Databroker shall send UNAVAILABLE if terminating subscriptions on shutdown

[BjoernAtBosch]

The broker currently sends gRPC status code OK to consumers if terminating running subscriptions on a clean shutdown (e.g. caused by SigInt).

Instead it should send the code UNAVAILABLE because the result of a shutdown of the broker will be its unavailability.

Reasoning:

OK isn't a proper status code for that situation. A status OK signals that the stream has regularly ended. It makes sense in case where something finite is streamed, like file contents or a movie. When the end is reached the provider sends the ok to signal that all data has been send.

In the databroker case, a stream of signal updates is as by its nature an endless stream. Therefore, there is no reason for the broker to terminate an existing subscription - except in error cases. Also, from a consumer's perspective a subscription should live as long as the consumer is not cancelling it.

A shutdown of the broker - regardless of being intentionally (clean shutdown) as well as caused by an unexpected failure - is from that perspective always an exceptional situation for all the involved components, i.e. consumers, providers, and the broker itself: They basically expect to "run forever". (Of course they need to be prepared for possible unavailability of other components, but this is always an exceptional situation.

Therefore, I would go for the status code UNAVAILABLE.

I wouldn't use ABORTED because

1. any error returned will abort the subscription - so this gives no extra meaning,
2. it wouldn't fully match the description of ABORTED as given by the gRPC documentation of status codes.