@@ -13,10 +13,21 @@ import (

 // V2MetadataService maps layer IDs to a set of known metadata for

 // the layer.

-type V2MetadataService struct {

+type V2MetadataService interface {

+	GetMetadata(diffID layer.DiffID) ([]V2Metadata, error)

+	GetDiffID(dgst digest.Digest) (layer.DiffID, error)

+	Add(diffID layer.DiffID, metadata V2Metadata) error

+	TagAndAdd(diffID layer.DiffID, hmacKey []byte, metadata V2Metadata) error

+	Remove(metadata V2Metadata) error

+}

+

+// v2MetadataService implements V2MetadataService

+type v2MetadataService struct {

 	store Store

 }

+var _ V2MetadataService = &v2MetadataService{}

+

 // V2Metadata contains the digest and source repository information for a layer.

 type V2Metadata struct {

 	Digest           digest.Digest

@@ -90,30 +101,30 @@ type authConfigKeyInput struct {

 const maxMetadata = 50

 // NewV2MetadataService creates a new diff ID to v2 metadata mapping service.

-func NewV2MetadataService(store Store) *V2MetadataService {

-	return &V2MetadataService{

+func NewV2MetadataService(store Store) V2MetadataService {

+	return &v2MetadataService{

 		store: store,

 	}

 }

-func (serv *V2MetadataService) diffIDNamespace() string {

+func (serv *v2MetadataService) diffIDNamespace() string {

 	return "v2metadata-by-diffid"

 }

-func (serv *V2MetadataService) digestNamespace() string {

+func (serv *v2MetadataService) digestNamespace() string {

 	return "diffid-by-digest"

 }

-func (serv *V2MetadataService) diffIDKey(diffID layer.DiffID) string {

+func (serv *v2MetadataService) diffIDKey(diffID layer.DiffID) string {

 	return string(digest.Digest(diffID).Algorithm()) + "/" + digest.Digest(diffID).Hex()

 }

-func (serv *V2MetadataService) digestKey(dgst digest.Digest) string {

+func (serv *v2MetadataService) digestKey(dgst digest.Digest) string {

 	return string(dgst.Algorithm()) + "/" + dgst.Hex()

 }

 // GetMetadata finds the metadata associated with a layer DiffID.

-func (serv *V2MetadataService) GetMetadata(diffID layer.DiffID) ([]V2Metadata, error) {

+func (serv *v2MetadataService) GetMetadata(diffID layer.DiffID) ([]V2Metadata, error) {

 	jsonBytes, err := serv.store.Get(serv.diffIDNamespace(), serv.diffIDKey(diffID))

 	if err != nil {

 		return nil, err

@@ -128,7 +139,7 @@ func (serv *V2MetadataService) GetMetadata(diffID layer.DiffID) ([]V2Metadata, e

 }

 // GetDiffID finds a layer DiffID from a digest.

-func (serv *V2MetadataService) GetDiffID(dgst digest.Digest) (layer.DiffID, error) {

+func (serv *v2MetadataService) GetDiffID(dgst digest.Digest) (layer.DiffID, error) {

 	diffIDBytes, err := serv.store.Get(serv.digestNamespace(), serv.digestKey(dgst))

 	if err != nil {

 		return layer.DiffID(""), err

@@ -139,7 +150,7 @@ func (serv *V2MetadataService) GetDiffID(dgst digest.Digest) (layer.DiffID, erro

 // Add associates metadata with a layer DiffID. If too many metadata entries are

 // present, the oldest one is dropped.

-func (serv *V2MetadataService) Add(diffID layer.DiffID, metadata V2Metadata) error {

+func (serv *v2MetadataService) Add(diffID layer.DiffID, metadata V2Metadata) error {

 	oldMetadata, err := serv.GetMetadata(diffID)

 	if err != nil {

 		oldMetadata = nil

@@ -174,13 +185,13 @@ func (serv *V2MetadataService) Add(diffID layer.DiffID, metadata V2Metadata) err

 // TagAndAdd amends the given "meta" for hmac hashed by the given "hmacKey" and associates it with a layer

 // DiffID. If too many metadata entries are present, the oldest one is dropped.

-func (serv *V2MetadataService) TagAndAdd(diffID layer.DiffID, hmacKey []byte, meta V2Metadata) error {

+func (serv *v2MetadataService) TagAndAdd(diffID layer.DiffID, hmacKey []byte, meta V2Metadata) error {

 	meta.HMAC = ComputeV2MetadataHMAC(hmacKey, &meta)

 	return serv.Add(diffID, meta)

 }

 // Remove unassociates a metadata entry from a layer DiffID.

-func (serv *V2MetadataService) Remove(metadata V2Metadata) error {

+func (serv *v2MetadataService) Remove(metadata V2Metadata) error {

 	diffID, err := serv.GetDiffID(metadata.Digest)

 	if err != nil {

 		return err

@@ -50,7 +50,7 @@ func (e ImageConfigPullError) Error() string {

 }

 type v2Puller struct {

-	V2MetadataService *metadata.V2MetadataService

+	V2MetadataService metadata.V2MetadataService

 	endpoint          registry.APIEndpoint

 	config            *ImagePullConfig

 	repoInfo          *registry.RepositoryInfo

@@ -134,7 +134,7 @@ type v2LayerDescriptor struct {

 	digest            digest.Digest

 	repoInfo          *registry.RepositoryInfo

 	repo              distribution.Repository

-	V2MetadataService *metadata.V2MetadataService

+	V2MetadataService metadata.V2MetadataService

 	tmpFile           *os.File

 	verifier          digest.Verifier

 	src               distribution.Descriptor

@@ -41,7 +41,7 @@ type PushResult struct {

 }

 type v2Pusher struct {

-	v2MetadataService *metadata.V2MetadataService

+	v2MetadataService metadata.V2MetadataService

 	ref               reference.Named

 	endpoint          registry.APIEndpoint

 	repoInfo          *registry.RepositoryInfo

@@ -243,7 +243,7 @@ func manifestFromBuilder(ctx context.Context, builder distribution.ManifestBuild

 type v2PushDescriptor struct {

 	layer             layer.Layer

-	v2MetadataService *metadata.V2MetadataService

+	v2MetadataService metadata.V2MetadataService

 	hmacKey           []byte

 	repoInfo          reference.Named

 	ref               reference.Named