golang protobuf SetExtension

func SetExtension(pb Message, extension *ExtensionDesc, value interface{}) error {
	epb, ok := extendable(pb)
	if !ok {
		return errors.New("proto: not an extendable proto")
	}
	if err := checkExtensionTypes(epb, extension); err != nil {
		return err
	}
	typ := reflect.TypeOf(extension.ExtensionType)
	if typ != reflect.TypeOf(value) {
		return errors.New("proto: bad extension value type")
	}
	// nil extension values need to be caught early, because the
	// encoder can‘t distinguish an ErrNil due to a nil extension
	// from an ErrNil due to a missing field. Extensions are
	// always optional, so the encoder would just swallow the error
	// and drop all the extensions from the encoded message.
	if reflect.ValueOf(value).IsNil() {
		return fmt.Errorf("proto: SetExtension called with nil value of type %T", value)
	}

	extmap := epb.extensionsWrite()
	extmap[extension.Field] = Extension{desc: extension, value: value}
	return nil
}