NameResolver.Listener2 is underspecified

[jdcormie]

• onResult2() now returns a Status but its javadoc API contract says nothing about what it would mean for an implementation to return an error.
• ManagedChannelImpl's Listener2 demands that onResult2() be called from the synchronization context ... but this rule isn't mentioned anywhere in the API contract.
• Listener2 exposes two methods for reporting errors: onError(Status) and a StatusOr<> field in ResolutionResult. But the API contract says nothing about the difference or why a caller would use one or the other.

[jdcormie]

Thanks for fixing this Kannan! As a NameResolver author, though, I still don't quite know what it would mean for onResult2() to reject my update with a non-OK Status. Can you say more about this part?

[kannanjgithub]

The NameResolver listener can use the return value to convey to the NameResolver whether the addresses and / service config were accepted by the loadbalancer, which is usually what the listener will call. Some loadbalancers may consider an empty address list as an error condition, but others may not, for example. The onResult2 return value can potentially be used to retrigger a name resolution by push based name resolvers, and let them know about a bad config push for example.