encoding/textpb: add tests for nil message in repeated and map

Updates golang/protobuf#798 by adding testcases to show the intention.

Also, slightly move code blocks in encode.go w/o affecting logic to make
it cleaner.

Change-Id: I14575f6e7139a0908483bd318b599339c2daf8ad
Reviewed-on: https://go-review.googlesource.com/c/161717
Reviewed-by: Damien Neil <dneil@google.com>

encoding/textpb/encode.go

@@ -46,9 +46,7 @@ func (o MarshalOptions) Marshal(m proto.Message) ([]byte, error) {
 	}
 
 	var nerr errors.NonFatal
-	var v text.Value
-	var err error
-	v, err = o.marshalMessage(m.ProtoReflect())
+	v, err := o.marshalMessage(m.ProtoReflect())
 	if !nerr.Merge(err) {
 		return nil, err
 	}
@@ -99,10 +97,14 @@ func (o MarshalOptions) marshalMessage(m pref.Message) (text.Value, error) {
 			continue
 		}
 
-		tname := text.ValueOf(fd.Name())
+		name := text.ValueOf(fd.Name())
+		// Use type name for group field name.
+		if fd.Kind() == pref.GroupKind {
+			name = text.ValueOf(fd.MessageType().Name())
+		}
 		pval := knownFields.Get(num)
 		var err error
-		msgFields, err = o.appendField(msgFields, tname, pval, fd)
+		msgFields, err = o.appendField(msgFields, name, pval, fd)
 		if !nerr.Merge(err) {
 			return text.Value{}, err
 		}
@@ -126,14 +128,9 @@ func (o MarshalOptions) marshalMessage(m pref.Message) (text.Value, error) {
 }
 
 // appendField marshals a protoreflect.Value and appends it to the given [][2]text.Value.
-func (o MarshalOptions) appendField(msgFields [][2]text.Value, tname text.Value, pval pref.Value, fd pref.FieldDescriptor) ([][2]text.Value, error) {
+func (o MarshalOptions) appendField(msgFields [][2]text.Value, name text.Value, pval pref.Value, fd pref.FieldDescriptor) ([][2]text.Value, error) {
 	var nerr errors.NonFatal
 
-	// Use type name for group field name.
-	if fd.Kind() == pref.GroupKind {
-		tname = text.ValueOf(fd.MessageType().Name())
-	}
-
 	if fd.Cardinality() == pref.Repeated {
 		// Map or repeated fields.
 		var items []text.Value
@@ -152,7 +149,7 @@ func (o MarshalOptions) appendField(msgFields [][2]text.Value, tname text.Value,
 
 		// Add each item as key: value field.
 		for _, item := range items {
-			msgFields = append(msgFields, [2]text.Value{tname, item})
+			msgFields = append(msgFields, [2]text.Value{name, item})
 		}
 	} else {
 		// Required or optional fields.
@@ -160,7 +157,7 @@ func (o MarshalOptions) appendField(msgFields [][2]text.Value, tname text.Value,
 		if !nerr.Merge(err) {
 			return msgFields, err
 		}
-		msgFields = append(msgFields, [2]text.Value{tname, tval})
+		msgFields = append(msgFields, [2]text.Value{name, tval})
 	}
 
 	return msgFields, nerr.E

encoding/textpb/encode_test.go

@@ -529,6 +529,14 @@ rpt_nested: {
   }
 }
 rpt_nested: {}
+`,
+	}, {
+		desc: "repeated nested messages contains nil value",
+		input: &pb2.Nests{
+			RptNested: []*pb2.Nested{nil, {}},
+		},
+		want: `rpt_nested: {}
+rpt_nested: {}
 `,
 	}, {
 		desc: "repeated group fields",
@@ -677,6 +685,18 @@ str_to_oneofs: {
     str: "hello"
   }
 }
+`,
+	}, {
+		desc: "map field nil message value",
+		input: &pb2.Maps{
+			StrToNested: map[string]*pb2.Nested{
+				"nil": nil,
+			},
+		},
+		want: `str_to_nested: {
+  key: "nil"
+  value: {}
+}
 `,
 	}, {
 		desc:    "proto2 required fields not set",