Add DiffOpt to exclude new paths. (#552)

ygot/diff.go

@@ -353,6 +353,25 @@ type DiffOpt interface {
 	IsDiffOpt()
 }
 
+// IgnoreAdditions is a DiffOpt that indicates newly-added fields should be
+// ignored. The returned Notification will only contain the updates and
+// deletions from original to modified.
+type IgnoreAdditions struct{}
+
+func (*IgnoreAdditions) IsDiffOpt() {}
+
+// hasIgnoreAdditions returns the first IgnoreAdditions from an opts slice, or
+// nil if there isn't one.
+func hasIgnoreAdditions(opts []DiffOpt) *IgnoreAdditions {
+	for _, o := range opts {
+		switch v := o.(type) {
+		case *IgnoreAdditions:
+			return v
+		}
+	}
+	return nil
+}
+
 // DiffPathOpt is a DiffOpt that allows control of the path behaviour of the
 // Diff function.
 type DiffPathOpt struct {
@@ -432,7 +451,9 @@ func Diff(original, modified GoStruct, opts ...DiffOpt) (*gnmipb.Notification, e
 			n.Delete = append(n.Delete, origPath.gNMIPaths...)
 		}
 	}
-
+	if hasIgnoreAdditions(opts) != nil {
+		return n, nil
+	}
 	// Check that all paths that are in the modified struct have been examined, if
 	// not they are updates.
 	for modPath, modVal := range modLeaves {

ygot/diff_test.go

@@ -838,6 +838,34 @@ func TestDiff(t *testing.T) {
 				Val: &gnmipb.TypedValue{Value: &gnmipb.TypedValue_StringVal{"cabernet-sauvignon"}},
 			}},
 		},
+	}, {
+		desc: "one path each modified, deleted, and added with IgnoreNewPaths set",
+		inOrig: &renderExample{
+			IntVal:   Int32(5),
+			FloatVal: Float32(1.5),
+			Int64Val: Int64(100),
+		},
+		inMod: &renderExample{
+			IntVal:   Int32(10),
+			Str:      String("cabernet-sauvignon"),
+			Int64Val: Int64(100),
+		},
+		inOpts: []DiffOpt{&IgnoreAdditions{}},
+		want: &gnmipb.Notification{
+			Delete: []*gnmipb.Path{{
+				Elem: []*gnmipb.PathElem{{
+					Name: "floatval",
+				}},
+			}},
+			Update: []*gnmipb.Update{{
+				Path: &gnmipb.Path{
+					Elem: []*gnmipb.PathElem{{
+						Name: "int-val",
+					}},
+				},
+				Val: &gnmipb.TypedValue{Value: &gnmipb.TypedValue_IntVal{10}},
+			}},
+		},
 	}, {
 		desc:   "extra empty child struct in modified -- no difference",
 		inOrig: &renderExample{},