Add option to get content digest from manifest get

The client may need the content digest to delete a manifest using the digest used by the registry.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

docs/client/repository.go

@@ -394,11 +394,26 @@ func (o etagOption) Apply(ms distribution.ManifestService) error {
 	return fmt.Errorf("etag options is a client-only option")
 }
 
+// ReturnContentDigest allows a client to set a the content digest on
+// a successful request from the 'Docker-Content-Digest' header. This
+// returned digest is represents the digest which the registry uses
+// to refer to the content and can be used to delete the content.
+func ReturnContentDigest(dgst *digest.Digest) distribution.ManifestServiceOption {
+	return contentDigestOption{dgst}
+}
+
+type contentDigestOption struct{ digest *digest.Digest }
+
+func (o contentDigestOption) Apply(ms distribution.ManifestService) error {
+	return nil
+}
+
 func (ms *manifests) Get(ctx context.Context, dgst digest.Digest, options ...distribution.ManifestServiceOption) (distribution.Manifest, error) {
 	var (
 		digestOrTag string
 		ref         reference.Named
 		err         error
+		contentDgst *digest.Digest
 	)
 
 	for _, option := range options {
@@ -408,6 +423,8 @@ func (ms *manifests) Get(ctx context.Context, dgst digest.Digest, options ...dis
 			if err != nil {
 				return nil, err
 			}
+		} else if opt, ok := option.(contentDigestOption); ok {
+			contentDgst = opt.digest
 		} else {
 			err := option.Apply(ms)
 			if err != nil {
@@ -450,6 +467,12 @@ func (ms *manifests) Get(ctx context.Context, dgst digest.Digest, options ...dis
 	if resp.StatusCode == http.StatusNotModified {
 		return nil, distribution.ErrManifestNotModified
 	} else if SuccessStatus(resp.StatusCode) {
+		if contentDgst != nil {
+			dgst, err := digest.ParseDigest(resp.Header.Get("Docker-Content-Digest"))
+			if err == nil {
+				*contentDgst = dgst
+			}
+		}
 		mt := resp.Header.Get("Content-Type")
 		body, err := ioutil.ReadAll(resp.Body)
 

docs/client/repository_test.go

@@ -605,6 +605,14 @@ func addTestManifestWithEtag(repo reference.Named, reference string, content []b
 	*m = append(*m, testutil.RequestResponseMapping{Request: getReqWithEtag, Response: getRespWithEtag})
 }
 
+func contentDigestString(mediatype string, content []byte) string {
+	if mediatype == schema1.MediaTypeSignedManifest {
+		m, _, _ := distribution.UnmarshalManifest(mediatype, content)
+		content = m.(*schema1.SignedManifest).Canonical
+	}
+	return digest.Canonical.FromBytes(content).String()
+}
+
 func addTestManifest(repo reference.Named, reference string, mediatype string, content []byte, m *testutil.RequestResponseMap) {
 	*m = append(*m, testutil.RequestResponseMapping{
 		Request: testutil.Request{
@@ -618,6 +626,7 @@ func addTestManifest(repo reference.Named, reference string, mediatype string, c
 				"Content-Length":        {fmt.Sprint(len(content))},
 				"Last-Modified":         {time.Now().Add(-1 * time.Second).Format(time.ANSIC)},
 				"Content-Type":          {mediatype},
+				"Docker-Content-Digest": {contentDigestString(mediatype, content)},
 			}),
 		},
 	})
@@ -632,6 +641,7 @@ func addTestManifest(repo reference.Named, reference string, mediatype string, c
 				"Content-Length":        {fmt.Sprint(len(content))},
 				"Last-Modified":         {time.Now().Add(-1 * time.Second).Format(time.ANSIC)},
 				"Content-Type":          {mediatype},
+				"Docker-Content-Digest": {digest.Canonical.FromBytes(content).String()},
 			}),
 		},
 	})
@@ -710,7 +720,8 @@ func TestV1ManifestFetch(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	manifest, err = ms.Get(ctx, dgst, distribution.WithTag("latest"))
+	var contentDigest digest.Digest
+	manifest, err = ms.Get(ctx, dgst, distribution.WithTag("latest"), ReturnContentDigest(&contentDigest))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -723,6 +734,10 @@ func TestV1ManifestFetch(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if contentDigest != dgst {
+		t.Fatalf("Unexpected returned content digest %v, expected %v", contentDigest, dgst)
+	}
+
 	manifest, err = ms.Get(ctx, dgst, distribution.WithTag("badcontenttype"))
 	if err != nil {
 		t.Fatal(err)