pubsub: Report a non-nil error when shutting down. (#7309)

If a subscriber arrives while the pubsub service is shutting down, the existing
code will return a nil subscription without error. With unlucky timing, this
may lead to a nil indirection panic in the RPC service.

To avoid that problem, make sure that when a subscription fails for this
reason, we report a non-nil error so that the client will detect it and give up
gracefully.

libs/pubsub/pubsub.go

@@ -194,7 +194,7 @@ func (s *Server) subscribe(ctx context.Context, clientID string, query Query, ou
 	case <-ctx.Done():
 		return nil, ctx.Err()
 	case <-s.Quit():
-		return nil, nil
+		return nil, errors.New("service is shutting down")
 	}
 }