event hub: a start that found no durable on the bus is tried again at the next start, so a part is not lost for the order in which the bus makes its consumer (B7e; issue 1966)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

go/process/event_hub.go

@@ -19,11 +19,9 @@ import (
 type EventHub struct {
 	logf func(string, ...any)
 
-	mu       sync.RWMutex
-	routes   []hubRoute
-	started  bool
-	cons     jetstream.ConsumeContext
-	startErr error
+	mu     sync.RWMutex
+	routes []hubRoute
+	cons   jetstream.ConsumeContext
 }
 
 type hubRoute struct {
@@ -68,23 +66,23 @@ func (h *EventHub) Patterns() []string {
 	return out
 }
 
-// Start binds the module's durable on the shared events stream and pulls it; later calls do nothing and return the
-// first one's result. It makes no stream and no consumer.
+// Start binds the module's durable on the shared events stream and pulls it; a call after a start that took does
+// nothing. It makes no stream and no consumer: a consumer the bus does not hold yet is the error of this call, and a
+// later call (the module's next change of membership) binds it, so a part is not lost for the bus's order.
 func (h *EventHub) Start(ctx context.Context, js jetstream.JetStream, cred Account) error {
 	h.mu.Lock()
 	defer h.mu.Unlock()
-	if h.started {
-		return h.startErr
+	if h.cons != nil {
+		return nil
 	}
-	h.started = true
 	cons, err := cred.EventConsumer(ctx, js, EventsStream)
 	if err == nil {
 		h.cons, err = HearEvents(cons, h.hear, h.logf)
 	}
 	if err != nil {
-		h.startErr = fmt.Errorf("the consumer %s on %s is not heard: %w", cred.EventConsumerName(), EventsStream, err)
+		return fmt.Errorf("the consumer %s on %s is not heard: %w", cred.EventConsumerName(), EventsStream, err)
 	}
-	return h.startErr
+	return nil
 }
 
 // Stop ends the pull.