Message compatibility

[ndantam]

Looking at the latest release, a field was added to the middle of hubo_state_t. This will break binary compatibility with applications compiled against the old header. The normal way to handle binary compatibility is with a library version number. However, since hubo-ach and its clients are separate processes, library versioning (alone) won't work for this.

What is a good way to handle message compatibility?
Some options:

• Punt. Users must recompile.
• Switch to a full-blow serialization library, e.g., protobuf.
• Add version number to messages
• Only append fields. Clients must be able to accept messages larger than what they were expecting, and must ignore the updated end of the message.