jsonpb should support custom JSON serialization

[jhump]

This would be done by delegating to the message if it implements json.Marshaler or json.Unmarshaler.

FWIW, this could simplify how the well-known types are handled, e.g. no need to special-case them in this package. (So, in a way, this issue relates to #169.)

Both the binary and text encodings already support the same concept, where a message can implement a particular interface (proto.Marshaler/proto.Unmarshaler, encoding.TextMarshaler/encoding.TextUnmarshaler) to customize serialization/de-serialization. This just extends that same concept to JSON encoding/decoding.

I've also opened a pull request that implements the change: #325

I think consistency with binary and text marshaling is a reasonable justification for the change. But I also have my own reason for the change: allow users of a dynamic message library to use jsonpb as they would with any other proto.Message.

[jhump]

ping

[jhump]

@bcmills, pinging you since git blame identifies you as the last to touch the jsonpb package. Mind giving this a brief glance?

[bcmills]

"Last to touch the package" isn't a great heuristic. I think @dsnet, @cherrymui, and @randall77 have been working on proto stuff lately.

[jhump]

"Last to touch the package" isn't a great heuristic.

Agreed, it isn't. But outsiders have pretty poor visibility into anything better, and I was hoping to get some feedback on this vs. letting it languish like so many other issues and pull requests.

Thanks for tagging the relevant folks!

[dsnet]

\cc @cybrcodr, who will be handling some of the jsonpb-related PRs and issues.

[jhump]

@cybrcodr: ping

[cybrcodr]

I think this sounds reasonable for dynamic messages. But it would be better for WKT's JSON (de)serialization be as implemented right now. I'll review your PR soon.

[dsnet]

Fixed in #325