Support Thread Safety

[iszak]

Currently it appears go-operational is not thread safe, in particular the Status struct. Example https://github.com/utilitywarehouse/go-operational/blob/master/op/opstatus.go#L47-L50 I think we should clarify that it's not thread safe (although this is usually assumed the default) or add support for it. Additional work would need to be done around handlers to ensure they assume that e.g. checks could be registered at any time.

My use case for adding support is it will allow starting a process which spawns multiple go routines in parallel where they have a need to register a status check. e.g. gRPC service, message consumer and ops server as separate go routines. This should improve start up time.

The downside of adding locking is a performance penalty.

[mjgarton]

Consider an application with two checks, which are added as each of two parts of the application start up concurrently.

What do you expect the health check to return when one but not the other check has been added?

It's possible one part gets blocked and so never adds the check. To me, until both parts have started, it can't be considered healthy.

How do you propose to ensure the application is "unhealthy" until all of the expected parts have added their checks and are healthy?