 
 			orderedNames := ic.Config.(*config).Order
-			ordered := make([]diff.Differ, len(orderedNames))
+			ordered := make([]diff.DiffApplier, len(orderedNames))
 			for i, n := range orderedNames {
 				differp, ok := differs[n]
 				if !ok {
@@ -49,7 +49,10 @@ func init() {
 					return nil, errors.Wrapf(err, "could not load required differ due plugin init error: %s", n)
 				}
 
-				ordered[i] = differ.(diff.Differ)
+				ordered[i], ok = differ.(diff.DiffApplier)
+				if !ok {
+					return nil, errors.Errorf("differ does not implement diff.DiffApplier interface: %s", n)
+				}
 			}
 
 			return &service{
@@ -60,7 +63,7 @@ func init() {
 }
 
 type service struct {
-	differs []diff.Differ
+	differs []diff.DiffApplier
 }
 
 func (s *service) Register(gs *grpc.Server) error {
@@ -113,7 +116,7 @@ func (s *service) Diff(ctx context.Context, dr *diffapi.DiffRequest) (*diffapi.D
 	}
 
 	for _, differ := range s.differs {
-		ocidesc, err = differ.DiffMounts(ctx, aMounts, bMounts, opts...)
+		ocidesc, err = differ.Diff(ctx, aMounts, bMounts, opts...)
 		if !errdefs.IsNotImplemented(err) {
 			break
 		}