The node agent waits for a backup's end on the holder's event, not by polling; the manifest consumes it (R6-A A5)

internal/link/watch.go

@@ -127,3 +127,50 @@ func (q *Session) WhenLinked(ctx context.Context) (context.Context, context.Canc
 		}
 	}
 }
+
+// EventBus is a link that can hear an event on a subject of core NATS: the node agent waits for a backup's end with it,
+// and never polls.
+type EventBus interface {
+	// Hear yields the payloads published on the subject until stop is called or the link ends.
+	Hear(subject string) (events <-chan []byte, stop func(), err error)
+}
+
+func (b OverNATS) Hear(subject string) (<-chan []byte, func(), error) {
+	out := make(chan []byte, 16)
+	sub, err := b.Conn.Subscribe(subject, func(m *nats.Msg) {
+		select {
+		case out <- m.Data:
+		default:
+		}
+	})
+	if err != nil {
+		return nil, nil, err
+	}
+	if err := b.Conn.Flush(); err != nil {
+		_ = sub.Unsubscribe()
+		return nil, nil, err
+	}
+	return out, func() { _ = sub.Unsubscribe() }, nil
+}
+
+func (l *natsLink) Hear(subject string) (<-chan []byte, func(), error) {
+	return OverNATS{Conn: l.conn, JS: l.js}.Hear(subject)
+}
+
+var (
+	_ EventBus = OverNATS{}
+	_ EventBus = (*natsLink)(nil)
+)
+
+// Hear listens for an event on the link open now, from before the caller asks for what it waits on.
+func (q *Session) Hear(subject string) (<-chan []byte, func(), error) {
+	q.init()
+	q.mu.Lock()
+	bus := q.bus
+	q.mu.Unlock()
+	h, ok := bus.(EventBus)
+	if bus == nil || !ok {
+		return nil, nil, ErrNoLink
+	}
+	return h.Hear(subject)
+}