Cross platform backwards compatibility

[staticgears]

Version of protoc (protoc --version)

libprotoc 3.0.0

Version of ProtocolBuffers.framework

ProtoBuf3.0-Swift2.0 19ac094

.proto file to reproduce

syntax = "proto3";
package protocol;

import 'src/main/swift/swift-descriptor.proto';
option (.google.protobuf.swift_file_options).entities_access_control = PublicEntities;

message Envelope {
    enum Type {
        CONTENT          = 0;
       //Added in a second version of the protocol
        READ                 = 1;
   }
}

Description

Protobuf 3 enum compatibility

Your implementation differs from the Java implementation in the following two key ways

1. The protoc java implementation generates an UNKNOWN value for enums and when ever an unknown type is encountered the resulting Object has this UNKNOWN value for the enum.
2. The protoc java implementation does not write out default values

The result of these two differences is that if I have a Swift client built using the first version of the protocol (ie without the READ value) and it receives a message from a Java client which sent the READ value that is indistinguishable from the case in which the Java client sends the CONTENT value.

It is worth noting that if all of the clients involved are using your Swift generator this ambiguity does not arise because the sender will write out the actual CONTENT value in the bytes (as opposed to the Java implementation which sends empty bytes and relies on the default object on the other side).The receiver can use the generated hasType property to differentiate the two cases if the full bytes are written.