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>

docs/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)
+		}
+	}
+
+}