Implement oneof support.

This includes the code generation changes,
and the infrastructure to wire it up to the encode/decode machinery.

The overall API changes are these:
  - oneofs in a message are replaced by a single interface field
  - each field in a oneof gets a distinguished type that satisfies
    the corresponding interface
  - a type switch may be used to distinguish between oneof fields

Fixes #29.

Makefile

@@ -51,5 +51,6 @@ nuke:
 regenerate:
 	make -C protoc-gen-go/descriptor regenerate
 	make -C protoc-gen-go/plugin regenerate
+	make -C protoc-gen-go/testdata regenerate
 	make -C proto/testdata regenerate
 	make -C jsonpb/jsonpb_test_proto regenerate

jsonpb/jsonpb.go

@@ -111,6 +111,18 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent string
 			}
 		}
 
+		// Oneof fields need special handling.
+		if valueField.Tag.Get("protobuf_oneof") != "" {
+			// value is an interface containing &T{real_value}.
+			sv := value.Elem().Elem() // interface -> *T -> T
+			value = sv.Field(0)
+			valueField = sv.Type().Field(0)
+
+			var p proto.Properties
+			p.Parse(sv.Type().Field(0).Tag.Get("protobuf"))
+			fieldName = p.OrigName
+		}
+
 		out.write(writeBeforeField)
 		if m.Indent != "" {
 			out.write(indent)
@@ -319,6 +331,44 @@ func unmarshalValue(target reflect.Value, inputValue json.RawMessage) error {
 				delete(jsonFields, fieldName)
 			}
 		}
+		// Check for any oneof fields.
+		// This might be slow; we can optimise it if it becomes a problem.
+		type oneofMessage interface {
+			XXX_OneofFuncs() (func(proto.Message, *proto.Buffer) error, func(proto.Message, int, int, *proto.Buffer) (bool, error), []interface{})
+		}
+		var oneofTypes []interface{}
+		if om, ok := reflect.Zero(reflect.PtrTo(targetType)).Interface().(oneofMessage); ok {
+			_, _, oneofTypes = om.XXX_OneofFuncs()
+		}
+		for fname, raw := range jsonFields {
+			for _, oot := range oneofTypes {
+				sp := reflect.ValueOf(oot).Type() // *T
+				var props proto.Properties
+				props.Parse(sp.Elem().Field(0).Tag.Get("protobuf"))
+				if props.OrigName != fname {
+					continue
+				}
+				nv := reflect.New(sp.Elem())
+				// There will be exactly one interface field that
+				// this new value is assignable to.
+				for i := 0; i < targetType.NumField(); i++ {
+					f := targetType.Field(i)
+					if f.Type.Kind() != reflect.Interface {
+						continue
+					}
+					if !nv.Type().AssignableTo(f.Type) {
+						continue
+					}
+					target.Field(i).Set(nv)
+					break
+				}
+				if err := unmarshalValue(nv.Elem().Field(0), raw); err != nil {
+					return err
+				}
+				delete(jsonFields, fname)
+				break
+			}
+		}
 		if len(jsonFields) > 0 {
 			// Pick any field to be the scapegoat.
 			var f string

jsonpb/jsonpb_test.go

@@ -32,6 +32,7 @@
 package jsonpb
 
 import (
+	"reflect"
 	"testing"
 
 	pb "github.com/golang/protobuf/jsonpb/jsonpb_test_proto"
@@ -291,6 +292,8 @@ var marshalingTests = []struct {
 	{"proto2 map<bool, Object>", marshaler,
 		&pb.Maps{MBoolSimple: map[bool]*pb.Simple{true: &pb.Simple{OInt32: proto.Int32(1)}}},
 		`{"m_bool_simple":{"true":{"o_int32":1}}}`},
+	{"oneof, not set", marshaler, &pb.MsgWithOneof{}, `{}`},
+	{"oneof, set", marshaler, &pb.MsgWithOneof{Union: &pb.MsgWithOneof_Title{"Grand Poobah"}}, `{"title":"Grand Poobah"}`},
 }
 
 func TestMarshaling(t *testing.T) {
@@ -325,13 +328,13 @@ var unmarshalingTests = []struct {
 	{"map<int64, int32>", `{"nummy":{"1":2,"3":4}}`, &pb.Mappy{Nummy: map[int64]int32{1: 2, 3: 4}}},
 	{"map<string, string>", `{"strry":{"\"one\"":"two","three":"four"}}`, &pb.Mappy{Strry: map[string]string{`"one"`: "two", "three": "four"}}},
 	{"map<int32, Object>", `{"objjy":{"1":{"dub":1}}}`, &pb.Mappy{Objjy: map[int32]*pb.Simple3{1: &pb.Simple3{Dub: 1}}}},
+	{"oneof", `{"salary":31000}`, &pb.MsgWithOneof{Union: &pb.MsgWithOneof_Salary{31000}}},
 }
 
 func TestUnmarshaling(t *testing.T) {
 	for _, tt := range unmarshalingTests {
 		// Make a new instance of the type of our expected object.
-		p := proto.Clone(tt.pb)
-		p.Reset()
+		p := reflect.New(reflect.TypeOf(tt.pb).Elem()).Interface().(proto.Message)
 
 		err := UnmarshalString(tt.json, p)
 		if err != nil {

jsonpb/jsonpb_test_proto/more_test_objects.pb.go

@@ -16,9 +16,13 @@ It has these top-level messages:
 package jsonpb
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
+import math "math"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
 
 type Simple3 struct {
 	Dub float64 `protobuf:"fixed64,1,opt,name=dub" json:"dub,omitempty"`
@@ -74,6 +78,3 @@ func (m *Mappy) GetBooly() map[bool]bool {
 	}
 	return nil
 }
-
-func init() {
-}

jsonpb/jsonpb_test_proto/test_objects.pb.go

@@ -5,10 +5,12 @@
 package jsonpb
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
 import math "math"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
 var _ = math.Inf
 
 type Widget_Color int32
@@ -322,6 +324,100 @@ func (m *Maps) GetMBoolSimple() map[bool]*Simple {
 	return nil
 }
 
+type MsgWithOneof struct {
+	// Types that are valid to be assigned to Union:
+	//	*MsgWithOneof_Title
+	//	*MsgWithOneof_Salary
+	Union            isMsgWithOneof_Union `protobuf_oneof:"union"`
+	XXX_unrecognized []byte               `json:"-"`
+}
+
+func (m *MsgWithOneof) Reset()         { *m = MsgWithOneof{} }
+func (m *MsgWithOneof) String() string { return proto.CompactTextString(m) }
+func (*MsgWithOneof) ProtoMessage()    {}
+
+type isMsgWithOneof_Union interface {
+	isMsgWithOneof_Union()
+}
+
+type MsgWithOneof_Title struct {
+	Title string `protobuf:"bytes,1,opt,name=title"`
+}
+type MsgWithOneof_Salary struct {
+	Salary int64 `protobuf:"varint,2,opt,name=salary"`
+}
+
+func (*MsgWithOneof_Title) isMsgWithOneof_Union()  {}
+func (*MsgWithOneof_Salary) isMsgWithOneof_Union() {}
+
+func (m *MsgWithOneof) GetUnion() isMsgWithOneof_Union {
+	if m != nil {
+		return m.Union
+	}
+	return nil
+}
+
+func (m *MsgWithOneof) GetTitle() string {
+	if x, ok := m.GetUnion().(*MsgWithOneof_Title); ok {
+		return x.Title
+	}
+	return ""
+}
+
+func (m *MsgWithOneof) GetSalary() int64 {
+	if x, ok := m.GetUnion().(*MsgWithOneof_Salary); ok {
+		return x.Salary
+	}
+	return 0
+}
+
+// XXX_OneofFuncs is for the internal use of the proto package.
+func (*MsgWithOneof) XXX_OneofFuncs() (func(msg proto.Message, b *proto.Buffer) error, func(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error), []interface{}) {
+	return _MsgWithOneof_OneofMarshaler, _MsgWithOneof_OneofUnmarshaler, []interface{}{
+		(*MsgWithOneof_Title)(nil),
+		(*MsgWithOneof_Salary)(nil),
+	}
+}
+
+func _MsgWithOneof_OneofMarshaler(msg proto.Message, b *proto.Buffer) error {
+	m := msg.(*MsgWithOneof)
+	// union
+	switch x := m.Union.(type) {
+	case *MsgWithOneof_Title:
+		b.EncodeVarint(1<<3 | proto.WireBytes)
+		b.EncodeStringBytes(x.Title)
+	case *MsgWithOneof_Salary:
+		b.EncodeVarint(2<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Salary))
+	case nil:
+	default:
+		return fmt.Errorf("MsgWithOneof.Union has unexpected type %T", x)
+	}
+	return nil
+}
+
+func _MsgWithOneof_OneofUnmarshaler(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error) {
+	m := msg.(*MsgWithOneof)
+	switch tag {
+	case 1: // union.title
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeStringBytes()
+		m.Union = &MsgWithOneof_Title{x}
+		return true, err
+	case 2: // union.salary
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &MsgWithOneof_Salary{int64(x)}
+		return true, err
+	default:
+		return false, nil
+	}
+}
+
 func init() {
 	proto.RegisterEnum("jsonpb.Widget_Color", Widget_Color_name, Widget_Color_value)
 }

jsonpb/jsonpb_test_proto/test_objects.proto

@@ -84,3 +84,10 @@ message Maps {
   map<int64, string> m_int64_str = 1;
   map<bool, Simple> m_bool_simple = 2;
 }
+
+message MsgWithOneof {
+  oneof union {
+    string title = 1;
+    int64 salary = 2;
+  }
+}

proto/all_test.go

@@ -1958,6 +1958,58 @@ func TestMapFieldWithNil(t *testing.T) {
 	}
 }
 
+func TestOneof(t *testing.T) {
+	m := &Communique{}
+	b, err := Marshal(m)
+	if err != nil {
+		t.Fatalf("Marshal of empty message with oneof: %v", err)
+	}
+	if len(b) != 0 {
+		t.Errorf("Marshal of empty message yielded too many bytes: %v", b)
+	}
+
+	m = &Communique{
+		Union: &Communique_Name{"Barry"},
+	}
+
+	// Round-trip.
+	b, err = Marshal(m)
+	if err != nil {
+		t.Fatalf("Marshal of message with oneof: %v", err)
+	}
+	if len(b) != 7 { // name tag/wire (1) + name len (1) + name (5)
+		t.Errorf("Incorrect marshal of message with oneof: %v", b)
+	}
+	m.Reset()
+	if err := Unmarshal(b, m); err != nil {
+		t.Fatalf("Unmarshal of message with oneof: %v", err)
+	}
+	if x, ok := m.Union.(*Communique_Name); !ok || x.Name != "Barry" {
+		t.Errorf("After round trip, Union = %+v", m.Union)
+	}
+	if name := m.GetName(); name != "Barry" {
+		t.Errorf("After round trip, GetName = %q, want %q", name, "Barry")
+	}
+
+	// Let's try with a message in the oneof.
+	m.Union = &Communique_Msg{&Strings{StringField: String("deep deep string")}}
+	b, err = Marshal(m)
+	if err != nil {
+		t.Fatalf("Marshal of message with oneof set to message: %v", err)
+	}
+	if len(b) != 20 { // msg tag/wire (1) + msg len (1) + msg (1 + 1 + 16)
+		t.Errorf("Incorrect marshal of message with oneof set to message: %v", b)
+	}
+	m.Reset()
+	if err := Unmarshal(b, m); err != nil {
+		t.Fatalf("Unmarshal of message with oneof set to message: %v", err)
+	}
+	ss, ok := m.Union.(*Communique_Msg)
+	if !ok || ss.Msg.GetStringField() != "deep deep string" {
+		t.Errorf("After round trip with oneof set to message, Union = %+v", m.Union)
+	}
+}
+
 // Benchmarks
 
 func testMsg() *GoTest {

proto/clone.go

@@ -120,6 +120,13 @@ func mergeAny(out, in reflect.Value, viaPtr bool, prop *Properties) {
 			return
 		}
 		out.Set(in)
+	case reflect.Interface:
+		// Probably a oneof field; copy non-nil values.
+		if in.IsNil() {
+			return
+		}
+		out.Set(reflect.New(in.Elem().Elem().Type())) // interface -> *T -> T -> new(T)
+		mergeAny(out.Elem(), in.Elem(), false, nil)
 	case reflect.Map:
 		if in.Len() == 0 {
 			return

proto/clone_test.go

@@ -232,6 +232,28 @@ var mergeTests = []struct {
 			Data:       []byte("texas!"),
 		},
 	},
+	// Oneof fields should merge by assignment.
+	{
+		src: &pb.Communique{
+			Union: &pb.Communique_Number{41},
+		},
+		dst: &pb.Communique{
+			Union: &pb.Communique_Name{"Bobby Tables"},
+		},
+		want: &pb.Communique{
+			Union: &pb.Communique_Number{41},
+		},
+	},
+	// Oneof nil is the same as not set.
+	{
+		src: &pb.Communique{},
+		dst: &pb.Communique{
+			Union: &pb.Communique_Name{"Bobby Tables"},
+		},
+		want: &pb.Communique{
+			Union: &pb.Communique_Name{"Bobby Tables"},
+		},
+	},
 }
 
 func TestMerge(t *testing.T) {

proto/decode.go

@@ -46,6 +46,10 @@ import (
 // errOverflow is returned when an integer is too large to be represented.
 var errOverflow = errors.New("proto: integer overflow")
 
+// ErrInternalBadWireType is returned by generated code when an incorrect
+// wire type is encountered. It does not get returned to user code.
+var ErrInternalBadWireType = errors.New("proto: internal error: bad wiretype for oneof")
+
 // The fundamental decoders that interpret bytes on the wire.
 // Those that take integer types all return uint64 and are
 // therefore of type valueDecoder.
@@ -314,6 +318,24 @@ func UnmarshalMerge(buf []byte, pb Message) error {
 	return NewBuffer(buf).Unmarshal(pb)
 }
 
+// DecodeMessage reads a count-delimited message from the Buffer.
+func (p *Buffer) DecodeMessage(pb Message) error {
+	enc, err := p.DecodeRawBytes(false)
+	if err != nil {
+		return err
+	}
+	return NewBuffer(enc).Unmarshal(pb)
+}
+
+// DecodeGroup reads a tag-delimited group from the Buffer.
+func (p *Buffer) DecodeGroup(pb Message) error {
+	typ, base, err := getbase(pb)
+	if err != nil {
+		return err
+	}
+	return p.unmarshalType(typ.Elem(), GetProperties(typ.Elem()), true, base)
+}
+
 // Unmarshal parses the protocol buffer representation in the
 // Buffer and places the decoded result in pb.  If the struct
 // underlying pb does not match the data in the buffer, the results can be
@@ -377,6 +399,20 @@ func (o *Buffer) unmarshalType(st reflect.Type, prop *StructProperties, is_group
 					continue
 				}
 			}
+			// Maybe it's a oneof?
+			if prop.oneofUnmarshaler != nil {
+				m := structPointer_Interface(base, st).(Message)
+				// First return value indicates whether tag is a oneof field.
+				ok, err = prop.oneofUnmarshaler(m, tag, wire, o)
+				if err == ErrInternalBadWireType {
+					// Map the error to something more descriptive.
+					// Do the formatting here to save generated code space.
+					err = fmt.Errorf("bad wiretype for oneof field in %T", m)
+				}
+				if ok {
+					continue
+				}
+			}
 			err = o.skipAndSave(st, tag, wire, base, prop.unrecField)
 			continue
 		}

proto/encode.go

@@ -228,6 +228,20 @@ func Marshal(pb Message) ([]byte, error) {
 	return p.buf, err
 }
 
+// EncodeMessage writes the protocol buffer to the Buffer,
+// prefixed by a varint-encoded length.
+func (p *Buffer) EncodeMessage(pb Message) error {
+	t, base, err := getbase(pb)
+	if structPointer_IsNil(base) {
+		return ErrNil
+	}
+	if err == nil {
+		var state errorState
+		err = p.enc_len_struct(GetProperties(t.Elem()), base, &state)
+	}
+	return err
+}
+
 // Marshal takes the protocol buffer
 // and encodes it into the wire format, writing the result to the
 // Buffer.
@@ -1201,6 +1215,14 @@ func (o *Buffer) enc_struct(prop *StructProperties, base structPointer) error {
 		}
 	}
 
+	// Do oneof fields.
+	if prop.oneofMarshaler != nil {
+		m := structPointer_Interface(base, prop.stype).(Message)
+		if err := prop.oneofMarshaler(m, o); err != nil {
+			return err
+		}
+	}
+
 	// Add unrecognized fields at the end.
 	if prop.unrecField.IsValid() {
 		v := *structPointer_Bytes(base, prop.unrecField)
@@ -1226,6 +1248,27 @@ func size_struct(prop *StructProperties, base structPointer) (n int) {
 		n += len(v)
 	}
 
+	// Factor in any oneof fields.
+	// TODO: This could be faster and use less reflection.
+	if prop.oneofMarshaler != nil {
+		sv := reflect.ValueOf(structPointer_Interface(base, prop.stype)).Elem()
+		for i := 0; i < prop.stype.NumField(); i++ {
+			fv := sv.Field(i)
+			if fv.Kind() != reflect.Interface || fv.IsNil() {
+				continue
+			}
+			if prop.stype.Field(i).Tag.Get("protobuf_oneof") == "" {
+				continue
+			}
+			spv := fv.Elem()         // interface -> *T
+			sv := spv.Elem()         // *T -> T
+			sf := sv.Type().Field(0) // StructField inside T
+			var prop Properties
+			prop.Init(sf.Type, "whatever", sf.Tag.Get("protobuf"), &sf)
+			n += prop.size(&prop, toStructPointer(spv))
+		}
+	}
+
 	return
 }
 

proto/equal.go

@@ -154,6 +154,17 @@ func equalAny(v1, v2 reflect.Value) bool {
 		return v1.Float() == v2.Float()
 	case reflect.Int32, reflect.Int64:
 		return v1.Int() == v2.Int()
+	case reflect.Interface:
+		// Probably a oneof field; compare the inner values.
+		n1, n2 := v1.IsNil(), v2.IsNil()
+		if n1 || n2 {
+			return n1 == n2
+		}
+		e1, e2 := v1.Elem(), v2.Elem()
+		if e1.Type() != e2.Type() {
+			return false
+		}
+		return equalAny(e1, e2)
 	case reflect.Map:
 		if v1.Len() != v2.Len() {
 			return false

proto/equal_test.go

@@ -180,6 +180,24 @@ var EqualTests = []struct {
 		&pb.MessageWithMap{NameMapping: map[int32]string{1: "Rob"}},
 		false,
 	},
+	{
+		"oneof same",
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		true,
+	},
+	{
+		"oneof one nil",
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		&pb.Communique{},
+		false,
+	},
+	{
+		"oneof different",
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		&pb.Communique{Union: &pb.Communique_Name{"Bobby Tables"}},
+		false,
+	},
 }
 
 func TestEqual(t *testing.T) {

proto/lib.go

@@ -66,6 +66,8 @@ for a protocol buffer variable v:
 	that contain it (if any) followed by the CamelCased name of the
 	extension field itself.  HasExtension, ClearExtension, GetExtension
 	and SetExtension are functions for manipulating extensions.
+  - Oneof field sets are given a single field in their message,
+	with distinguished wrapper types for each possible field value.
   - Marshal and Unmarshal are functions to encode and decode the wire format.
 
 The simplest way to describe this is to see an example.

proto/properties.go

@@ -84,6 +84,12 @@ type decoder func(p *Buffer, prop *Properties, base structPointer) error
 // A valueDecoder decodes a single integer in a particular encoding.
 type valueDecoder func(o *Buffer) (x uint64, err error)
 
+// A oneofMarshaler does the marshaling for all oneof fields in a message.
+type oneofMarshaler func(Message, *Buffer) error
+
+// A oneofUnmarshaler does the unmarshaling for a oneof field in a message.
+type oneofUnmarshaler func(Message, int, int, *Buffer) (bool, error)
+
 // tagMap is an optimization over map[int]int for typical protocol buffer
 // use-cases. Encoded protocol buffers are often in tag order with small tag
 // numbers.
@@ -132,6 +138,11 @@ type StructProperties struct {
 	order            []int          // list of struct field numbers in tag order
 	unrecField       field          // field id of the XXX_unrecognized []byte field
 	extendable       bool           // is this an extendable proto
+
+	oneofMarshaler   oneofMarshaler
+	oneofUnmarshaler oneofUnmarshaler
+	stype            reflect.Type
+	oneofTypes       []interface{}
 }
 
 // Implement the sorting interface so we can sort the fields in tag order, as recommended by the spec.
@@ -665,6 +676,7 @@ func getPropertiesLocked(t reflect.Type) *StructProperties {
 		if f.Name == "XXX_unrecognized" { // special case
 			prop.unrecField = toField(&f)
 		}
+		oneof := f.Tag.Get("protobuf_oneof") != "" // special case
 		prop.Prop[i] = p
 		prop.order[i] = i
 		if debug {
@@ -674,7 +686,7 @@ func getPropertiesLocked(t reflect.Type) *StructProperties {
 			}
 			print("\n")
 		}
-		if p.enc == nil && !strings.HasPrefix(f.Name, "XXX_") {
+		if p.enc == nil && !strings.HasPrefix(f.Name, "XXX_") && !oneof {
 			fmt.Fprintln(os.Stderr, "proto: no encoder for", f.Name, f.Type.String(), "[GetProperties]")
 		}
 	}
@@ -682,6 +694,14 @@ func getPropertiesLocked(t reflect.Type) *StructProperties {
 	// Re-order prop.order.
 	sort.Sort(prop)
 
+	type oneofMessage interface {
+		XXX_OneofFuncs() (func(Message, *Buffer) error, func(Message, int, int, *Buffer) (bool, error), []interface{})
+	}
+	if om, ok := reflect.Zero(reflect.PtrTo(t)).Interface().(oneofMessage); ok {
+		prop.oneofMarshaler, prop.oneofUnmarshaler, prop.oneofTypes = om.XXX_OneofFuncs()
+		prop.stype = t
+	}
+
 	// build required counts
 	// build tags
 	reqCount := 0

proto/size_test.go

@@ -124,6 +124,10 @@ var SizeTests = []struct {
 	{"map field with big entry", &pb.MessageWithMap{NameMapping: map[int32]string{8: strings.Repeat("x", 125)}}},
 	{"map field with big key and val", &pb.MessageWithMap{StrToStr: map[string]string{strings.Repeat("x", 70): strings.Repeat("y", 70)}}},
 	{"map field with big numeric key", &pb.MessageWithMap{NameMapping: map[int32]string{0xf00d: "om nom nom"}}},
+
+	{"oneof not set", &pb.Communique{}},
+	{"oneof int32", &pb.Communique{Union: &pb.Communique_Number{3}}},
+	{"oneof string", &pb.Communique{Union: &pb.Communique_Name{"Rhythmic Fman"}}},
 }
 
 func TestSize(t *testing.T) {

proto/testdata/test.pb.go

@@ -35,14 +35,17 @@ It has these top-level messages:
 	GroupNew
 	FloatingPoint
 	MessageWithMap
+	Communique
 */
 package testdata
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
 import math "math"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
 var _ = math.Inf
 
 type FOO int32
@@ -1986,6 +1989,205 @@ func (m *MessageWithMap) GetStrToStr() map[string]string {
 	return nil
 }
 
+type Communique struct {
+	MakeMeCry *bool `protobuf:"varint,1,opt,name=make_me_cry" json:"make_me_cry,omitempty"`
+	// This is a oneof, called "union".
+	//
+	// Types that are valid to be assigned to Union:
+	//	*Communique_Number
+	//	*Communique_Name
+	//	*Communique_Data
+	//	*Communique_TempC
+	//	*Communique_Col
+	//	*Communique_Msg
+	Union            isCommunique_Union `protobuf_oneof:"union"`
+	XXX_unrecognized []byte             `json:"-"`
+}
+
+func (m *Communique) Reset()         { *m = Communique{} }
+func (m *Communique) String() string { return proto.CompactTextString(m) }
+func (*Communique) ProtoMessage()    {}
+
+type isCommunique_Union interface {
+	isCommunique_Union()
+}
+
+type Communique_Number struct {
+	Number int32 `protobuf:"varint,5,opt,name=number"`
+}
+type Communique_Name struct {
+	Name string `protobuf:"bytes,6,opt,name=name"`
+}
+type Communique_Data struct {
+	Data []byte `protobuf:"bytes,7,opt,name=data"`
+}
+type Communique_TempC struct {
+	TempC float64 `protobuf:"fixed64,8,opt,name=temp_c"`
+}
+type Communique_Col struct {
+	Col MyMessage_Color `protobuf:"varint,9,opt,name=col,enum=testdata.MyMessage_Color"`
+}
+type Communique_Msg struct {
+	Msg *Strings `protobuf:"bytes,10,opt,name=msg"`
+}
+
+func (*Communique_Number) isCommunique_Union() {}
+func (*Communique_Name) isCommunique_Union()   {}
+func (*Communique_Data) isCommunique_Union()   {}
+func (*Communique_TempC) isCommunique_Union()  {}
+func (*Communique_Col) isCommunique_Union()    {}
+func (*Communique_Msg) isCommunique_Union()    {}
+
+func (m *Communique) GetUnion() isCommunique_Union {
+	if m != nil {
+		return m.Union
+	}
+	return nil
+}
+
+func (m *Communique) GetMakeMeCry() bool {
+	if m != nil && m.MakeMeCry != nil {
+		return *m.MakeMeCry
+	}
+	return false
+}
+
+func (m *Communique) GetNumber() int32 {
+	if x, ok := m.GetUnion().(*Communique_Number); ok {
+		return x.Number
+	}
+	return 0
+}
+
+func (m *Communique) GetName() string {
+	if x, ok := m.GetUnion().(*Communique_Name); ok {
+		return x.Name
+	}
+	return ""
+}
+
+func (m *Communique) GetData() []byte {
+	if x, ok := m.GetUnion().(*Communique_Data); ok {
+		return x.Data
+	}
+	return nil
+}
+
+func (m *Communique) GetTempC() float64 {
+	if x, ok := m.GetUnion().(*Communique_TempC); ok {
+		return x.TempC
+	}
+	return 0
+}
+
+func (m *Communique) GetCol() MyMessage_Color {
+	if x, ok := m.GetUnion().(*Communique_Col); ok {
+		return x.Col
+	}
+	return MyMessage_RED
+}
+
+func (m *Communique) GetMsg() *Strings {
+	if x, ok := m.GetUnion().(*Communique_Msg); ok {
+		return x.Msg
+	}
+	return nil
+}
+
+// XXX_OneofFuncs is for the internal use of the proto package.
+func (*Communique) XXX_OneofFuncs() (func(msg proto.Message, b *proto.Buffer) error, func(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error), []interface{}) {
+	return _Communique_OneofMarshaler, _Communique_OneofUnmarshaler, []interface{}{
+		(*Communique_Number)(nil),
+		(*Communique_Name)(nil),
+		(*Communique_Data)(nil),
+		(*Communique_TempC)(nil),
+		(*Communique_Col)(nil),
+		(*Communique_Msg)(nil),
+	}
+}
+
+func _Communique_OneofMarshaler(msg proto.Message, b *proto.Buffer) error {
+	m := msg.(*Communique)
+	// union
+	switch x := m.Union.(type) {
+	case *Communique_Number:
+		b.EncodeVarint(5<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Number))
+	case *Communique_Name:
+		b.EncodeVarint(6<<3 | proto.WireBytes)
+		b.EncodeStringBytes(x.Name)
+	case *Communique_Data:
+		b.EncodeVarint(7<<3 | proto.WireBytes)
+		b.EncodeRawBytes(x.Data)
+	case *Communique_TempC:
+		b.EncodeVarint(8<<3 | proto.WireFixed64)
+		b.EncodeFixed64(math.Float64bits(x.TempC))
+	case *Communique_Col:
+		b.EncodeVarint(9<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Col))
+	case *Communique_Msg:
+		b.EncodeVarint(10<<3 | proto.WireBytes)
+		if err := b.EncodeMessage(x.Msg); err != nil {
+			return err
+		}
+	case nil:
+	default:
+		return fmt.Errorf("Communique.Union has unexpected type %T", x)
+	}
+	return nil
+}
+
+func _Communique_OneofUnmarshaler(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error) {
+	m := msg.(*Communique)
+	switch tag {
+	case 5: // union.number
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Number{int32(x)}
+		return true, err
+	case 6: // union.name
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeStringBytes()
+		m.Union = &Communique_Name{x}
+		return true, err
+	case 7: // union.data
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeRawBytes(true)
+		m.Union = &Communique_Data{x}
+		return true, err
+	case 8: // union.temp_c
+		if wire != proto.WireFixed64 {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeFixed64()
+		m.Union = &Communique_TempC{math.Float64frombits(x)}
+		return true, err
+	case 9: // union.col
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Col{MyMessage_Color(x)}
+		return true, err
+	case 10: // union.msg
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		msg := new(Strings)
+		err := b.DecodeMessage(msg)
+		m.Union = &Communique_Msg{msg}
+		return true, err
+	default:
+		return false, nil
+	}
+}
+
 var E_Greeting = &proto.ExtensionDesc{
 	ExtendedType:  (*MyMessage)(nil),
 	ExtensionType: ([]string)(nil),

proto/testdata/test.proto

@@ -478,3 +478,17 @@ message MessageWithMap {
   map<bool, bytes> byte_mapping = 3;
   map<string, string> str_to_str = 4;
 }
+
+message Communique {
+  optional bool make_me_cry = 1;
+
+  // This is a oneof, called "union".
+  oneof union {
+    int32 number = 5;
+    string name = 6;
+    bytes data = 7;
+    double temp_c = 8;
+    MyMessage.Color col = 9;
+    Strings msg = 10;
+  }
+}

proto/text.go

@@ -322,6 +322,23 @@ func writeStruct(w *textWriter, sv reflect.Value) error {
 			}
 		}
 
+		if fv.Kind() == reflect.Interface {
+			// Check if it is a oneof.
+			if st.Field(i).Tag.Get("protobuf_oneof") != "" {
+				// fv is nil, or holds a pointer to generated struct.
+				// That generated struct has exactly one field,
+				// which has a protobuf struct tag.
+				if fv.IsNil() {
+					continue
+				}
+				inner := fv.Elem().Elem() // interface -> *T -> T
+				tag := inner.Type().Field(0).Tag.Get("protobuf")
+				props.Parse(tag) // Overwrite the outer props.
+				// Write the value in the oneof, not the oneof itself.
+				fv = inner.Field(0)
+			}
+		}
+
 		if err := writeName(w, props); err != nil {
 			return err
 		}

proto/text_parser.go

@@ -385,8 +385,7 @@ func (p *textParser) missingRequiredFieldError(sv reflect.Value) *RequiredNotSet
 }
 
 // Returns the index in the struct for the named field, as well as the parsed tag properties.
-func structFieldByName(st reflect.Type, name string) (int, *Properties, bool) {
-	sprops := GetProperties(st)
+func structFieldByName(sprops *StructProperties, name string) (int, *Properties, bool) {
 	i, ok := sprops.decoderOrigNames[name]
 	if ok {
 		return i, sprops.Prop[i], true
@@ -438,7 +437,8 @@ func (p *textParser) checkForColon(props *Properties, typ reflect.Type) *ParseEr
 
 func (p *textParser) readStruct(sv reflect.Value, terminator string) error {
 	st := sv.Type()
-	reqCount := GetProperties(st).reqCount
+	sprops := GetProperties(st)
+	reqCount := sprops.reqCount
 	var reqFieldErr error
 	fieldSet := make(map[string]bool)
 	// A struct is a sequence of "name: value", terminated by one of
@@ -520,95 +520,129 @@ func (p *textParser) readStruct(sv reflect.Value, terminator string) error {
 				sl = reflect.Append(sl, ext)
 				SetExtension(ep, desc, sl.Interface())
 			}
-		} else {
-			// This is a normal, non-extension field.
-			name := tok.value
-			fi, props, ok := structFieldByName(st, name)
-			if !ok {
-				return p.errorf("unknown field name %q in %v", name, st)
+			if err := p.consumeOptionalSeparator(); err != nil {
+				return err
 			}
+			continue
+		}
 
-			dst := sv.Field(fi)
-
-			if dst.Kind() == reflect.Map {
-				// Consume any colon.
-				if err := p.checkForColon(props, dst.Type()); err != nil {
-					return err
-				}
-
-				// Construct the map if it doesn't already exist.
-				if dst.IsNil() {
-					dst.Set(reflect.MakeMap(dst.Type()))
-				}
-				key := reflect.New(dst.Type().Key()).Elem()
-				val := reflect.New(dst.Type().Elem()).Elem()
-
-				// The map entry should be this sequence of tokens:
-				//	< key : KEY value : VALUE >
-				// Technically the "key" and "value" could come in any order,
-				// but in practice they won't.
-
-				tok := p.next()
-				var terminator string
-				switch tok.value {
-				case "<":
-					terminator = ">"
-				case "{":
-					terminator = "}"
-				default:
-					return p.errorf("expected '{' or '<', found %q", tok.value)
-				}
-				if err := p.consumeToken("key"); err != nil {
-					return err
-				}
-				if err := p.consumeToken(":"); err != nil {
-					return err
-				}
-				if err := p.readAny(key, props.mkeyprop); err != nil {
-					return err
-				}
-				if err := p.consumeOptionalSeparator(); err != nil {
-					return err
-				}
-				if err := p.consumeToken("value"); err != nil {
-					return err
-				}
-				if err := p.checkForColon(props.mvalprop, dst.Type().Elem()); err != nil {
-					return err
-				}
-				if err := p.readAny(val, props.mvalprop); err != nil {
-					return err
-				}
-				if err := p.consumeOptionalSeparator(); err != nil {
-					return err
+		// This is a normal, non-extension field.
+		name := tok.value
+		var dst reflect.Value
+		fi, props, ok := structFieldByName(sprops, name)
+		if ok {
+			dst = sv.Field(fi)
+		} else {
+			// Maybe it is a oneof.
+			// TODO: If this shows in profiles, cache the mapping.
+			for _, oot := range sprops.oneofTypes {
+				sp := reflect.ValueOf(oot).Type() // *T
+				var p Properties
+				p.Parse(sp.Elem().Field(0).Tag.Get("protobuf"))
+				if p.OrigName != name {
+					continue
 				}
-				if err := p.consumeToken(terminator); err != nil {
-					return err
+				nv := reflect.New(sp.Elem())
+				dst = nv.Elem().Field(0)
+				props = &p
+				// There will be exactly one interface field that
+				// this new value is assignable to.
+				for i := 0; i < st.NumField(); i++ {
+					f := st.Field(i)
+					if f.Type.Kind() != reflect.Interface {
+						continue
+					}
+					if !nv.Type().AssignableTo(f.Type) {
+						continue
+					}
+					sv.Field(i).Set(nv)
+					break
 				}
-
-				dst.SetMapIndex(key, val)
-				continue
+				break
 			}
+		}
+		if !dst.IsValid() {
+			return p.errorf("unknown field name %q in %v", name, st)
+		}
 
-			// Check that it's not already set if it's not a repeated field.
-			if !props.Repeated && fieldSet[name] {
-				return p.errorf("non-repeated field %q was repeated", name)
+		if dst.Kind() == reflect.Map {
+			// Consume any colon.
+			if err := p.checkForColon(props, dst.Type()); err != nil {
+				return err
 			}
 
-			if err := p.checkForColon(props, st.Field(fi).Type); err != nil {
+			// Construct the map if it doesn't already exist.
+			if dst.IsNil() {
+				dst.Set(reflect.MakeMap(dst.Type()))
+			}
+			key := reflect.New(dst.Type().Key()).Elem()
+			val := reflect.New(dst.Type().Elem()).Elem()
+
+			// The map entry should be this sequence of tokens:
+			//	< key : KEY value : VALUE >
+			// Technically the "key" and "value" could come in any order,
+			// but in practice they won't.
+
+			tok := p.next()
+			var terminator string
+			switch tok.value {
+			case "<":
+				terminator = ">"
+			case "{":
+				terminator = "}"
+			default:
+				return p.errorf("expected '{' or '<', found %q", tok.value)
+			}
+			if err := p.consumeToken("key"); err != nil {
+				return err
+			}
+			if err := p.consumeToken(":"); err != nil {
+				return err
+			}
+			if err := p.readAny(key, props.mkeyprop); err != nil {
+				return err
+			}
+			if err := p.consumeOptionalSeparator(); err != nil {
+				return err
+			}
+			if err := p.consumeToken("value"); err != nil {
+				return err
+			}
+			if err := p.checkForColon(props.mvalprop, dst.Type().Elem()); err != nil {
+				return err
+			}
+			if err := p.readAny(val, props.mvalprop); err != nil {
+				return err
+			}
+			if err := p.consumeOptionalSeparator(); err != nil {
+				return err
+			}
+			if err := p.consumeToken(terminator); err != nil {
 				return err
 			}
 
-			// Parse into the field.
-			fieldSet[name] = true
-			if err := p.readAny(dst, props); err != nil {
-				if _, ok := err.(*RequiredNotSetError); !ok {
-					return err
-				}
-				reqFieldErr = err
-			} else if props.Required {
-				reqCount--
+			dst.SetMapIndex(key, val)
+			continue
+		}
+
+		// Check that it's not already set if it's not a repeated field.
+		if !props.Repeated && fieldSet[name] {
+			return p.errorf("non-repeated field %q was repeated", name)
+		}
+
+		if err := p.checkForColon(props, dst.Type()); err != nil {
+			return err
+		}
+
+		// Parse into the field.
+		fieldSet[name] = true
+		if err := p.readAny(dst, props); err != nil {
+			if _, ok := err.(*RequiredNotSetError); !ok {
+				return err
 			}
+			reqFieldErr = err
+		} else if props.Required {
+			reqCount--
 		}
 
 		if err := p.consumeOptionalSeparator(); err != nil {

proto/text_parser_test.go

@@ -486,6 +486,18 @@ func TestMapParsing(t *testing.T) {
 	}
 }
 
+func TestOneofParsing(t *testing.T) {
+	const in = `name:"Shrek"`
+	m := new(Communique)
+	want := &Communique{Union: &Communique_Name{"Shrek"}}
+	if err := UnmarshalText(in, m); err != nil {
+		t.Fatal(err)
+	}
+	if !Equal(m, want) {
+		t.Errorf("\n got %v\nwant %v", m, want)
+	}
+}
+
 var benchInput string
 
 func init() {

proto/text_test.go

@@ -208,6 +208,28 @@ func TestMarshalTextUnknownEnum(t *testing.T) {
 	}
 }
 
+func TestTextOneof(t *testing.T) {
+	tests := []struct {
+		m    proto.Message
+		want string
+	}{
+		// zero message
+		{&pb.Communique{}, ``},
+		// scalar field
+		{&pb.Communique{Union: &pb.Communique_Number{4}}, `number:4`},
+		// message field
+		{&pb.Communique{Union: &pb.Communique_Msg{
+			&pb.Strings{StringField: proto.String("why hello!")},
+		}}, `msg:<string_field:"why hello!" >`},
+	}
+	for _, test := range tests {
+		got := strings.TrimSpace(test.m.String())
+		if got != test.want {
+			t.Errorf("\n got %s\nwant %s", got, test.want)
+		}
+	}
+}
+
 func BenchmarkMarshalTextBuffered(b *testing.B) {
 	buf := new(bytes.Buffer)
 	m := newTestMessage()

protoc-gen-go/descriptor/descriptor.pb.go

@@ -31,10 +31,12 @@ It has these top-level messages:
 package descriptor
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
 import math "math"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
 var _ = math.Inf
 
 type FieldDescriptorProto_Type int32

protoc-gen-go/generator/generator.go

@@ -109,6 +109,7 @@ type Descriptor struct {
 	*descriptor.DescriptorProto
 	parent   *Descriptor            // The containing message, if any.
 	nested   []*Descriptor          // Inner messages, if any.
+	enums    []*EnumDescriptor      // Inner enums, if any.
 	ext      []*ExtensionDescriptor // Extensions, if any.
 	typename []string               // Cached typename vector.
 	index    int                    // The index into the container, whether the file or another message.
@@ -658,6 +659,7 @@ func (g *Generator) SetPackageNames() {
 	// Register the support package names. They might collide with the
 	// name of a package we import.
 	g.Pkg = map[string]string{
+		"fmt":   RegisterUniquePackageName("fmt", nil),
 		"math":  RegisterUniquePackageName("math", nil),
 		"proto": RegisterUniquePackageName("proto", nil),
 	}
@@ -691,6 +693,7 @@ func (g *Generator) WrapTypes() {
 		descs := wrapDescriptors(f)
 		g.buildNestedDescriptors(descs)
 		enums := wrapEnumDescriptors(f, descs)
+		g.buildNestedEnums(descs, enums)
 		exts := wrapExtensions(f)
 		imps := wrapImported(f, g)
 		fd := &FileDescriptor{
@@ -726,21 +729,33 @@ FindFiles:
 func (g *Generator) buildNestedDescriptors(descs []*Descriptor) {
 	for _, desc := range descs {
 		if len(desc.NestedType) != 0 {
-			desc.nested = make([]*Descriptor, len(desc.NestedType))
-			n := 0
 			for _, nest := range descs {
 				if nest.parent == desc {
-					desc.nested[n] = nest
-					n++
+					desc.nested = append(desc.nested, nest)
 				}
 			}
-			if n != len(desc.NestedType) {
+			if len(desc.nested) != len(desc.NestedType) {
 				g.Fail("internal error: nesting failure for", desc.GetName())
 			}
 		}
 	}
 }
 
+func (g *Generator) buildNestedEnums(descs []*Descriptor, enums []*EnumDescriptor) {
+	for _, desc := range descs {
+		if len(desc.EnumType) != 0 {
+			for _, enum := range enums {
+				if enum.parent == desc {
+					desc.enums = append(desc.enums, enum)
+				}
+			}
+			if len(desc.enums) != len(desc.EnumType) {
+				g.Fail("internal error: enum nesting failure for", desc.GetName())
+			}
+		}
+	}
+}
+
 // Construct the Descriptor
 func newDescriptor(desc *descriptor.DescriptorProto, parent *Descriptor, file *descriptor.FileDescriptorProto, index int) *Descriptor {
 	d := &Descriptor{
@@ -1131,14 +1146,17 @@ func (g *Generator) generateHeader() {
 
 // PrintComments prints any comments from the source .proto file.
 // The path is a comma-separated list of integers.
+// It returns an indication of whether any comments were printed.
 // See descriptor.proto for its format.
-func (g *Generator) PrintComments(path string) {
+func (g *Generator) PrintComments(path string) bool {
 	if loc, ok := g.file.comments[path]; ok {
 		text := strings.TrimSuffix(loc.GetLeadingComments(), "\n")
 		for _, line := range strings.Split(text, "\n") {
 			g.P("// ", strings.TrimPrefix(line, " "))
 		}
+		return true
 	}
+	return false
 }
 
 func (g *Generator) fileByName(filename string) *FileDescriptor {
@@ -1164,12 +1182,10 @@ func (g *Generator) weak(i int32) bool {
 func (g *Generator) generateImports() {
 	// We almost always need a proto import.  Rather than computing when we
 	// do, which is tricky when there's a plugin, just import it and
-	// reference it later. The same argument applies to the math package,
-	// for handling bit patterns for floating-point numbers.
+	// reference it later. The same argument applies to the fmt and math packages.
 	g.P("import " + g.Pkg["proto"] + " " + strconv.Quote(g.ImportPrefix+"github.com/golang/protobuf/proto"))
-	if !g.file.proto3 {
-		g.P("import " + g.Pkg["math"] + ` "math"`)
-	}
+	g.P("import " + g.Pkg["fmt"] + ` "fmt"`)
+	g.P("import " + g.Pkg["math"] + ` "math"`)
 	for i, s := range g.file.Dependency {
 		fd := g.fileByName(s)
 		// Do not import our own package.
@@ -1206,9 +1222,8 @@ func (g *Generator) generateImports() {
 	}
 	g.P("// Reference imports to suppress errors if they are not otherwise used.")
 	g.P("var _ = ", g.Pkg["proto"], ".Marshal")
-	if !g.file.proto3 {
-		g.P("var _ = ", g.Pkg["math"], ".Inf")
-	}
+	g.P("var _ = ", g.Pkg["fmt"], ".Errorf")
+	g.P("var _ = ", g.Pkg["math"], ".Inf")
 	g.P()
 }
 
@@ -1501,6 +1516,8 @@ func (g *Generator) GoType(message *Descriptor, field *descriptor.FieldDescripto
 		typ = "[]" + typ
 	} else if message != nil && message.proto3() {
 		return
+	} else if field.OneofIndex != nil && message != nil {
+		return
 	} else if needsStar(*field.Type) {
 		typ = "*" + typ
 	}
@@ -1541,25 +1558,59 @@ func (g *Generator) generateMessage(message *Descriptor) {
 	}
 	fieldNames := make(map[*descriptor.FieldDescriptorProto]string)
 	fieldGetterNames := make(map[*descriptor.FieldDescriptorProto]string)
+	fieldTypes := make(map[*descriptor.FieldDescriptorProto]string)
 	mapFieldTypes := make(map[*descriptor.FieldDescriptorProto]string)
 
+	oneofFieldName := make(map[int32]string)                           // indexed by oneof_index field of FieldDescriptorProto
+	oneofDisc := make(map[int32]string)                                // name of discriminator method
+	oneofTypeName := make(map[*descriptor.FieldDescriptorProto]string) // without star
+	oneofInsertPoints := make(map[int32]int)                           // oneof_index => offset of g.Buffer
+
 	g.PrintComments(message.path)
 	g.P("type ", ccTypeName, " struct {")
 	g.In()
 
-	for i, field := range message.Field {
-		g.PrintComments(fmt.Sprintf("%s,%d,%d", message.path, messageFieldPath, i))
-
-		fieldName := CamelCase(*field.Name)
-		for usedNames[fieldName] {
-			fieldName += "_"
+	allocName := func(basis string) string {
+		n := CamelCase(basis)
+		for usedNames[n] {
+			n += "_"
 		}
+		usedNames[n] = true
+		return n
+	}
+
+	for i, field := range message.Field {
+		fieldName := allocName(*field.Name)
 		fieldGetterName := fieldName
-		usedNames[fieldName] = true
 		typename, wiretype := g.GoType(message, field)
 		jsonName := *field.Name
 		tag := fmt.Sprintf("protobuf:%s json:%q", g.goTag(message, field, wiretype), jsonName+",omitempty")
 
+		fieldNames[field] = fieldName
+		fieldGetterNames[field] = fieldGetterName
+
+		oneof := field.OneofIndex != nil
+		if oneof && oneofFieldName[*field.OneofIndex] == "" {
+			odp := message.OneofDecl[int(*field.OneofIndex)]
+			fname := allocName(odp.GetName())
+
+			// This is the first field of a oneof we haven't seen before.
+			// Generate the union field.
+			com := g.PrintComments(fmt.Sprintf("%s,%d,%d", message.path, messageOneofPath, *field.OneofIndex))
+			if com {
+				g.P("//")
+			}
+			g.P("// Types that are valid to be assigned to ", fname, ":")
+			// Generate the rest of this comment later,
+			// when we've computed any disambiguation.
+			oneofInsertPoints[*field.OneofIndex] = g.Buffer.Len()
+
+			dname := "is" + ccTypeName + "_" + fname
+			oneofFieldName[*field.OneofIndex] = fname
+			oneofDisc[*field.OneofIndex] = dname
+			g.P(fname, " ", dname, " `protobuf_oneof:\"", odp.GetName(), "\"`")
+		}
+
 		if *field.Type == descriptor.FieldDescriptorProto_TYPE_MESSAGE {
 			desc := g.ObjectNamed(field.GetTypeName())
 			if d, ok := desc.(*Descriptor); ok && d.GetOptions().GetMapEntry() {
@@ -1590,8 +1641,38 @@ func (g *Generator) generateMessage(message *Descriptor) {
 			}
 		}
 
-		fieldNames[field] = fieldName
-		fieldGetterNames[field] = fieldGetterName
+		fieldTypes[field] = typename
+
+		if oneof {
+			tname := ccTypeName + "_" + fieldName
+			// It is possible for this to collide with a message or enum
+			// nested in this message. Check for collisions.
+			for {
+				ok := true
+				for _, desc := range message.nested {
+					if strings.Join(desc.TypeName(), "_") == tname {
+						ok = false
+						break
+					}
+				}
+				for _, enum := range message.enums {
+					if strings.Join(enum.TypeName(), "_") == tname {
+						ok = false
+						break
+					}
+				}
+				if !ok {
+					tname += "_"
+					continue
+				}
+				break
+			}
+
+			oneofTypeName[field] = tname
+			continue
+		}
+
+		g.PrintComments(fmt.Sprintf("%s,%d,%d", message.path, messageFieldPath, i))
 		g.P(fieldName, "\t", typename, "\t`", tag, "`")
 		g.RecordTypeUse(field.GetTypeName())
 	}
@@ -1604,6 +1685,23 @@ func (g *Generator) generateMessage(message *Descriptor) {
 	g.Out()
 	g.P("}")
 
+	// Update g.Buffer to list valid oneof types.
+	// We do this down here, after we've disambiguated the oneof type names.
+	// We go in reverse order of insertion point to avoid invalidating offsets.
+	for oi := int32(len(message.OneofDecl)); oi >= 0; oi-- {
+		ip := oneofInsertPoints[oi]
+		all := g.Buffer.Bytes()
+		rem := all[ip:]
+		g.Buffer = bytes.NewBuffer(all[:ip:ip]) // set cap so we don't scribble on rem
+		for _, field := range message.Field {
+			if field.OneofIndex == nil || *field.OneofIndex != oi {
+				continue
+			}
+			g.P("//\t*", oneofTypeName[field])
+		}
+		g.Buffer.Write(rem)
+	}
+
 	// Reset, String and ProtoMessage methods.
 	g.P("func (m *", ccTypeName, ") Reset() { *m = ", ccTypeName, "{} }")
 	g.P("func (m *", ccTypeName, ") String() string { return ", g.Pkg["proto"], ".CompactTextString(m) }")
@@ -1724,9 +1822,48 @@ func (g *Generator) generateMessage(message *Descriptor) {
 	}
 	g.P()
 
+	// Oneof per-field types, discriminants and getters.
+	//
+	// Generate unexported named types for the discriminant interfaces.
+	// We shouldn't have to do this, but there was (~19 Aug 2015) a compiler/linker bug
+	// that was triggered by using anonymous interfaces here.
+	// TODO: Revisit this and consider reverting back to anonymous interfaces.
+	for oi := range message.OneofDecl {
+		dname := oneofDisc[int32(oi)]
+		g.P("type ", dname, " interface { ", dname, "() }")
+	}
+	g.P()
+	for _, field := range message.Field {
+		if field.OneofIndex == nil {
+			continue
+		}
+		_, wiretype := g.GoType(message, field)
+		tag := "protobuf:" + g.goTag(message, field, wiretype)
+		g.P("type ", oneofTypeName[field], " struct{ ", fieldNames[field], " ", fieldTypes[field], " `", tag, "` }")
+		g.RecordTypeUse(field.GetTypeName())
+	}
+	g.P()
+	for _, field := range message.Field {
+		if field.OneofIndex == nil {
+			continue
+		}
+		g.P("func (*", oneofTypeName[field], ") ", oneofDisc[*field.OneofIndex], "() {}")
+	}
+	g.P()
+	for oi := range message.OneofDecl {
+		fname := oneofFieldName[int32(oi)]
+		g.P("func (m *", ccTypeName, ") Get", fname, "() ", oneofDisc[int32(oi)], " {")
+		g.P("if m != nil { return m.", fname, " }")
+		g.P("return nil")
+		g.P("}")
+	}
+	g.P()
+
 	// Field getters
 	var getters []getterSymbol
 	for _, field := range message.Field {
+		oneof := field.OneofIndex != nil
+
 		fname := fieldNames[field]
 		typename, _ := g.GoType(message, field)
 		if t, ok := mapFieldTypes[field]; ok {
@@ -1739,8 +1876,8 @@ func (g *Generator) generateMessage(message *Descriptor) {
 			star = "*"
 		}
 
-		// In proto3, only generate getters for message fields.
-		if message.proto3() && *field.Type != descriptor.FieldDescriptorProto_TYPE_MESSAGE {
+		// In proto3, only generate getters for message fields and oneof fields.
+		if message.proto3() && *field.Type != descriptor.FieldDescriptorProto_TYPE_MESSAGE && !oneof {
 			continue
 		}
 
@@ -1787,7 +1924,7 @@ func (g *Generator) generateMessage(message *Descriptor) {
 		if isRepeated(field) {
 			typeDefaultIsNil = true
 		}
-		if typeDefaultIsNil {
+		if typeDefaultIsNil && !oneof {
 			// A bytes field with no explicit default needs less generated code,
 			// as does a message or group field, or a repeated field.
 			g.P("if m != nil {")
@@ -1801,11 +1938,19 @@ func (g *Generator) generateMessage(message *Descriptor) {
 			g.P()
 			continue
 		}
-		g.P("if m != nil && m." + fname + " != nil {")
-		g.In()
-		g.P("return " + star + "m." + fname)
-		g.Out()
-		g.P("}")
+		if !oneof {
+			g.P("if m != nil && m." + fname + " != nil {")
+			g.In()
+			g.P("return " + star + "m." + fname)
+			g.Out()
+			g.P("}")
+		} else {
+			uname := oneofFieldName[*field.OneofIndex]
+			tname := oneofTypeName[field]
+			g.P("if x, ok := m.Get", uname, "().(*", tname, "); ok {")
+			g.P("return x.", fname)
+			g.P("}")
+		}
 		if hasDef {
 			if *field.Type != descriptor.FieldDescriptorProto_TYPE_BYTES {
 				g.P("return " + def)
@@ -1820,6 +1965,11 @@ func (g *Generator) generateMessage(message *Descriptor) {
 				g.P("return false")
 			case descriptor.FieldDescriptorProto_TYPE_STRING:
 				g.P(`return ""`)
+			case descriptor.FieldDescriptorProto_TYPE_GROUP,
+				descriptor.FieldDescriptorProto_TYPE_MESSAGE,
+				descriptor.FieldDescriptorProto_TYPE_BYTES:
+				// This is only possible for oneof fields.
+				g.P("return nil")
 			case descriptor.FieldDescriptorProto_TYPE_ENUM:
 				// The default default for an enum is the first value in the enum,
 				// not zero.
@@ -1855,6 +2005,207 @@ func (g *Generator) generateMessage(message *Descriptor) {
 		g.file.addExport(message, ms)
 	}
 
+	// Oneof functions
+	if len(message.OneofDecl) > 0 {
+		fieldWire := make(map[*descriptor.FieldDescriptorProto]string)
+
+		// method
+		enc := "_" + ccTypeName + "_OneofMarshaler"
+		dec := "_" + ccTypeName + "_OneofUnmarshaler"
+		encSig := "(msg " + g.Pkg["proto"] + ".Message, b *" + g.Pkg["proto"] + ".Buffer) error"
+		decSig := "(msg " + g.Pkg["proto"] + ".Message, tag, wire int, b *" + g.Pkg["proto"] + ".Buffer) (bool, error)"
+
+		g.P("// XXX_OneofFuncs is for the internal use of the proto package.")
+		g.P("func (*", ccTypeName, ") XXX_OneofFuncs() (func", encSig, ", func", decSig, ", []interface{}) {")
+		g.P("return ", enc, ", ", dec, ", []interface{}{")
+		for _, field := range message.Field {
+			if field.OneofIndex == nil {
+				continue
+			}
+			g.P("(*", oneofTypeName[field], ")(nil),")
+		}
+		g.P("}")
+		g.P("}")
+		g.P()
+
+		// marshaler
+		g.P("func ", enc, encSig, " {")
+		g.P("m := msg.(*", ccTypeName, ")")
+		for oi, odp := range message.OneofDecl {
+			g.P("// ", odp.GetName())
+			fname := oneofFieldName[int32(oi)]
+			g.P("switch x := m.", fname, ".(type) {")
+			for _, field := range message.Field {
+				if field.OneofIndex == nil || int(*field.OneofIndex) != oi {
+					continue
+				}
+				g.P("case *", oneofTypeName[field], ":")
+				var wire, pre, post string
+				val := "x." + fieldNames[field] // overridden for TYPE_BOOL
+				canFail := false                // only TYPE_MESSAGE and TYPE_GROUP can fail
+				switch *field.Type {
+				case descriptor.FieldDescriptorProto_TYPE_DOUBLE:
+					wire = "WireFixed64"
+					pre = "b.EncodeFixed64(" + g.Pkg["math"] + ".Float64bits("
+					post = "))"
+				case descriptor.FieldDescriptorProto_TYPE_FLOAT:
+					wire = "WireFixed32"
+					pre = "b.EncodeFixed32(uint64(" + g.Pkg["math"] + ".Float32bits("
+					post = ")))"
+				case descriptor.FieldDescriptorProto_TYPE_INT64,
+					descriptor.FieldDescriptorProto_TYPE_UINT64:
+					wire = "WireVarint"
+					pre, post = "b.EncodeVarint(uint64(", "))"
+				case descriptor.FieldDescriptorProto_TYPE_INT32,
+					descriptor.FieldDescriptorProto_TYPE_UINT32,
+					descriptor.FieldDescriptorProto_TYPE_ENUM:
+					wire = "WireVarint"
+					pre, post = "b.EncodeVarint(uint64(", "))"
+				case descriptor.FieldDescriptorProto_TYPE_FIXED64,
+					descriptor.FieldDescriptorProto_TYPE_SFIXED64:
+					wire = "WireFixed64"
+					pre, post = "b.EncodeFixed64(uint64(", "))"
+				case descriptor.FieldDescriptorProto_TYPE_FIXED32,
+					descriptor.FieldDescriptorProto_TYPE_SFIXED32:
+					wire = "WireFixed32"
+					pre, post = "b.EncodeFixed32(uint64(", "))"
+				case descriptor.FieldDescriptorProto_TYPE_BOOL:
+					// bool needs special handling.
+					g.P("t := uint64(0)")
+					g.P("if ", val, " { t = 1 }")
+					val = "t"
+					wire = "WireVarint"
+					pre, post = "b.EncodeVarint(", ")"
+				case descriptor.FieldDescriptorProto_TYPE_STRING:
+					wire = "WireBytes"
+					pre, post = "b.EncodeStringBytes(", ")"
+				case descriptor.FieldDescriptorProto_TYPE_GROUP:
+					wire = "WireStartGroup"
+					pre, post = "b.Marshal(", ")"
+					canFail = true
+				case descriptor.FieldDescriptorProto_TYPE_MESSAGE:
+					wire = "WireBytes"
+					pre, post = "b.EncodeMessage(", ")"
+					canFail = true
+				case descriptor.FieldDescriptorProto_TYPE_BYTES:
+					wire = "WireBytes"
+					pre, post = "b.EncodeRawBytes(", ")"
+				case descriptor.FieldDescriptorProto_TYPE_SINT32:
+					wire = "WireVarint"
+					pre, post = "b.EncodeZigzag32(uint64(", "))"
+				case descriptor.FieldDescriptorProto_TYPE_SINT64:
+					wire = "WireVarint"
+					pre, post = "b.EncodeZigzag64(uint64(", "))"
+				default:
+					g.Fail("unhandled oneof field type ", field.Type.String())
+				}
+				fieldWire[field] = wire
+				g.P("b.EncodeVarint(", field.Number, "<<3|", g.Pkg["proto"], ".", wire, ")")
+				if !canFail {
+					g.P(pre, val, post)
+				} else {
+					g.P("if err := ", pre, val, post, "; err != nil {")
+					g.P("return err")
+					g.P("}")
+				}
+				if *field.Type == descriptor.FieldDescriptorProto_TYPE_GROUP {
+					g.P("b.EncodeVarint(", field.Number, "<<3|", g.Pkg["proto"], ".WireEndGroup)")
+				}
+			}
+			g.P("case nil:")
+			g.P("default: return ", g.Pkg["fmt"], `.Errorf("`, ccTypeName, ".", fname, ` has unexpected type %T", x)`)
+			g.P("}")
+		}
+		g.P("return nil")
+		g.P("}")
+		g.P()
+
+		// unmarshaler
+		g.P("func ", dec, decSig, " {")
+		g.P("m := msg.(*", ccTypeName, ")")
+		g.P("switch tag {")
+		for _, field := range message.Field {
+			if field.OneofIndex == nil {
+				continue
+			}
+			odp := message.OneofDecl[int(*field.OneofIndex)]
+			g.P("case ", field.Number, ": // ", odp.GetName(), ".", *field.Name)
+			g.P("if wire != ", g.Pkg["proto"], ".", fieldWire[field], " {")
+			g.P("return true, ", g.Pkg["proto"], ".ErrInternalBadWireType")
+			g.P("}")
+			lhs := "x, err" // overridden for TYPE_MESSAGE and TYPE_GROUP
+			var dec, cast, cast2 string
+			switch *field.Type {
+			case descriptor.FieldDescriptorProto_TYPE_DOUBLE:
+				dec, cast = "b.DecodeFixed64()", g.Pkg["math"]+".Float64frombits"
+			case descriptor.FieldDescriptorProto_TYPE_FLOAT:
+				dec, cast, cast2 = "b.DecodeFixed32()", "uint32", g.Pkg["math"]+".Float32frombits"
+			case descriptor.FieldDescriptorProto_TYPE_INT64:
+				dec, cast = "b.DecodeVarint()", "int64"
+			case descriptor.FieldDescriptorProto_TYPE_UINT64:
+				dec = "b.DecodeVarint()"
+			case descriptor.FieldDescriptorProto_TYPE_INT32:
+				dec, cast = "b.DecodeVarint()", "int32"
+			case descriptor.FieldDescriptorProto_TYPE_FIXED64:
+				dec = "b.DecodeFixed64()"
+			case descriptor.FieldDescriptorProto_TYPE_FIXED32:
+				dec, cast = "b.DecodeFixed32()", "uint32"
+			case descriptor.FieldDescriptorProto_TYPE_BOOL:
+				dec = "b.DecodeVarint()"
+				// handled specially below
+			case descriptor.FieldDescriptorProto_TYPE_STRING:
+				dec = "b.DecodeStringBytes()"
+			case descriptor.FieldDescriptorProto_TYPE_GROUP:
+				g.P("msg := new(", fieldTypes[field][1:], ")") // drop star
+				lhs = "err"
+				dec = "b.DecodeGroup(msg)"
+				// handled specially below
+			case descriptor.FieldDescriptorProto_TYPE_MESSAGE:
+				g.P("msg := new(", fieldTypes[field][1:], ")") // drop star
+				lhs = "err"
+				dec = "b.DecodeMessage(msg)"
+				// handled specially below
+			case descriptor.FieldDescriptorProto_TYPE_BYTES:
+				dec = "b.DecodeRawBytes(true)"
+			case descriptor.FieldDescriptorProto_TYPE_UINT32:
+				dec, cast = "b.DecodeVarint()", "uint32"
+			case descriptor.FieldDescriptorProto_TYPE_ENUM:
+				dec, cast = "b.DecodeVarint()", fieldTypes[field]
+			case descriptor.FieldDescriptorProto_TYPE_SFIXED32:
+				dec, cast = "b.DecodeFixed32()", "int32"
+			case descriptor.FieldDescriptorProto_TYPE_SFIXED64:
+				dec, cast = "b.DecodeFixed64()", "int64"
+			case descriptor.FieldDescriptorProto_TYPE_SINT32:
+				dec, cast = "b.DecodeZigzag32()", "int32"
+			case descriptor.FieldDescriptorProto_TYPE_SINT64:
+				dec, cast = "b.DecodeZigzag64()", "int64"
+			default:
+				g.Fail("unhandled oneof field type ", field.Type.String())
+			}
+			g.P(lhs, " := ", dec)
+			val := "x"
+			if cast != "" {
+				val = cast + "(" + val + ")"
+			}
+			if cast2 != "" {
+				val = cast2 + "(" + val + ")"
+			}
+			switch *field.Type {
+			case descriptor.FieldDescriptorProto_TYPE_BOOL:
+				val += " != 0"
+			case descriptor.FieldDescriptorProto_TYPE_GROUP,
+				descriptor.FieldDescriptorProto_TYPE_MESSAGE:
+				val = "msg"
+			}
+			g.P("m.", oneofFieldName[*field.OneofIndex], " = &", oneofTypeName[field], "{", val, "}")
+			g.P("return true, err")
+		}
+		g.P("default: return false, nil")
+		g.P("}")
+		g.P("}")
+		g.P()
+	}
+
 	for _, ext := range message.ext {
 		g.generateExtension(ext)
 	}
@@ -2093,6 +2444,7 @@ const (
 	messageFieldPath   = 2 // field
 	messageMessagePath = 3 // nested_type
 	messageEnumPath    = 4 // enum_type
+	messageOneofPath   = 8 // oneof_decl
 	// tag numbers in EnumDescriptorProto
 	enumValuePath = 2 // value
 )

protoc-gen-go/plugin/plugin.pb.go

@@ -15,11 +15,13 @@ It has these top-level messages:
 package google_protobuf_compiler
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
 import math "math"
 import google_protobuf "github.com/golang/protobuf/protoc-gen-go/descriptor"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
 var _ = math.Inf
 
 // An encoded CodeGeneratorRequest is written to the plugin's stdin.
@@ -184,6 +186,3 @@ func (m *CodeGeneratorResponse_File) GetContent() string {
 	}
 	return ""
 }
-
-func init() {
-}

protoc-gen-go/testdata/my_test/test.pb.go

@@ -17,16 +17,19 @@ It has these top-level messages:
 	ReplyExtensions
 	OtherReplyExtensions
 	OldReply
+	Communique
 */
 package my_test
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
 import math "math"
 
 // discarding unused import multitest2 "multi"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
 var _ = math.Inf
 
 type HatType int32
@@ -458,6 +461,329 @@ func (m *OldReply) ExtensionMap() map[int32]proto.Extension {
 	return m.XXX_extensions
 }
 
+type Communique struct {
+	MakeMeCry *bool `protobuf:"varint,1,opt,name=make_me_cry" json:"make_me_cry,omitempty"`
+	// This is a oneof, called "union".
+	//
+	// Types that are valid to be assigned to Union:
+	//	*Communique_Number
+	//	*Communique_Name
+	//	*Communique_Data
+	//	*Communique_TempC
+	//	*Communique_Height
+	//	*Communique_Today
+	//	*Communique_Maybe
+	//	*Communique_Delta_
+	//	*Communique_Msg
+	//	*Communique_Somegroup
+	Union            isCommunique_Union `protobuf_oneof:"union"`
+	XXX_unrecognized []byte             `json:"-"`
+}
+
+func (m *Communique) Reset()         { *m = Communique{} }
+func (m *Communique) String() string { return proto.CompactTextString(m) }
+func (*Communique) ProtoMessage()    {}
+
+type isCommunique_Union interface {
+	isCommunique_Union()
+}
+
+type Communique_Number struct {
+	Number int32 `protobuf:"varint,5,opt,name=number"`
+}
+type Communique_Name struct {
+	Name string `protobuf:"bytes,6,opt,name=name"`
+}
+type Communique_Data struct {
+	Data []byte `protobuf:"bytes,7,opt,name=data"`
+}
+type Communique_TempC struct {
+	TempC float64 `protobuf:"fixed64,8,opt,name=temp_c"`
+}
+type Communique_Height struct {
+	Height float32 `protobuf:"fixed32,9,opt,name=height"`
+}
+type Communique_Today struct {
+	Today Days `protobuf:"varint,10,opt,name=today,enum=my.test.Days"`
+}
+type Communique_Maybe struct {
+	Maybe bool `protobuf:"varint,11,opt,name=maybe"`
+}
+type Communique_Delta_ struct {
+	Delta int32 `protobuf:"zigzag32,12,opt,name=delta"`
+}
+type Communique_Msg struct {
+	Msg *Reply `protobuf:"bytes,13,opt,name=msg"`
+}
+type Communique_Somegroup struct {
+	Somegroup *Communique_SomeGroup `protobuf:"group,14,opt,name=SomeGroup"`
+}
+
+func (*Communique_Number) isCommunique_Union()    {}
+func (*Communique_Name) isCommunique_Union()      {}
+func (*Communique_Data) isCommunique_Union()      {}
+func (*Communique_TempC) isCommunique_Union()     {}
+func (*Communique_Height) isCommunique_Union()    {}
+func (*Communique_Today) isCommunique_Union()     {}
+func (*Communique_Maybe) isCommunique_Union()     {}
+func (*Communique_Delta_) isCommunique_Union()    {}
+func (*Communique_Msg) isCommunique_Union()       {}
+func (*Communique_Somegroup) isCommunique_Union() {}
+
+func (m *Communique) GetUnion() isCommunique_Union {
+	if m != nil {
+		return m.Union
+	}
+	return nil
+}
+
+func (m *Communique) GetMakeMeCry() bool {
+	if m != nil && m.MakeMeCry != nil {
+		return *m.MakeMeCry
+	}
+	return false
+}
+
+func (m *Communique) GetNumber() int32 {
+	if x, ok := m.GetUnion().(*Communique_Number); ok {
+		return x.Number
+	}
+	return 0
+}
+
+func (m *Communique) GetName() string {
+	if x, ok := m.GetUnion().(*Communique_Name); ok {
+		return x.Name
+	}
+	return ""
+}
+
+func (m *Communique) GetData() []byte {
+	if x, ok := m.GetUnion().(*Communique_Data); ok {
+		return x.Data
+	}
+	return nil
+}
+
+func (m *Communique) GetTempC() float64 {
+	if x, ok := m.GetUnion().(*Communique_TempC); ok {
+		return x.TempC
+	}
+	return 0
+}
+
+func (m *Communique) GetHeight() float32 {
+	if x, ok := m.GetUnion().(*Communique_Height); ok {
+		return x.Height
+	}
+	return 0
+}
+
+func (m *Communique) GetToday() Days {
+	if x, ok := m.GetUnion().(*Communique_Today); ok {
+		return x.Today
+	}
+	return Days_MONDAY
+}
+
+func (m *Communique) GetMaybe() bool {
+	if x, ok := m.GetUnion().(*Communique_Maybe); ok {
+		return x.Maybe
+	}
+	return false
+}
+
+func (m *Communique) GetDelta() int32 {
+	if x, ok := m.GetUnion().(*Communique_Delta_); ok {
+		return x.Delta
+	}
+	return 0
+}
+
+func (m *Communique) GetMsg() *Reply {
+	if x, ok := m.GetUnion().(*Communique_Msg); ok {
+		return x.Msg
+	}
+	return nil
+}
+
+func (m *Communique) GetSomegroup() *Communique_SomeGroup {
+	if x, ok := m.GetUnion().(*Communique_Somegroup); ok {
+		return x.Somegroup
+	}
+	return nil
+}
+
+// XXX_OneofFuncs is for the internal use of the proto package.
+func (*Communique) XXX_OneofFuncs() (func(msg proto.Message, b *proto.Buffer) error, func(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error), []interface{}) {
+	return _Communique_OneofMarshaler, _Communique_OneofUnmarshaler, []interface{}{
+		(*Communique_Number)(nil),
+		(*Communique_Name)(nil),
+		(*Communique_Data)(nil),
+		(*Communique_TempC)(nil),
+		(*Communique_Height)(nil),
+		(*Communique_Today)(nil),
+		(*Communique_Maybe)(nil),
+		(*Communique_Delta_)(nil),
+		(*Communique_Msg)(nil),
+		(*Communique_Somegroup)(nil),
+	}
+}
+
+func _Communique_OneofMarshaler(msg proto.Message, b *proto.Buffer) error {
+	m := msg.(*Communique)
+	// union
+	switch x := m.Union.(type) {
+	case *Communique_Number:
+		b.EncodeVarint(5<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Number))
+	case *Communique_Name:
+		b.EncodeVarint(6<<3 | proto.WireBytes)
+		b.EncodeStringBytes(x.Name)
+	case *Communique_Data:
+		b.EncodeVarint(7<<3 | proto.WireBytes)
+		b.EncodeRawBytes(x.Data)
+	case *Communique_TempC:
+		b.EncodeVarint(8<<3 | proto.WireFixed64)
+		b.EncodeFixed64(math.Float64bits(x.TempC))
+	case *Communique_Height:
+		b.EncodeVarint(9<<3 | proto.WireFixed32)
+		b.EncodeFixed32(uint64(math.Float32bits(x.Height)))
+	case *Communique_Today:
+		b.EncodeVarint(10<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Today))
+	case *Communique_Maybe:
+		t := uint64(0)
+		if x.Maybe {
+			t = 1
+		}
+		b.EncodeVarint(11<<3 | proto.WireVarint)
+		b.EncodeVarint(t)
+	case *Communique_Delta_:
+		b.EncodeVarint(12<<3 | proto.WireVarint)
+		b.EncodeZigzag32(uint64(x.Delta))
+	case *Communique_Msg:
+		b.EncodeVarint(13<<3 | proto.WireBytes)
+		if err := b.EncodeMessage(x.Msg); err != nil {
+			return err
+		}
+	case *Communique_Somegroup:
+		b.EncodeVarint(14<<3 | proto.WireStartGroup)
+		if err := b.Marshal(x.Somegroup); err != nil {
+			return err
+		}
+		b.EncodeVarint(14<<3 | proto.WireEndGroup)
+	case nil:
+	default:
+		return fmt.Errorf("Communique.Union has unexpected type %T", x)
+	}
+	return nil
+}
+
+func _Communique_OneofUnmarshaler(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error) {
+	m := msg.(*Communique)
+	switch tag {
+	case 5: // union.number
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Number{int32(x)}
+		return true, err
+	case 6: // union.name
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeStringBytes()
+		m.Union = &Communique_Name{x}
+		return true, err
+	case 7: // union.data
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeRawBytes(true)
+		m.Union = &Communique_Data{x}
+		return true, err
+	case 8: // union.temp_c
+		if wire != proto.WireFixed64 {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeFixed64()
+		m.Union = &Communique_TempC{math.Float64frombits(x)}
+		return true, err
+	case 9: // union.height
+		if wire != proto.WireFixed32 {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeFixed32()
+		m.Union = &Communique_Height{math.Float32frombits(uint32(x))}
+		return true, err
+	case 10: // union.today
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Today{Days(x)}
+		return true, err
+	case 11: // union.maybe
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Maybe{x != 0}
+		return true, err
+	case 12: // union.delta
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeZigzag32()
+		m.Union = &Communique_Delta_{int32(x)}
+		return true, err
+	case 13: // union.msg
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		msg := new(Reply)
+		err := b.DecodeMessage(msg)
+		m.Union = &Communique_Msg{msg}
+		return true, err
+	case 14: // union.somegroup
+		if wire != proto.WireStartGroup {
+			return true, proto.ErrInternalBadWireType
+		}
+		msg := new(Communique_SomeGroup)
+		err := b.DecodeGroup(msg)
+		m.Union = &Communique_Somegroup{msg}
+		return true, err
+	default:
+		return false, nil
+	}
+}
+
+type Communique_SomeGroup struct {
+	Member           *string `protobuf:"bytes,15,opt,name=member" json:"member,omitempty"`
+	XXX_unrecognized []byte  `json:"-"`
+}
+
+func (m *Communique_SomeGroup) Reset()         { *m = Communique_SomeGroup{} }
+func (m *Communique_SomeGroup) String() string { return proto.CompactTextString(m) }
+func (*Communique_SomeGroup) ProtoMessage()    {}
+
+func (m *Communique_SomeGroup) GetMember() string {
+	if m != nil && m.Member != nil {
+		return *m.Member
+	}
+	return ""
+}
+
+type Communique_Delta struct {
+	XXX_unrecognized []byte `json:"-"`
+}
+
+func (m *Communique_Delta) Reset()         { *m = Communique_Delta{} }
+func (m *Communique_Delta) String() string { return proto.CompactTextString(m) }
+func (*Communique_Delta) ProtoMessage()    {}
+
 var E_Tag = &proto.ExtensionDesc{
 	ExtendedType:  (*Reply)(nil),
 	ExtensionType: (*string)(nil),

protoc-gen-go/testdata/my_test/test.pb.go.golden

@@ -17,16 +17,19 @@ It has these top-level messages:
 	ReplyExtensions
 	OtherReplyExtensions
 	OldReply
+	Communique
 */
 package my_test
 
 import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
 import math "math"
 
 // discarding unused import multitest2 "multi"
 
 // Reference imports to suppress errors if they are not otherwise used.
 var _ = proto.Marshal
+var _ = fmt.Errorf
 var _ = math.Inf
 
 type HatType int32
@@ -458,6 +461,329 @@ func (m *OldReply) ExtensionMap() map[int32]proto.Extension {
 	return m.XXX_extensions
 }
 
+type Communique struct {
+	MakeMeCry *bool `protobuf:"varint,1,opt,name=make_me_cry" json:"make_me_cry,omitempty"`
+	// This is a oneof, called "union".
+	//
+	// Types that are valid to be assigned to Union:
+	//	*Communique_Number
+	//	*Communique_Name
+	//	*Communique_Data
+	//	*Communique_TempC
+	//	*Communique_Height
+	//	*Communique_Today
+	//	*Communique_Maybe
+	//	*Communique_Delta_
+	//	*Communique_Msg
+	//	*Communique_Somegroup
+	Union            isCommunique_Union `protobuf_oneof:"union"`
+	XXX_unrecognized []byte             `json:"-"`
+}
+
+func (m *Communique) Reset()         { *m = Communique{} }
+func (m *Communique) String() string { return proto.CompactTextString(m) }
+func (*Communique) ProtoMessage()    {}
+
+type isCommunique_Union interface {
+	isCommunique_Union()
+}
+
+type Communique_Number struct {
+	Number int32 `protobuf:"varint,5,opt,name=number"`
+}
+type Communique_Name struct {
+	Name string `protobuf:"bytes,6,opt,name=name"`
+}
+type Communique_Data struct {
+	Data []byte `protobuf:"bytes,7,opt,name=data"`
+}
+type Communique_TempC struct {
+	TempC float64 `protobuf:"fixed64,8,opt,name=temp_c"`
+}
+type Communique_Height struct {
+	Height float32 `protobuf:"fixed32,9,opt,name=height"`
+}
+type Communique_Today struct {
+	Today Days `protobuf:"varint,10,opt,name=today,enum=my.test.Days"`
+}
+type Communique_Maybe struct {
+	Maybe bool `protobuf:"varint,11,opt,name=maybe"`
+}
+type Communique_Delta_ struct {
+	Delta int32 `protobuf:"zigzag32,12,opt,name=delta"`
+}
+type Communique_Msg struct {
+	Msg *Reply `protobuf:"bytes,13,opt,name=msg"`
+}
+type Communique_Somegroup struct {
+	Somegroup *Communique_SomeGroup `protobuf:"group,14,opt,name=SomeGroup"`
+}
+
+func (*Communique_Number) isCommunique_Union()    {}
+func (*Communique_Name) isCommunique_Union()      {}
+func (*Communique_Data) isCommunique_Union()      {}
+func (*Communique_TempC) isCommunique_Union()     {}
+func (*Communique_Height) isCommunique_Union()    {}
+func (*Communique_Today) isCommunique_Union()     {}
+func (*Communique_Maybe) isCommunique_Union()     {}
+func (*Communique_Delta_) isCommunique_Union()    {}
+func (*Communique_Msg) isCommunique_Union()       {}
+func (*Communique_Somegroup) isCommunique_Union() {}
+
+func (m *Communique) GetUnion() isCommunique_Union {
+	if m != nil {
+		return m.Union
+	}
+	return nil
+}
+
+func (m *Communique) GetMakeMeCry() bool {
+	if m != nil && m.MakeMeCry != nil {
+		return *m.MakeMeCry
+	}
+	return false
+}
+
+func (m *Communique) GetNumber() int32 {
+	if x, ok := m.GetUnion().(*Communique_Number); ok {
+		return x.Number
+	}
+	return 0
+}
+
+func (m *Communique) GetName() string {
+	if x, ok := m.GetUnion().(*Communique_Name); ok {
+		return x.Name
+	}
+	return ""
+}
+
+func (m *Communique) GetData() []byte {
+	if x, ok := m.GetUnion().(*Communique_Data); ok {
+		return x.Data
+	}
+	return nil
+}
+
+func (m *Communique) GetTempC() float64 {
+	if x, ok := m.GetUnion().(*Communique_TempC); ok {
+		return x.TempC
+	}
+	return 0
+}
+
+func (m *Communique) GetHeight() float32 {
+	if x, ok := m.GetUnion().(*Communique_Height); ok {
+		return x.Height
+	}
+	return 0
+}
+
+func (m *Communique) GetToday() Days {
+	if x, ok := m.GetUnion().(*Communique_Today); ok {
+		return x.Today
+	}
+	return Days_MONDAY
+}
+
+func (m *Communique) GetMaybe() bool {
+	if x, ok := m.GetUnion().(*Communique_Maybe); ok {
+		return x.Maybe
+	}
+	return false
+}
+
+func (m *Communique) GetDelta() int32 {
+	if x, ok := m.GetUnion().(*Communique_Delta_); ok {
+		return x.Delta
+	}
+	return 0
+}
+
+func (m *Communique) GetMsg() *Reply {
+	if x, ok := m.GetUnion().(*Communique_Msg); ok {
+		return x.Msg
+	}
+	return nil
+}
+
+func (m *Communique) GetSomegroup() *Communique_SomeGroup {
+	if x, ok := m.GetUnion().(*Communique_Somegroup); ok {
+		return x.Somegroup
+	}
+	return nil
+}
+
+// XXX_OneofFuncs is for the internal use of the proto package.
+func (*Communique) XXX_OneofFuncs() (func(msg proto.Message, b *proto.Buffer) error, func(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error), []interface{}) {
+	return _Communique_OneofMarshaler, _Communique_OneofUnmarshaler, []interface{}{
+		(*Communique_Number)(nil),
+		(*Communique_Name)(nil),
+		(*Communique_Data)(nil),
+		(*Communique_TempC)(nil),
+		(*Communique_Height)(nil),
+		(*Communique_Today)(nil),
+		(*Communique_Maybe)(nil),
+		(*Communique_Delta_)(nil),
+		(*Communique_Msg)(nil),
+		(*Communique_Somegroup)(nil),
+	}
+}
+
+func _Communique_OneofMarshaler(msg proto.Message, b *proto.Buffer) error {
+	m := msg.(*Communique)
+	// union
+	switch x := m.Union.(type) {
+	case *Communique_Number:
+		b.EncodeVarint(5<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Number))
+	case *Communique_Name:
+		b.EncodeVarint(6<<3 | proto.WireBytes)
+		b.EncodeStringBytes(x.Name)
+	case *Communique_Data:
+		b.EncodeVarint(7<<3 | proto.WireBytes)
+		b.EncodeRawBytes(x.Data)
+	case *Communique_TempC:
+		b.EncodeVarint(8<<3 | proto.WireFixed64)
+		b.EncodeFixed64(math.Float64bits(x.TempC))
+	case *Communique_Height:
+		b.EncodeVarint(9<<3 | proto.WireFixed32)
+		b.EncodeFixed32(uint64(math.Float32bits(x.Height)))
+	case *Communique_Today:
+		b.EncodeVarint(10<<3 | proto.WireVarint)
+		b.EncodeVarint(uint64(x.Today))
+	case *Communique_Maybe:
+		t := uint64(0)
+		if x.Maybe {
+			t = 1
+		}
+		b.EncodeVarint(11<<3 | proto.WireVarint)
+		b.EncodeVarint(t)
+	case *Communique_Delta_:
+		b.EncodeVarint(12<<3 | proto.WireVarint)
+		b.EncodeZigzag32(uint64(x.Delta))
+	case *Communique_Msg:
+		b.EncodeVarint(13<<3 | proto.WireBytes)
+		if err := b.EncodeMessage(x.Msg); err != nil {
+			return err
+		}
+	case *Communique_Somegroup:
+		b.EncodeVarint(14<<3 | proto.WireStartGroup)
+		if err := b.Marshal(x.Somegroup); err != nil {
+			return err
+		}
+		b.EncodeVarint(14<<3 | proto.WireEndGroup)
+	case nil:
+	default:
+		return fmt.Errorf("Communique.Union has unexpected type %T", x)
+	}
+	return nil
+}
+
+func _Communique_OneofUnmarshaler(msg proto.Message, tag, wire int, b *proto.Buffer) (bool, error) {
+	m := msg.(*Communique)
+	switch tag {
+	case 5: // union.number
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Number{int32(x)}
+		return true, err
+	case 6: // union.name
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeStringBytes()
+		m.Union = &Communique_Name{x}
+		return true, err
+	case 7: // union.data
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeRawBytes(true)
+		m.Union = &Communique_Data{x}
+		return true, err
+	case 8: // union.temp_c
+		if wire != proto.WireFixed64 {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeFixed64()
+		m.Union = &Communique_TempC{math.Float64frombits(x)}
+		return true, err
+	case 9: // union.height
+		if wire != proto.WireFixed32 {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeFixed32()
+		m.Union = &Communique_Height{math.Float32frombits(uint32(x))}
+		return true, err
+	case 10: // union.today
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Today{Days(x)}
+		return true, err
+	case 11: // union.maybe
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeVarint()
+		m.Union = &Communique_Maybe{x != 0}
+		return true, err
+	case 12: // union.delta
+		if wire != proto.WireVarint {
+			return true, proto.ErrInternalBadWireType
+		}
+		x, err := b.DecodeZigzag32()
+		m.Union = &Communique_Delta_{int32(x)}
+		return true, err
+	case 13: // union.msg
+		if wire != proto.WireBytes {
+			return true, proto.ErrInternalBadWireType
+		}
+		msg := new(Reply)
+		err := b.DecodeMessage(msg)
+		m.Union = &Communique_Msg{msg}
+		return true, err
+	case 14: // union.somegroup
+		if wire != proto.WireStartGroup {
+			return true, proto.ErrInternalBadWireType
+		}
+		msg := new(Communique_SomeGroup)
+		err := b.DecodeGroup(msg)
+		m.Union = &Communique_Somegroup{msg}
+		return true, err
+	default:
+		return false, nil
+	}
+}
+
+type Communique_SomeGroup struct {
+	Member           *string `protobuf:"bytes,15,opt,name=member" json:"member,omitempty"`
+	XXX_unrecognized []byte  `json:"-"`
+}
+
+func (m *Communique_SomeGroup) Reset()         { *m = Communique_SomeGroup{} }
+func (m *Communique_SomeGroup) String() string { return proto.CompactTextString(m) }
+func (*Communique_SomeGroup) ProtoMessage()    {}
+
+func (m *Communique_SomeGroup) GetMember() string {
+	if m != nil && m.Member != nil {
+		return *m.Member
+	}
+	return ""
+}
+
+type Communique_Delta struct {
+	XXX_unrecognized []byte `json:"-"`
+}
+
+func (m *Communique_Delta) Reset()         { *m = Communique_Delta{} }
+func (m *Communique_Delta) String() string { return proto.CompactTextString(m) }
+func (*Communique_Delta) ProtoMessage()    {}
+
 var E_Tag = &proto.ExtensionDesc{
 	ExtendedType:  (*Reply)(nil),
 	ExtensionType: (*string)(nil),

protoc-gen-go/testdata/my_test/test.proto

@@ -130,3 +130,25 @@ message OldReply {
   extensions 100 to max;
 }
 
+message Communique {
+  optional bool make_me_cry = 1;
+
+  // This is a oneof, called "union".
+  oneof union {
+    int32 number = 5;
+    string name = 6;
+    bytes data = 7;
+    double temp_c = 8;
+    float height = 9;
+    Days today = 10;
+    bool maybe = 11;
+    sint32 delta = 12;  // name will conflict with Delta below
+    Reply msg = 13;
+    group SomeGroup = 14 {
+      optional string member = 15;
+    }
+  }
+
+  message Delta {}
+}
+