Only encode & send advisories when there is interest (#6341)

This stops us from spending time & CPU cycles encoding advisory JSONs
when there is no one listening for them.

Signed-off-by: Neil Twigg <neil@nats.io>

server/consumer.go

@@ -1588,8 +1588,23 @@ func (o *consumer) unsubscribe(sub *subscription) {
 
 // We need to make sure we protect access to the outq.
 // Do all advisory sends here.
-func (o *consumer) sendAdvisory(subj string, msg []byte) {
-	o.outq.sendMsg(subj, msg)
+func (o *consumer) sendAdvisory(subject string, e any) {
+	if o.acc == nil {
+		return
+	}
+
+	// If there is no one listening for this advisory then save ourselves the effort
+	// and don't bother encoding the JSON or sending it.
+	if sl := o.acc.sl; (sl != nil && !sl.HasInterest(subject)) && !o.srv.hasGatewayInterest(o.acc.Name, subject) {
+		return
+	}
+
+	j, err := json.Marshal(e)
+	if err != nil {
+		return
+	}
+
+	o.outq.sendMsg(subject, j)
 }
 
 func (o *consumer) sendDeleteAdvisoryLocked() {
@@ -1605,13 +1620,8 @@ func (o *consumer) sendDeleteAdvisoryLocked() {
 		Domain:   o.srv.getOpts().JetStreamDomain,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
 	subj := JSAdvisoryConsumerDeletedPre + "." + o.stream + "." + o.name
-	o.sendAdvisory(subj, j)
+	o.sendAdvisory(subj, e)
 }
 
 func (o *consumer) sendPinnedAdvisoryLocked(group string) {
@@ -1629,13 +1639,8 @@ func (o *consumer) sendPinnedAdvisoryLocked(group string) {
 		Group:          group,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
 	subj := JSAdvisoryConsumerPinnedPre + "." + o.stream + "." + o.name
-	o.sendAdvisory(subj, j)
+	o.sendAdvisory(subj, e)
 
 }
 func (o *consumer) sendUnpinnedAdvisoryLocked(group string, reason string) {
@@ -1653,13 +1658,8 @@ func (o *consumer) sendUnpinnedAdvisoryLocked(group string, reason string) {
 		Reason:   reason,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
 	subj := JSAdvisoryConsumerUnpinnedPre + "." + o.stream + "." + o.name
-	o.sendAdvisory(subj, j)
+	o.sendAdvisory(subj, e)
 
 }
 
@@ -1679,13 +1679,8 @@ func (o *consumer) sendCreateAdvisory() {
 		Domain:   o.srv.getOpts().JetStreamDomain,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
 	subj := JSAdvisoryConsumerCreatedPre + "." + o.stream + "." + o.name
-	o.sendAdvisory(subj, j)
+	o.sendAdvisory(subj, e)
 }
 
 func (o *consumer) sendPauseAdvisoryLocked(cfg *ConsumerConfig) {
@@ -1705,13 +1700,8 @@ func (o *consumer) sendPauseAdvisoryLocked(cfg *ConsumerConfig) {
 		e.Paused = time.Now().Before(e.PauseUntil)
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
 	subj := JSAdvisoryConsumerPausePre + "." + o.stream + "." + o.name
-	o.sendAdvisory(subj, j)
+	o.sendAdvisory(subj, e)
 }
 
 // Created returns created time.
@@ -2652,12 +2642,7 @@ func (o *consumer) processNak(sseq, dseq, dc uint64, nak []byte) {
 		Domain:      o.srv.getOpts().JetStreamDomain,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
-	o.sendAdvisory(o.nakEventT, j)
+	o.sendAdvisory(o.nakEventT, e)
 
 	// Check to see if we have delays attached.
 	if len(nak) > len(AckNak) {
@@ -2732,15 +2717,8 @@ func (o *consumer) processTerm(sseq, dseq, dc uint64, reason, reply string) bool
 		Domain:      o.srv.getOpts().JetStreamDomain,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		// We had an error during the marshal, so we can't send the advisory,
-		// but we still need to tell the caller that the ack was processed.
-		return ackedInPlace
-	}
-
 	subj := JSAdvisoryConsumerMsgTerminatedPre + "." + o.stream + "." + o.name
-	o.sendAdvisory(subj, j)
+	o.sendAdvisory(subj, e)
 	return ackedInPlace
 }
 
@@ -3052,12 +3030,7 @@ func (o *consumer) sampleAck(sseq, dseq, dc uint64) {
 		Domain:      o.srv.getOpts().JetStreamDomain,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
-	o.sendAdvisory(o.ackEventT, j)
+	o.sendAdvisory(o.ackEventT, e)
 }
 
 // Process an ACK.
@@ -3946,12 +3919,7 @@ func (o *consumer) notifyDeliveryExceeded(sseq, dc uint64) {
 		Domain:     o.srv.getOpts().JetStreamDomain,
 	}
 
-	j, err := json.Marshal(e)
-	if err != nil {
-		return
-	}
-
-	o.sendAdvisory(o.deliveryExcEventT, j)
+	o.sendAdvisory(o.deliveryExcEventT, e)
 }
 
 // Check if the candidate subject matches a filter if its present.

server/jetstream_cluster_4_test.go

@@ -5229,3 +5229,34 @@ func TestJetStreamClusterMetaStepdownPreferred(t *testing.T) {
 		require_Equal(t, ErrorIdentifier(apiresp.Error.ErrCode), JSClusterNoPeersErrF)
 	})
 }
+
+func TestJetStreamClusterOnlyPublishAdvisoriesWhenInterest(t *testing.T) {
+	c := createJetStreamClusterExplicit(t, "R3S", 3)
+	defer c.shutdown()
+
+	subj := "$JS.ADVISORY.TEST"
+	s1 := c.servers[0]
+	s2 := c.servers[1]
+
+	// On the first server, see if we think the advisory will be published.
+	require_False(t, s1.publishAdvisory(s1.GlobalAccount(), subj, "test"))
+
+	// On the second server, subscribe to the advisory subject.
+	nc, _ := jsClientConnect(t, s2)
+	defer nc.Close()
+
+	_, err := nc.Subscribe(subj, func(_ *nats.Msg) {})
+	require_NoError(t, err)
+
+	// Wait for the interest to propagate to the first server.
+	checkFor(t, time.Second, 25*time.Millisecond, func() error {
+		if !s1.GlobalAccount().sl.HasInterest(subj) {
+			return fmt.Errorf("expected interest in %q, not yet found", subj)
+		}
+		return nil
+	})
+
+	// On the first server, try and publish the advisory again. THis time
+	// it should succeed.
+	require_True(t, s1.publishAdvisory(s1.GlobalAccount(), subj, "test"))
+}

server/jetstream_events.go

@@ -18,13 +18,22 @@ import (
 	"time"
 )
 
-func (s *Server) publishAdvisory(acc *Account, subject string, adv any) {
+// publishAdvisory sends the given advisory into the account. Returns true if
+// it was sent, false if not (i.e. due to lack of interest or a marshal error).
+func (s *Server) publishAdvisory(acc *Account, subject string, adv any) bool {
 	if acc == nil {
 		acc = s.SystemAccount()
 		if acc == nil {
-			return
+			return false
 		}
 	}
+
+	// If there is no one listening for this advisory then save ourselves the effort
+	// and don't bother encoding the JSON or sending it.
+	if sl := acc.sl; (sl != nil && !sl.HasInterest(subject)) && !s.hasGatewayInterest(acc.Name, subject) {
+		return false
+	}
+
 	ej, err := json.Marshal(adv)
 	if err == nil {
 		err = s.sendInternalAccountMsg(acc, subject, ej)
@@ -34,6 +43,7 @@ func (s *Server) publishAdvisory(acc *Account, subject string, adv any) {
 	} else {
 		s.Warnf("Advisory could not be serialized for account %q: %v", acc.Name, err)
 	}
+	return err == nil
 }
 
 // JSAPIAudit is an advisory about administrative actions taken on JetStream