diff --git a/jsonpb/jsonpb.go b/jsonpb/jsonpb.go
index 91dcd26af5..71985e64ff 100644
--- a/jsonpb/jsonpb.go
+++ b/jsonpb/jsonpb.go
@@ -233,12 +233,14 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeU
 	}
 
 	// Handle proto2 extensions.
-	if ep, ok := v.(extendableProto); ok {
+	if ep, ok := v.(proto.Message); ok {
 		extensions := proto.RegisteredExtensions(v)
-		extensionMap := ep.ExtensionMap()
 		// Sort extensions for stable output.
-		ids := make([]int32, 0, len(extensionMap))
-		for id := range extensionMap {
+		ids := make([]int32, 0, len(extensions))
+		for id, desc := range extensions {
+			if !proto.HasExtension(ep, desc) {
+				continue
+			}
 			ids = append(ids, id)
 		}
 		sort.Sort(int32Slice(ids))
@@ -767,13 +769,6 @@ func acceptedJSONFieldNames(prop *proto.Properties) fieldNames {
 	return opts
 }
 
-// extendableProto is an interface implemented by any protocol buffer that may be extended.
-type extendableProto interface {
-	proto.Message
-	ExtensionRangeArray() []proto.ExtensionRange
-	ExtensionMap() map[int32]proto.Extension
-}
-
 // Writer wrapper inspired by https://blog.golang.org/errors-are-values
 type errWriter struct {
 	writer io.Writer
diff --git a/proto/clone.go b/proto/clone.go
index e98ddec981..e392575b35 100644
--- a/proto/clone.go
+++ b/proto/clone.go
@@ -84,9 +84,15 @@ func mergeStruct(out, in reflect.Value) {
 		mergeAny(out.Field(i), in.Field(i), false, sprop.Prop[i])
 	}
 
-	if emIn, ok := in.Addr().Interface().(extendableProto); ok {
-		emOut := out.Addr().Interface().(extendableProto)
-		mergeExtension(emOut.ExtensionMap(), emIn.ExtensionMap())
+	if emIn, ok := extendable(in.Addr().Interface()); ok {
+		emOut, _ := extendable(out.Addr().Interface())
+		mIn, muIn := emIn.extensionsRead()
+		if mIn != nil {
+			mOut := emOut.extensionsWrite()
+			muIn.Lock()
+			mergeExtension(mOut, mIn)
+			muIn.Unlock()
+		}
 	}
 
 	uf := in.FieldByName("XXX_unrecognized")
diff --git a/proto/decode.go b/proto/decode.go
index f94b9f416e..07288a250b 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -390,11 +390,12 @@ func (o *Buffer) unmarshalType(st reflect.Type, prop *StructProperties, is_group
 		if !ok {
 			// Maybe it's an extension?
 			if prop.extendable {
-				if e := structPointer_Interface(base, st).(extendableProto); isExtensionField(e, int32(tag)) {
+				if e, _ := extendable(structPointer_Interface(base, st)); isExtensionField(e, int32(tag)) {
 					if err = o.skip(st, tag, wire); err == nil {
-						ext := e.ExtensionMap()[int32(tag)] // may be missing
+						extmap := e.extensionsWrite()
+						ext := extmap[int32(tag)] // may be missing
 						ext.enc = append(ext.enc, o.buf[oi:o.index]...)
-						e.ExtensionMap()[int32(tag)] = ext
+						extmap[int32(tag)] = ext
 					}
 					continue
 				}
diff --git a/proto/encode.go b/proto/encode.go
index 401c1143c8..1b5578d9db 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -1073,10 +1073,25 @@ func size_slice_struct_group(p *Properties, base structPointer) (n int) {
 
 // Encode an extension map.
 func (o *Buffer) enc_map(p *Properties, base structPointer) error {
-	v := *structPointer_ExtMap(base, p.field)
-	if err := encodeExtensionMap(v); err != nil {
+	exts := structPointer_ExtMap(base, p.field)
+	if err := encodeExtensionsMap(*exts); err != nil {
 		return err
 	}
+
+	return o.enc_map_body(*exts)
+}
+
+func (o *Buffer) enc_exts(p *Properties, base structPointer) error {
+	exts := structPointer_Extensions(base, p.field)
+	if err := encodeExtensions(exts); err != nil {
+		return err
+	}
+	v, _ := exts.extensionsRead()
+
+	return o.enc_map_body(v)
+}
+
+func (o *Buffer) enc_map_body(v map[int32]Extension) error {
 	// Fast-path for common cases: zero or one extensions.
 	if len(v) <= 1 {
 		for _, e := range v {
@@ -1099,8 +1114,13 @@ func (o *Buffer) enc_map(p *Properties, base structPointer) error {
 }
 
 func size_map(p *Properties, base structPointer) int {
-	v := *structPointer_ExtMap(base, p.field)
-	return sizeExtensionMap(v)
+	v := structPointer_ExtMap(base, p.field)
+	return extensionsMapSize(*v)
+}
+
+func size_exts(p *Properties, base structPointer) int {
+	v := structPointer_Extensions(base, p.field)
+	return extensionsSize(v)
 }
 
 // Encode a map field.
diff --git a/proto/equal.go b/proto/equal.go
index f5db1def3c..8c6fa85371 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -121,9 +121,9 @@ func equalStruct(v1, v2 reflect.Value) bool {
 		}
 	}
 
-	if em1 := v1.FieldByName("XXX_extensions"); em1.IsValid() {
-		em2 := v2.FieldByName("XXX_extensions")
-		if !equalExtensions(v1.Type(), em1.Interface().(map[int32]Extension), em2.Interface().(map[int32]Extension)) {
+	if em1 := v1.FieldByName("XXX_InternalExtensions"); em1.IsValid() {
+		em2 := v2.FieldByName("XXX_InternalExtensions")
+		if !equalExtensions(v1.Type(), em1.Interface().(XXX_InternalExtensions), em2.Interface().(XXX_InternalExtensions)) {
 			return false
 		}
 	}
@@ -223,8 +223,10 @@ func equalAny(v1, v2 reflect.Value, prop *Properties) bool {
 }
 
 // base is the struct type that the extensions are based on.
-// em1 and em2 are extension maps.
-func equalExtensions(base reflect.Type, em1, em2 map[int32]Extension) bool {
+// x1 and x2 are InternalExtensions.
+func equalExtensions(base reflect.Type, x1, x2 XXX_InternalExtensions) bool {
+	em1, _ := x1.extensionsRead()
+	em2, _ := x2.extensionsRead()
 	if len(em1) != len(em2) {
 		return false
 	}
diff --git a/proto/extensions.go b/proto/extensions.go
index 0de0b424f8..9f484f53a3 100644
--- a/proto/extensions.go
+++ b/proto/extensions.go
@@ -52,14 +52,99 @@ type ExtensionRange struct {
 	Start, End int32 // both inclusive
 }
 
-// extendableProto is an interface implemented by any protocol buffer that may be extended.
+// extendableProto is an interface implemented by any protocol buffer generated by the current
+// proto compiler that may be extended.
 type extendableProto interface {
+	Message
+	ExtensionRangeArray() []ExtensionRange
+	extensionsWrite() map[int32]Extension
+	extensionsRead() (map[int32]Extension, sync.Locker)
+}
+
+// extendableProtoV1 is an interface implemented by a protocol buffer generated by the previous
+// version of the proto compiler that may be extended.
+type extendableProtoV1 interface {
 	Message
 	ExtensionRangeArray() []ExtensionRange
 	ExtensionMap() map[int32]Extension
 }
 
+// extensionAdapter is a wrapper around extendableProtoV1 that implements extendableProto.
+type extensionAdapter struct {
+	extendableProtoV1
+}
+
+func (e extensionAdapter) extensionsWrite() map[int32]Extension {
+	return e.ExtensionMap()
+}
+
+func (e extensionAdapter) extensionsRead() (map[int32]Extension, sync.Locker) {
+	return e.ExtensionMap(), notLocker{}
+}
+
+// notLocker is a sync.Locker whose Lock and Unlock methods are nops.
+type notLocker struct{}
+
+func (n notLocker) Lock()   {}
+func (n notLocker) Unlock() {}
+
+// extendable returns the extendableProto interface for the given generated proto message.
+// If the proto message has the old extension format, it returns a wrapper that implements
+// the extendableProto interface.
+func extendable(p interface{}) (extendableProto, bool) {
+	if ep, ok := p.(extendableProto); ok {
+		return ep, ok
+	}
+	if ep, ok := p.(extendableProtoV1); ok {
+		return extensionAdapter{ep}, ok
+	}
+	return nil, false
+}
+
+// XXX_InternalExtensions is an internal representation of proto extensions.
+//
+// Each generated message struct type embeds an anonymous XXX_InternalExtensions field,
+// thus gaining the unexported 'extensions' method, which can be called only from the proto package.
+//
+// The methods of XXX_InternalExtensions are not concurrency safe in general,
+// but calls to logically read-only methods such as has and get may be executed concurrently.
+type XXX_InternalExtensions struct {
+	// The struct must be indirect so that if a user inadvertently copies a
+	// generated message and its embedded XXX_InternalExtensions, they
+	// avoid the mayhem of a copied mutex.
+	//
+	// The mutex serializes all logically read-only operations to p.extensionMap.
+	// It is up to the client to ensure that write operations to p.extensionMap are
+	// mutually exclusive with other accesses.
+	p *struct {
+		mu           sync.Mutex
+		extensionMap map[int32]Extension
+	}
+}
+
+// extensionsWrite returns the extension map, creating it on first use.
+func (e *XXX_InternalExtensions) extensionsWrite() map[int32]Extension {
+	if e.p == nil {
+		e.p = new(struct {
+			mu           sync.Mutex
+			extensionMap map[int32]Extension
+		})
+		e.p.extensionMap = make(map[int32]Extension)
+	}
+	return e.p.extensionMap
+}
+
+// extensionsRead returns the extensions map for read-only use.  It may be nil.
+// The caller must hold the returned mutex's lock when accessing Elements within the map.
+func (e *XXX_InternalExtensions) extensionsRead() (map[int32]Extension, sync.Locker) {
+	if e.p == nil {
+		return nil, nil
+	}
+	return e.p.extensionMap, &e.p.mu
+}
+
 var extendableProtoType = reflect.TypeOf((*extendableProto)(nil)).Elem()
+var extendableProtoV1Type = reflect.TypeOf((*extendableProtoV1)(nil)).Elem()
 
 // ExtensionDesc represents an extension specification.
 // Used in generated code from the protocol compiler.
@@ -93,11 +178,12 @@ type Extension struct {
 
 // SetRawExtension is for testing only.
 func SetRawExtension(base Message, id int32, b []byte) {
-	epb, ok := base.(extendableProto)
+	epb, ok := extendable(base)
 	if !ok {
 		return
 	}
-	epb.ExtensionMap()[id] = Extension{enc: b}
+	extmap := epb.extensionsWrite()
+	extmap[id] = Extension{enc: b}
 }
 
 // isExtensionField returns true iff the given field number is in an extension range.
@@ -112,8 +198,12 @@ func isExtensionField(pb extendableProto, field int32) bool {
 
 // checkExtensionTypes checks that the given extension is valid for pb.
 func checkExtensionTypes(pb extendableProto, extension *ExtensionDesc) error {
+	var pbi interface{} = pb
 	// Check the extended type.
-	if a, b := reflect.TypeOf(pb), reflect.TypeOf(extension.ExtendedType); a != b {
+	if ea, ok := pbi.(extensionAdapter); ok {
+		pbi = ea.extendableProtoV1
+	}
+	if a, b := reflect.TypeOf(pbi), reflect.TypeOf(extension.ExtendedType); a != b {
 		return errors.New("proto: bad extended type; " + b.String() + " does not extend " + a.String())
 	}
 	// Check the range.
@@ -159,8 +249,19 @@ func extensionProperties(ed *ExtensionDesc) *Properties {
 	return prop
 }
 
-// encodeExtensionMap encodes any unmarshaled (unencoded) extensions in m.
-func encodeExtensionMap(m map[int32]Extension) error {
+// encode encodes any unmarshaled (unencoded) extensions in e.
+func encodeExtensions(e *XXX_InternalExtensions) error {
+	m, mu := e.extensionsRead()
+	if m == nil {
+		return nil // fast path
+	}
+	mu.Lock()
+	defer mu.Unlock()
+	return encodeExtensionsMap(m)
+}
+
+// encode encodes any unmarshaled (unencoded) extensions in e.
+func encodeExtensionsMap(m map[int32]Extension) error {
 	for k, e := range m {
 		if e.value == nil || e.desc == nil {
 			// Extension is only in its encoded form.
@@ -188,7 +289,17 @@ func encodeExtensionMap(m map[int32]Extension) error {
 	return nil
 }
 
-func sizeExtensionMap(m map[int32]Extension) (n int) {
+func extensionsSize(e *XXX_InternalExtensions) (n int) {
+	m, mu := e.extensionsRead()
+	if m == nil {
+		return 0
+	}
+	mu.Lock()
+	defer mu.Unlock()
+	return extensionsMapSize(m)
+}
+
+func extensionsMapSize(m map[int32]Extension) (n int) {
 	for _, e := range m {
 		if e.value == nil || e.desc == nil {
 			// Extension is only in its encoded form.
@@ -215,28 +326,35 @@ func sizeExtensionMap(m map[int32]Extension) (n int) {
 // HasExtension returns whether the given extension is present in pb.
 func HasExtension(pb Message, extension *ExtensionDesc) bool {
 	// TODO: Check types, field numbers, etc.?
-	epb, ok := pb.(extendableProto)
+	epb, ok := extendable(pb)
 	if !ok {
 		return false
 	}
-	_, ok = epb.ExtensionMap()[extension.Field]
+	extmap, mu := epb.extensionsRead()
+	if extmap == nil {
+		return false
+	}
+	mu.Lock()
+	_, ok = extmap[extension.Field]
+	mu.Unlock()
 	return ok
 }
 
 // ClearExtension removes the given extension from pb.
 func ClearExtension(pb Message, extension *ExtensionDesc) {
-	epb, ok := pb.(extendableProto)
+	epb, ok := extendable(pb)
 	if !ok {
 		return
 	}
 	// TODO: Check types, field numbers, etc.?
-	delete(epb.ExtensionMap(), extension.Field)
+	extmap := epb.extensionsWrite()
+	delete(extmap, extension.Field)
 }
 
 // GetExtension parses and returns the given extension of pb.
 // If the extension is not present and has no default value it returns ErrMissingExtension.
 func GetExtension(pb Message, extension *ExtensionDesc) (interface{}, error) {
-	epb, ok := pb.(extendableProto)
+	epb, ok := extendable(pb)
 	if !ok {
 		return nil, errors.New("proto: not an extendable proto")
 	}
@@ -245,7 +363,12 @@ func GetExtension(pb Message, extension *ExtensionDesc) (interface{}, error) {
 		return nil, err
 	}
 
-	emap := epb.ExtensionMap()
+	emap, mu := epb.extensionsRead()
+	if emap == nil {
+		return defaultExtensionValue(extension)
+	}
+	mu.Lock()
+	defer mu.Unlock()
 	e, ok := emap[extension.Field]
 	if !ok {
 		// defaultExtensionValue returns the default value or
@@ -349,7 +472,7 @@ func decodeExtension(b []byte, extension *ExtensionDesc) (interface{}, error) {
 // GetExtensions returns a slice of the extensions present in pb that are also listed in es.
 // The returned slice has the same length as es; missing extensions will appear as nil elements.
 func GetExtensions(pb Message, es []*ExtensionDesc) (extensions []interface{}, err error) {
-	epb, ok := pb.(extendableProto)
+	epb, ok := extendable(pb)
 	if !ok {
 		return nil, errors.New("proto: not an extendable proto")
 	}
@@ -368,7 +491,7 @@ func GetExtensions(pb Message, es []*ExtensionDesc) (extensions []interface{}, e
 
 // SetExtension sets the specified extension of pb to the specified value.
 func SetExtension(pb Message, extension *ExtensionDesc, value interface{}) error {
-	epb, ok := pb.(extendableProto)
+	epb, ok := extendable(pb)
 	if !ok {
 		return errors.New("proto: not an extendable proto")
 	}
@@ -388,17 +511,18 @@ func SetExtension(pb Message, extension *ExtensionDesc, value interface{}) error
 		return fmt.Errorf("proto: SetExtension called with nil value of type %T", value)
 	}
 
-	epb.ExtensionMap()[extension.Field] = Extension{desc: extension, value: value}
+	extmap := epb.extensionsWrite()
+	extmap[extension.Field] = Extension{desc: extension, value: value}
 	return nil
 }
 
 // ClearAllExtensions clears all extensions from pb.
 func ClearAllExtensions(pb Message) {
-	epb, ok := pb.(extendableProto)
+	epb, ok := extendable(pb)
 	if !ok {
 		return
 	}
-	m := epb.ExtensionMap()
+	m := epb.extensionsWrite()
 	for k := range m {
 		delete(m, k)
 	}
diff --git a/proto/lib.go b/proto/lib.go
index 0de8f8dffd..170b8e87d2 100644
--- a/proto/lib.go
+++ b/proto/lib.go
@@ -889,6 +889,10 @@ func isProto3Zero(v reflect.Value) bool {
 	return false
 }
 
+// ProtoPackageIsVersion2 is referenced from generated protocol buffer files
+// to assert that that code is compatible with this version of the proto package.
+const ProtoPackageIsVersion2 = true
+
 // ProtoPackageIsVersion1 is referenced from generated protocol buffer files
 // to assert that that code is compatible with this version of the proto package.
 const ProtoPackageIsVersion1 = true
diff --git a/proto/message_set.go b/proto/message_set.go
index e25e01e637..fd982decd6 100644
--- a/proto/message_set.go
+++ b/proto/message_set.go
@@ -149,9 +149,21 @@ func skipVarint(buf []byte) []byte {
 
 // MarshalMessageSet encodes the extension map represented by m in the message set wire format.
 // It is called by generated Marshal methods on protocol buffer messages with the message_set_wire_format option.
-func MarshalMessageSet(m map[int32]Extension) ([]byte, error) {
-	if err := encodeExtensionMap(m); err != nil {
-		return nil, err
+func MarshalMessageSet(exts interface{}) ([]byte, error) {
+	var m map[int32]Extension
+	switch exts := exts.(type) {
+	case *XXX_InternalExtensions:
+		if err := encodeExtensions(exts); err != nil {
+			return nil, err
+		}
+		m, _ = exts.extensionsRead()
+	case map[int32]Extension:
+		if err := encodeExtensionsMap(exts); err != nil {
+			return nil, err
+		}
+		m = exts
+	default:
+		return nil, errors.New("proto: not an extension map")
 	}
 
 	// Sort extension IDs to provide a deterministic encoding.
@@ -178,7 +190,17 @@ func MarshalMessageSet(m map[int32]Extension) ([]byte, error) {
 
 // UnmarshalMessageSet decodes the extension map encoded in buf in the message set wire format.
 // It is called by generated Unmarshal methods on protocol buffer messages with the message_set_wire_format option.
-func UnmarshalMessageSet(buf []byte, m map[int32]Extension) error {
+func UnmarshalMessageSet(buf []byte, exts interface{}) error {
+	var m map[int32]Extension
+	switch exts := exts.(type) {
+	case *XXX_InternalExtensions:
+		m = exts.extensionsWrite()
+	case map[int32]Extension:
+		m = exts
+	default:
+		return errors.New("proto: not an extension map")
+	}
+
 	ms := new(messageSet)
 	if err := Unmarshal(buf, ms); err != nil {
 		return err
@@ -209,7 +231,16 @@ func UnmarshalMessageSet(buf []byte, m map[int32]Extension) error {
 
 // MarshalMessageSetJSON encodes the extension map represented by m in JSON format.
 // It is called by generated MarshalJSON methods on protocol buffer messages with the message_set_wire_format option.
-func MarshalMessageSetJSON(m map[int32]Extension) ([]byte, error) {
+func MarshalMessageSetJSON(exts interface{}) ([]byte, error) {
+	var m map[int32]Extension
+	switch exts := exts.(type) {
+	case *XXX_InternalExtensions:
+		m, _ = exts.extensionsRead()
+	case map[int32]Extension:
+		m = exts
+	default:
+		return nil, errors.New("proto: not an extension map")
+	}
 	var b bytes.Buffer
 	b.WriteByte('{')
 
@@ -252,7 +283,7 @@ func MarshalMessageSetJSON(m map[int32]Extension) ([]byte, error) {
 
 // UnmarshalMessageSetJSON decodes the extension map encoded in buf in JSON format.
 // It is called by generated UnmarshalJSON methods on protocol buffer messages with the message_set_wire_format option.
-func UnmarshalMessageSetJSON(buf []byte, m map[int32]Extension) error {
+func UnmarshalMessageSetJSON(buf []byte, exts interface{}) error {
 	// Common-case fast path.
 	if len(buf) == 0 || bytes.Equal(buf, []byte("{}")) {
 		return nil
diff --git a/proto/message_set_test.go b/proto/message_set_test.go
index ab8ac2f0eb..353a3ea769 100644
--- a/proto/message_set_test.go
+++ b/proto/message_set_test.go
@@ -50,13 +50,13 @@ func TestUnmarshalMessageSetWithDuplicate(t *testing.T) {
 	}
 	t.Logf("Marshaled bytes: %q", b)
 
-	m := make(map[int32]Extension)
-	if err := UnmarshalMessageSet(b, m); err != nil {
+	var extensions XXX_InternalExtensions
+	if err := UnmarshalMessageSet(b, &extensions); err != nil {
 		t.Fatalf("UnmarshalMessageSet: %v", err)
 	}
-	ext, ok := m[12345]
+	ext, ok := extensions.p.extensionMap[12345]
 	if !ok {
-		t.Fatalf("Didn't retrieve extension 12345; map is %v", m)
+		t.Fatalf("Didn't retrieve extension 12345; map is %v", extensions.p.extensionMap)
 	}
 	// Skip wire type/field number and length varints.
 	got := skipVarint(skipVarint(ext.enc))
diff --git a/proto/pointer_reflect.go b/proto/pointer_reflect.go
index 989914177d..fb512e2e16 100644
--- a/proto/pointer_reflect.go
+++ b/proto/pointer_reflect.go
@@ -139,6 +139,11 @@ func structPointer_StringSlice(p structPointer, f field) *[]string {
 	return structPointer_ifield(p, f).(*[]string)
 }
 
+// Extensions returns the address of an extension map field in the struct.
+func structPointer_Extensions(p structPointer, f field) *XXX_InternalExtensions {
+	return structPointer_ifield(p, f).(*XXX_InternalExtensions)
+}
+
 // ExtMap returns the address of an extension map field in the struct.
 func structPointer_ExtMap(p structPointer, f field) *map[int32]Extension {
 	return structPointer_ifield(p, f).(*map[int32]Extension)
diff --git a/proto/pointer_unsafe.go b/proto/pointer_unsafe.go
index ceece772a2..6b5567d47c 100644
--- a/proto/pointer_unsafe.go
+++ b/proto/pointer_unsafe.go
@@ -126,6 +126,10 @@ func structPointer_StringSlice(p structPointer, f field) *[]string {
 }
 
 // ExtMap returns the address of an extension map field in the struct.
+func structPointer_Extensions(p structPointer, f field) *XXX_InternalExtensions {
+	return (*XXX_InternalExtensions)(unsafe.Pointer(uintptr(p) + uintptr(f)))
+}
+
 func structPointer_ExtMap(p structPointer, f field) *map[int32]Extension {
 	return (*map[int32]Extension)(unsafe.Pointer(uintptr(p) + uintptr(f)))
 }
diff --git a/proto/properties.go b/proto/properties.go
index 880eb22d8f..39edea32fd 100644
--- a/proto/properties.go
+++ b/proto/properties.go
@@ -682,7 +682,8 @@ func getPropertiesLocked(t reflect.Type) *StructProperties {
 	propertiesMap[t] = prop
 
 	// build properties
-	prop.extendable = reflect.PtrTo(t).Implements(extendableProtoType)
+	prop.extendable = reflect.PtrTo(t).Implements(extendableProtoType) ||
+		reflect.PtrTo(t).Implements(extendableProtoV1Type)
 	prop.unrecField = invalidField
 	prop.Prop = make([]*Properties, t.NumField())
 	prop.order = make([]int, t.NumField())
@@ -693,12 +694,15 @@ func getPropertiesLocked(t reflect.Type) *StructProperties {
 		name := f.Name
 		p.init(f.Type, name, f.Tag.Get("protobuf"), &f, false)
 
-		if f.Name == "XXX_extensions" { // special case
+		if f.Name == "XXX_InternalExtensions" { // special case
+			p.enc = (*Buffer).enc_exts
+			p.dec = nil // not needed
+			p.size = size_exts
+		} else if f.Name == "XXX_extensions" { // special case
 			p.enc = (*Buffer).enc_map
 			p.dec = nil // not needed
 			p.size = size_map
-		}
-		if f.Name == "XXX_unrecognized" { // special case
+		} else if f.Name == "XXX_unrecognized" { // special case
 			prop.unrecField = toField(&f)
 		}
 		oneof := f.Tag.Get("protobuf_oneof") // special case
diff --git a/proto/text.go b/proto/text.go
index 37c953570d..bd6f1ae5e4 100644
--- a/proto/text.go
+++ b/proto/text.go
@@ -455,7 +455,7 @@ func (tm *TextMarshaler) writeStruct(w *textWriter, sv reflect.Value) error {
 
 	// Extensions (the XXX_extensions field).
 	pv := sv.Addr()
-	if pv.Type().Implements(extendableProtoType) {
+	if _, ok := extendable(pv.Interface()); ok {
 		if err := tm.writeExtensions(w, pv); err != nil {
 			return err
 		}
@@ -689,17 +689,22 @@ func (s int32Slice) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 // pv is assumed to be a pointer to a protocol message struct that is extendable.
 func (tm *TextMarshaler) writeExtensions(w *textWriter, pv reflect.Value) error {
 	emap := extensionMaps[pv.Type().Elem()]
-	ep := pv.Interface().(extendableProto)
+	ep, _ := extendable(pv.Interface())
 
 	// Order the extensions by ID.
 	// This isn't strictly necessary, but it will give us
 	// canonical output, which will also make testing easier.
-	m := ep.ExtensionMap()
+	m, mu := ep.extensionsRead()
+	if m == nil {
+		return nil
+	}
+	mu.Lock()
 	ids := make([]int32, 0, len(m))
 	for id := range m {
 		ids = append(ids, id)
 	}
 	sort.Sort(int32Slice(ids))
+	mu.Unlock()
 
 	for _, extNum := range ids {
 		ext := m[extNum]
diff --git a/proto/text_parser.go b/proto/text_parser.go
index b5fba5b2ae..8fa9f3af6e 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -550,7 +550,7 @@ func (p *textParser) readStruct(sv reflect.Value, terminator string) error {
 				}
 				reqFieldErr = err
 			}
-			ep := sv.Addr().Interface().(extendableProto)
+			ep := sv.Addr().Interface().(Message)
 			if !rep {
 				SetExtension(ep, desc, ext.Interface())
 			} else {
diff --git a/protoc-gen-go/generator/generator.go b/protoc-gen-go/generator/generator.go
index 2994cc96b4..526919fdeb 100644
--- a/protoc-gen-go/generator/generator.go
+++ b/protoc-gen-go/generator/generator.go
@@ -62,7 +62,7 @@ import (
 // It is incremented whenever an incompatibility between the generated code and
 // proto package is introduced; the generated code references
 // a constant, proto.ProtoPackageIsVersionN (where N is generatedCodeVersion).
-const generatedCodeVersion = 1
+const generatedCodeVersion = 2
 
 // A Plugin provides functionality to add to the output during Go code generation,
 // such as to produce RPC stubs.
@@ -363,8 +363,6 @@ func (ms *messageSymbol) GenerateAlias(g *Generator, pkg string) {
 	if ms.hasExtensions {
 		g.P("func (*", ms.sym, ") ExtensionRangeArray() []", g.Pkg["proto"], ".ExtensionRange ",
 			"{ return (*", remoteSym, ")(nil).ExtensionRangeArray() }")
-		g.P("func (m *", ms.sym, ") ExtensionMap() map[int32]", g.Pkg["proto"], ".Extension ",
-			"{ return (*", remoteSym, ")(m).ExtensionMap() }")
 		if ms.isMessageSet {
 			g.P("func (m *", ms.sym, ") Marshal() ([]byte, error) ",
 				"{ return (*", remoteSym, ")(m).Marshal() }")
@@ -1173,7 +1171,9 @@ func (g *Generator) generate(file *FileDescriptor) {
 		// For one file in the package, assert version compatibility.
 		g.P("// This is a compile-time assertion to ensure that this generated file")
 		g.P("// is compatible with the proto package it is being compiled against.")
-		g.P("const _ = ", g.Pkg["proto"], ".ProtoPackageIsVersion", generatedCodeVersion)
+		g.P("// A compilation error at this line likely means your copy of the")
+		g.P("// proto package needs to be updated.")
+		g.P("const _ = ", g.Pkg["proto"], ".ProtoPackageIsVersion", generatedCodeVersion, " // please upgrade the proto package")
 		g.P()
 	}
 
@@ -1684,7 +1684,8 @@ func (g *Generator) RecordTypeUse(t string) {
 }
 
 // Method names that may be generated.  Fields with these names get an
-// underscore appended.
+// underscore appended. Any change to this set is a potential incompatible
+// API change because it changes generated field names.
 var methodNames = [...]string{
 	"Reset",
 	"String",
@@ -1869,7 +1870,7 @@ func (g *Generator) generateMessage(message *Descriptor) {
 		g.RecordTypeUse(field.GetTypeName())
 	}
 	if len(message.ExtensionRange) > 0 {
-		g.P("XXX_extensions\t\tmap[int32]", g.Pkg["proto"], ".Extension `json:\"-\"`")
+		g.P(g.Pkg["proto"], ".XXX_InternalExtensions `json:\"-\"`")
 	}
 	if !message.proto3() {
 		g.P("XXX_unrecognized\t[]byte `json:\"-\"`")
@@ -1919,22 +1920,22 @@ func (g *Generator) generateMessage(message *Descriptor) {
 			g.P()
 			g.P("func (m *", ccTypeName, ") Marshal() ([]byte, error) {")
 			g.In()
-			g.P("return ", g.Pkg["proto"], ".MarshalMessageSet(m.ExtensionMap())")
+			g.P("return ", g.Pkg["proto"], ".MarshalMessageSet(&m.XXX_InternalExtensions)")
 			g.Out()
 			g.P("}")
 			g.P("func (m *", ccTypeName, ") Unmarshal(buf []byte) error {")
 			g.In()
-			g.P("return ", g.Pkg["proto"], ".UnmarshalMessageSet(buf, m.ExtensionMap())")
+			g.P("return ", g.Pkg["proto"], ".UnmarshalMessageSet(buf, &m.XXX_InternalExtensions)")
 			g.Out()
 			g.P("}")
 			g.P("func (m *", ccTypeName, ") MarshalJSON() ([]byte, error) {")
 			g.In()
-			g.P("return ", g.Pkg["proto"], ".MarshalMessageSetJSON(m.XXX_extensions)")
+			g.P("return ", g.Pkg["proto"], ".MarshalMessageSetJSON(&m.XXX_InternalExtensions)")
 			g.Out()
 			g.P("}")
 			g.P("func (m *", ccTypeName, ") UnmarshalJSON(buf []byte) error {")
 			g.In()
-			g.P("return ", g.Pkg["proto"], ".UnmarshalMessageSetJSON(buf, m.XXX_extensions)")
+			g.P("return ", g.Pkg["proto"], ".UnmarshalMessageSetJSON(buf, &m.XXX_InternalExtensions)")
 			g.Out()
 			g.P("}")
 			g.P("// ensure ", ccTypeName, " satisfies proto.Marshaler and proto.Unmarshaler")
@@ -1956,16 +1957,6 @@ func (g *Generator) generateMessage(message *Descriptor) {
 		g.P("return extRange_", ccTypeName)
 		g.Out()
 		g.P("}")
-		g.P("func (m *", ccTypeName, ") ExtensionMap() map[int32]", g.Pkg["proto"], ".Extension {")
-		g.In()
-		g.P("if m.XXX_extensions == nil {")
-		g.In()
-		g.P("m.XXX_extensions = make(map[int32]", g.Pkg["proto"], ".Extension)")
-		g.Out()
-		g.P("}")
-		g.P("return m.XXX_extensions")
-		g.Out()
-		g.P("}")
 	}
 
 	// Default constants