protoutil: add OverwriteMasked method (#4651)

Add a method to copy selected fields from one proto message to another
(of the same type), using a FieldMask. This is intended for use in a new
databroker Patch method.

pkg/protoutil/fieldmasktree_test.go

@@ -0,0 +1,55 @@
+package protoutil
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"google.golang.org/protobuf/types/known/fieldmaskpb"
+)
+
+func TestFieldMaskTree(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		tr := newFieldMaskTree(&fieldmaskpb.FieldMask{})
+		assert.Equal(t, fieldMaskTree(nil), tr)
+	})
+	t.Run("basic", func(t *testing.T) {
+		tr := newFieldMaskTree(&fieldmaskpb.FieldMask{
+			Paths: []string{"foo", "bar", "baz"},
+		})
+		assert.Equal(t, fieldMaskTree{
+			"foo": {},
+			"bar": {},
+			"baz": {},
+		}, tr)
+	})
+	t.Run("nested", func(t *testing.T) {
+		tr := newFieldMaskTree(&fieldmaskpb.FieldMask{
+			Paths: []string{"foo.bar.baz", "foo.bar.xyz", "foo.quux"},
+		})
+		assert.Equal(t, fieldMaskTree{
+			"foo": {
+				"bar": {
+					"baz": {},
+					"xyz": {},
+				},
+				"quux": {},
+			},
+		}, tr)
+	})
+	t.Run("overlapping fields 1", func(t *testing.T) {
+		tr := newFieldMaskTree(&fieldmaskpb.FieldMask{
+			Paths: []string{"foo", "foo.bar"},
+		})
+		assert.Equal(t, fieldMaskTree{
+			"foo": {},
+		}, tr)
+	})
+	t.Run("overlapping fields 2", func(t *testing.T) {
+		tr := newFieldMaskTree(&fieldmaskpb.FieldMask{
+			Paths: []string{"foo.bar", "foo"},
+		})
+		assert.Equal(t, fieldMaskTree{
+			"foo": {},
+		}, tr)
+	})
+}