dns/dnsmessage: tweak the error value on nil resource body

Change-Id: I17444d1cc3ce03b6947aaae18d4cecdf9c36193f
Reviewed-on: https://go-review.googlesource.com/47110Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

dns/dnsmessage/message.go

@@ -80,6 +80,7 @@ var (
 	errReserved           = errors.New("segment prefix is reserved")
 	errTooManyPtr         = errors.New("too many pointers (>10)")
 	errInvalidPtr         = errors.New("invalid pointer")
+	errNilResouceBody     = errors.New("nil resource body")
 	errResourceLen        = errors.New("insufficient data for resource body length")
 	errSegTooLong         = errors.New("segment length too long")
 	errZeroSegLen         = errors.New("zero length segment")
@@ -281,7 +282,7 @@ type ResourceBody interface {
 
 func (r *Resource) pack(msg []byte, compression map[string]int) ([]byte, error) {
 	if r.Body == nil {
-		return msg, &nestedError{"Resource", errors.New("nil resource body")}
+		return msg, errNilResouceBody
 	}
 	oldMsg := msg
 	r.Header.Type = r.Body.realType()

dns/dnsmessage/message_test.go

@@ -535,8 +535,12 @@ func TestBuilder(t *testing.T) {
 }
 
 func TestResourcePack(t *testing.T) {
-	for _, m := range []Message{
+	for _, tt := range []struct {
+		m   Message
+		err error
+	}{
 		{
+			Message{
 				Questions: []Question{
 					{
 						Name:  mustNewName("."),
@@ -546,7 +550,10 @@ func TestResourcePack(t *testing.T) {
 				},
 				Answers: []Resource{{ResourceHeader{}, nil}},
 			},
+			&nestedError{"packing Answer", errNilResouceBody},
+		},
 		{
+			Message{
 				Questions: []Question{
 					{
 						Name:  mustNewName("."),
@@ -556,7 +563,14 @@ func TestResourcePack(t *testing.T) {
 				},
 				Authorities: []Resource{{ResourceHeader{}, (*NSResource)(nil)}},
 			},
+			&nestedError{"packing Authority",
+				&nestedError{"ResourceHeader",
+					&nestedError{"Name", errNonCanonicalName},
+				},
+			},
+		},
 		{
+			Message{
 				Questions: []Question{
 					{
 						Name:  mustNewName("."),
@@ -566,9 +580,12 @@ func TestResourcePack(t *testing.T) {
 				},
 				Additionals: []Resource{{ResourceHeader{}, nil}},
 			},
+			&nestedError{"packing Additional", errNilResouceBody},
+		},
 	} {
-		if _, err := m.Pack(); err == nil {
-			t.Errorf("should fail: %v", m)
+		_, err := tt.m.Pack()
+		if !reflect.DeepEqual(err, tt.err) {
+			t.Errorf("got %v for %v; want %v", err, tt.m, tt.err)
 		}
 	}
 }