Support HEAD requests without Docker-Content-Digest header

A statically hosted registry that responds correctly to GET with a
manifest will load the right digest (by looking at the manifest body and
calculating the digest). If the registry returns a HEAD without
`Docker-Content-Digest`, then the client Tags().Get() call will return
an empty digest.

This commit changes the client to fallback to loading the tag via GET if
the `Docker-Content-Digest` header is not set.

Signed-off-by: Clayton Coleman <ccoleman@redhat.com>

registry/client/repository.go

@@ -321,7 +321,8 @@ func (t *tags) Get(ctx context.Context, tag string) (distribution.Descriptor, er
 	defer resp.Body.Close()
 
 	switch {
-	case resp.StatusCode >= 200 && resp.StatusCode < 400:
+	case resp.StatusCode >= 200 && resp.StatusCode < 400 && len(resp.Header.Get("Docker-Content-Digest")) > 0:
+		// if the response is a success AND a Docker-Content-Digest can be retrieved from the headers
 		return descriptorFromResponse(resp)
 	default:
 		// if the response is an error - there will be no body to decode.

registry/client/repository_test.go

@@ -647,7 +647,38 @@ func addTestManifest(repo reference.Named, reference string, mediatype string, c
 			}),
 		},
 	})
+}
 
+func addTestManifestWithoutDigestHeader(repo reference.Named, reference string, mediatype string, content []byte, m *testutil.RequestResponseMap) {
+	*m = append(*m, testutil.RequestResponseMapping{
+		Request: testutil.Request{
+			Method: "GET",
+			Route:  "/v2/" + repo.Name() + "/manifests/" + reference,
+		},
+		Response: testutil.Response{
+			StatusCode: http.StatusOK,
+			Body:       content,
+			Headers: http.Header(map[string][]string{
+				"Content-Length": {fmt.Sprint(len(content))},
+				"Last-Modified":  {time.Now().Add(-1 * time.Second).Format(time.ANSIC)},
+				"Content-Type":   {mediatype},
+			}),
+		},
+	})
+	*m = append(*m, testutil.RequestResponseMapping{
+		Request: testutil.Request{
+			Method: "HEAD",
+			Route:  "/v2/" + repo.Name() + "/manifests/" + reference,
+		},
+		Response: testutil.Response{
+			StatusCode: http.StatusOK,
+			Headers: http.Header(map[string][]string{
+				"Content-Length": {fmt.Sprint(len(content))},
+				"Last-Modified":  {time.Now().Add(-1 * time.Second).Format(time.ANSIC)},
+				"Content-Type":   {mediatype},
+			}),
+		},
+	})
 }
 
 func checkEqualManifest(m1, m2 *schema1.SignedManifest) error {
@@ -994,6 +1025,36 @@ func TestObtainsErrorForMissingTag(t *testing.T) {
 	}
 }
 
+func TestObtainsManifestForTagWithoutHeaders(t *testing.T) {
+	repo, _ := reference.WithName("test.example.com/repo")
+
+	var m testutil.RequestResponseMap
+	m1, dgst, _ := newRandomSchemaV1Manifest(repo, "latest", 6)
+	_, pl, err := m1.Payload()
+	if err != nil {
+		t.Fatal(err)
+	}
+	addTestManifestWithoutDigestHeader(repo, "1.0.0", schema1.MediaTypeSignedManifest, pl, &m)
+
+	e, c := testServer(m)
+	defer c()
+
+	ctx := context.Background()
+	r, err := NewRepository(ctx, repo, e, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tagService := r.Tags(ctx)
+
+	desc, err := tagService.Get(ctx, "1.0.0")
+	if err != nil {
+		t.Fatalf("Expected no error")
+	}
+	if desc.Digest != dgst {
+		t.Fatalf("Unexpected digest")
+	}
+}
 func TestManifestTagsPaginated(t *testing.T) {
 	s := httptest.NewServer(http.NotFoundHandler())
 	defer s.Close()