C++ SDK: Use const-reference in WatchGameServer

[roberthbailey]

From #934:

Is it intentional to pass a GameServer object by value? I don't know if this is an expensive type to copy, but it looks like accepting this object by const-reference may be worthwhile.

We should consider changing the function signature of

grpc::Status SDK::WatchGameServer(
    const std::function<void(agones::dev::sdk::GameServer)>& callback) { ... }

Since this would be a breaking change to the CPP SDK, I'm marking this for milestone 0.12.0 and we should resolve to do this (or leave it) ASAP.

@devjgm

[roberthbailey]

@Kuqd / @markmandel - thoughts?

[markmandel]

I've no strong opinions. I basically wrote the C++ SDK as a sacrificial draft. Very happy to defer to opinions that have more C++ experience than me.

[devjgm]

A couple questions:

1. Does the GameServer argument to the callback need to be mutated? Or could it be const?
2. Is GameServer a small and cheap type that is most efficiently passed by value?

Depending on the answers to the above, I'd suggest a signature like:

grpc::Status SDK::WatchGameServer(
    const std::function<void(const agones::dev::sdk::GameServer&)>& callback);

[roberthbailey]

1. My understanding is that it should not need to be mutated as changes to it would not be reflected back into k8s (so mutating it would only be confusing).

2. I'm not sure where the cut-off is for small/cheap, but GameServer is a generated proto type, so I'm guessing the answer would be no.

[markmandel]

I can't think of any reason you would want to mutate the gameserver

[roberthbailey]

If there aren't any dissenting opinions by the end of the day tomorrow I'll send a PR to change the function signature.