Refactor specification of supported digests

To make the definition of supported digests more clear, we have refactored the
digest package to have a special Algorithm type. This represents the digest's
prefix and we associated various supported hash implementations through
function calls.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/client/repository.go

@@ -321,7 +321,7 @@ func (bs *blobs) Put(ctx context.Context, mediaType string, p []byte) (distribut
 	if err != nil {
 		return distribution.Descriptor{}, err
 	}
-	dgstr := digest.NewCanonicalDigester()
+	dgstr := digest.Canonical.New()
 	n, err := io.Copy(writer, io.TeeReader(bytes.NewReader(p), dgstr.Hash()))
 	if err != nil {
 		return distribution.Descriptor{}, err

docs/handlers/api_test.go

@@ -213,7 +213,7 @@ func TestBlobAPI(t *testing.T) {
 	// Now, push just a chunk
 	layerFile.Seek(0, 0)
 
-	canonicalDigester := digest.NewCanonicalDigester()
+	canonicalDigester := digest.Canonical.New()
 	if _, err := io.Copy(canonicalDigester.Hash(), layerFile); err != nil {
 		t.Fatalf("error copying to digest: %v", err)
 	}
@@ -637,7 +637,7 @@ func doPushLayer(t *testing.T, ub *v2.URLBuilder, name string, dgst digest.Diges
 
 // pushLayer pushes the layer content returning the url on success.
 func pushLayer(t *testing.T, ub *v2.URLBuilder, name string, dgst digest.Digest, uploadURLBase string, body io.Reader) string {
-	digester := digest.NewCanonicalDigester()
+	digester := digest.Canonical.New()
 
 	resp, err := doPushLayer(t, ub, name, dgst, uploadURLBase, io.TeeReader(body, digester.Hash()))
 	if err != nil {
@@ -702,7 +702,7 @@ func doPushChunk(t *testing.T, uploadURLBase string, body io.Reader) (*http.Resp
 
 	uploadURL := u.String()
 
-	digester := digest.NewCanonicalDigester()
+	digester := digest.Canonical.New()
 
 	req, err := http.NewRequest("PATCH", uploadURL, io.TeeReader(body, digester.Hash()))
 	if err != nil {

docs/storage/blobwriter.go

@@ -1,6 +1,7 @@
 package storage
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"time"
@@ -13,7 +14,7 @@ import (
 )
 
 var (
-	errResumableDigestNotAvailable = fmt.Errorf("resumable digest not available")
+	errResumableDigestNotAvailable = errors.New("resumable digest not available")
 )
 
 // layerWriter is used to control the various aspects of resumable
@@ -197,7 +198,7 @@ func (bw *blobWriter) validateBlob(ctx context.Context, desc distribution.Descri
 		// the same, we don't need to read the data from the backend. This is
 		// because we've written the entire file in the lifecycle of the
 		// current instance.
-		if bw.written == bw.size && digest.CanonicalAlgorithm == desc.Digest.Algorithm() {
+		if bw.written == bw.size && digest.Canonical == desc.Digest.Algorithm() {
 			canonical = bw.digester.Digest()
 			verified = desc.Digest == canonical
 		}
@@ -206,7 +207,7 @@ func (bw *blobWriter) validateBlob(ctx context.Context, desc distribution.Descri
 		// paths. We may be able to make the size-based check a stronger
 		// guarantee, so this may be defensive.
 		if !verified {
-			digester := digest.NewCanonicalDigester()
+			digester := digest.Canonical.New()
 
 			digestVerifier, err := digest.NewDigestVerifier(desc.Digest)
 			if err != nil {

docs/storage/linkedblobstore.go

@@ -164,7 +164,7 @@ func (lbs *linkedBlobStore) newBlobUpload(ctx context.Context, uuid, path string
 		blobStore:          lbs,
 		id:                 uuid,
 		startedAt:          startedAt,
-		digester:           digest.NewCanonicalDigester(),
+		digester:           digest.Canonical.New(),
 		bufferedFileWriter: *fw,
 	}
 

docs/storage/paths.go

@@ -262,7 +262,7 @@ func (pm *pathMapper) path(spec pathSpec) (string, error) {
 		if v.list {
 			offset = "" // Limit to the prefix for listing offsets.
 		}
-		return path.Join(append(repoPrefix, v.name, "_uploads", v.id, "hashstates", v.alg, offset)...), nil
+		return path.Join(append(repoPrefix, v.name, "_uploads", v.id, "hashstates", string(v.alg), offset)...), nil
 	case repositoriesRootPathSpec:
 		return path.Join(repoPrefix...), nil
 	default:
@@ -447,7 +447,7 @@ func (uploadStartedAtPathSpec) pathSpec() {}
 type uploadHashStatePathSpec struct {
 	name   string
 	id     string
-	alg    string
+	alg    digest.Algorithm
 	offset int64
 	list   bool
 }
@@ -479,7 +479,7 @@ func digestPathComponents(dgst digest.Digest, multilevel bool) ([]string, error)
 		return nil, err
 	}
 
-	algorithm := blobAlgorithmReplacer.Replace(dgst.Algorithm())
+	algorithm := blobAlgorithmReplacer.Replace(string(dgst.Algorithm()))
 	hex := dgst.Hex()
 	prefix := []string{algorithm}