encoding/gob: fix data race in Register

Fixes #4214.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://golang.org/cl/6637047

src/pkg/encoding/gob/decode.go

@@ -717,7 +717,9 @@ func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, p ui
 		errorf("name too long (%d bytes): %.20q...", len(name), name)
 	}
 	// The concrete type must be registered.
+	registerLock.RLock()
 	typ, ok := nameToConcreteType[name]
+	registerLock.RUnlock()
 	if !ok {
 		errorf("name not registered for interface: %q", name)
 	}

src/pkg/encoding/gob/encode.go

@@ -441,7 +441,9 @@ func (enc *Encoder) encodeInterface(b *bytes.Buffer, iv reflect.Value) {
 	}
 
 	ut := userType(iv.Elem().Type())
+	registerLock.RLock()
 	name, ok := concreteTypeToName[ut.base]
+	registerLock.RUnlock()
 	if !ok {
 		errorf("type not registered for interface: %s", ut.base)
 	}

src/pkg/encoding/gob/type.go

@@ -712,6 +712,7 @@ type GobDecoder interface {
 }
 
 var (
+	registerLock       sync.RWMutex
 	nameToConcreteType = make(map[string]reflect.Type)
 	concreteTypeToName = make(map[reflect.Type]string)
 )
@@ -723,6 +724,8 @@ func RegisterName(name string, value interface{}) {
 		// reserved for nil
 		panic("attempt to register empty name")
 	}
+	registerLock.Lock()
+	defer registerLock.Unlock()
 	ut := userType(reflect.TypeOf(value))
 	// Check for incompatible duplicates. The name must refer to the
 	// same user type, and vice versa.

src/pkg/encoding/gob/type_test.go

@@ -177,7 +177,10 @@ func TestRegistrationNaming(t *testing.T) {
 		Register(tc.t)
 
 		tct := reflect.TypeOf(tc.t)
-		if ct := nameToConcreteType[tc.name]; ct != tct {
+		registerLock.RLock()
+		ct := nameToConcreteType[tc.name]
+		registerLock.RUnlock()
+		if ct != tct {
 			t.Errorf("nameToConcreteType[%q] = %v, want %v", tc.name, ct, tct)
 		}
 		// concreteTypeToName is keyed off the base type.