Use correct path for manifest revision path

Unfortunately, the refactor used the incorrect path for manifest links within a
repository. While this didn't stop the registry from working, it did break
compatibility with 2.0 deployments for manifest fetches.

Tests were added to ensure these are locked down to the appropriate paths.

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

registry/storage/linkedblobstore.go

@@ -11,6 +11,10 @@ import (
 	"github.com/docker/distribution/uuid"
 )
 
+// linkPathFunc describes a function that can resolve a link based on the
+// repository name and digest.
+type linkPathFunc func(pm *pathMapper, name string, dgst digest.Digest) (string, error)
+
 // linkedBlobStore provides a full BlobService that namespaces the blobs to a
 // given repository. Effectively, it manages the links in a given repository
 // that grant access to the global blob store.
@@ -297,5 +301,5 @@ func blobLinkPath(pm *pathMapper, name string, dgst digest.Digest) (string, erro
 
 // manifestRevisionLinkPath provides the path to the manifest revision link.
 func manifestRevisionLinkPath(pm *pathMapper, name string, dgst digest.Digest) (string, error) {
-	return pm.path(layerLinkPathSpec{name: name, digest: dgst})
+	return pm.path(manifestRevisionLinkPathSpec{name: name, revision: dgst})
 }

registry/storage/manifeststore_test.go

@@ -362,3 +362,37 @@ func TestManifestStorage(t *testing.T) {
 		t.Errorf("Unexpected success deleting while disabled")
 	}
 }
+
+// TestLinkPathFuncs ensures that the link path functions behavior are locked
+// down and implemented as expected.
+func TestLinkPathFuncs(t *testing.T) {
+	for _, testcase := range []struct {
+		repo       string
+		digest     digest.Digest
+		linkPathFn linkPathFunc
+		expected   string
+	}{
+		{
+			repo:       "foo/bar",
+			digest:     "sha256:deadbeaf",
+			linkPathFn: blobLinkPath,
+			expected:   "/docker/registry/v2/repositories/foo/bar/_layers/sha256/deadbeaf/link",
+		},
+		{
+			repo:       "foo/bar",
+			digest:     "sha256:deadbeaf",
+			linkPathFn: manifestRevisionLinkPath,
+			expected:   "/docker/registry/v2/repositories/foo/bar/_manifests/revisions/sha256/deadbeaf/link",
+		},
+	} {
+		p, err := testcase.linkPathFn(defaultPathMapper, testcase.repo, testcase.digest)
+		if err != nil {
+			t.Fatalf("unexpected error calling linkPathFn(pm, %q, %q): %v", testcase.repo, testcase.digest, err)
+		}
+
+		if p != testcase.expected {
+			t.Fatalf("incorrect path returned: %q != %q", p, testcase.expected)
+		}
+	}
+
+}