fix(pubsub): make config call permission error in Receive transparent (…

…#3985)

* fix(pubsub): make config call permission error in Receive transparent

fix(pubsub): make config call permission error in Receive transparent

remove extra whitespace

clarify doc comments

fix typos

fix formatting

* make call to config only if we see an ordering key

* move comment to method doc

pubsub/subscription.go

@@ -48,6 +48,7 @@ type Subscription struct {
 	mu            sync.Mutex
 	receiveActive bool
 
+	once           sync.Once
 	enableOrdering bool
 }
 
@@ -224,7 +225,17 @@ type SubscriptionConfig struct {
 	// The set of labels for the subscription.
 	Labels map[string]string
 
-	// EnableMessageOrdering enables message ordering.
+	// EnableMessageOrdering enables message ordering on this subscription.
+	// This value is only used for subscription creation and update, and
+	// is not read locally in calls like Subscription.Receive().
+	//
+	// If set to false, even if messages are published with ordering keys,
+	// messages will not be delivered in order.
+	//
+	// When calling Subscription.Receive(), the client will check this
+	// value with a call to Subscription.Config(), which requires the
+	// roles/viewer or roles/pubsub.viewer role on your service account.
+	// If that call fails, mesages with ordering keys will be delivered in order.
 	EnableMessageOrdering bool
 
 	// DeadLetterPolicy specifies the conditions for dead lettering messages in
@@ -774,14 +785,6 @@ func (s *Subscription) Receive(ctx context.Context, f func(context.Context, *Mes
 	s.mu.Unlock()
 	defer func() { s.mu.Lock(); s.receiveActive = false; s.mu.Unlock() }()
 
-	// Check config to check EnableMessageOrdering field.
-	// See: https://github.com/googleapis/google-cloud-go/issues/3884
-	cfg, err := s.Config(ctx)
-	if err != nil {
-		return fmt.Errorf("sub.Config err: %v", err)
-	}
-	s.enableOrdering = cfg.EnableMessageOrdering
-
 	maxCount := s.ReceiveSettings.MaxOutstandingMessages
 	if maxCount == 0 {
 		maxCount = DefaultReceiveSettings.MaxOutstandingMessages
@@ -911,6 +914,9 @@ func (s *Subscription) Receive(ctx context.Context, f func(context.Context, *Mes
 						old(ackID, ack, receiveTime)
 					}
 					wg.Add(1)
+					if msg.OrderingKey != "" {
+						s.once.Do(s.checkOrdering)
+					}
 					// Make sure the subscription has ordering enabled before adding to scheduler.
 					var key string
 					if s.enableOrdering {
@@ -948,6 +954,21 @@ func (s *Subscription) Receive(ctx context.Context, f func(context.Context, *Mes
 	return group.Wait()
 }
 
+// checkOrdering calls Config to check theEnableMessageOrdering field.
+// If this call fails (e.g. because the service account doesn't have
+// the roles/viewer or roles/pubsub.viewer role) we will assume
+// EnableMessageOrdering to be true.
+// See: https://github.com/googleapis/google-cloud-go/issues/3884
+func (s *Subscription) checkOrdering() {
+	ctx := context.Background()
+	cfg, err := s.Config(ctx)
+	if err != nil {
+		s.enableOrdering = true
+	} else {
+		s.enableOrdering = cfg.EnableMessageOrdering
+	}
+}
+
 type pullOptions struct {
 	maxExtension       time.Duration // the maximum time to extend a message's ack deadline in total
 	maxExtensionPeriod time.Duration // the maximum time to extend a message's ack deadline per modack rpc