Add MaxDeliver for observables

Signed-off-by: Derek Collison <derek@nats.io>

server/observable.go

@@ -42,6 +42,7 @@ type ObservableConfig struct {
 	DeliverLast     bool          `json:"deliver_last,omitempty"`
 	AckPolicy       AckPolicy     `json:"ack_policy"`
 	AckWait         time.Duration `json:"ack_wait,omitempty"`
+	MaxDeliver      int           `json:"max_deliver,omitempty"`
 	Subject         string        `json:"subject,omitempty"`
 	ReplayPolicy    ReplayPolicy  `json:"replay_policy"`
 	SampleFrequency string        `json:"sample_frequency,omitempty"`
@@ -135,6 +136,7 @@ type Observable struct {
 	ptmr        *time.Timer
 	rdq         []uint64
 	rdc         map[uint64]uint64
+	maxdc       uint64
 	waiting     []string
 	config      ObservableConfig
 	store       ObservableStore
@@ -188,6 +190,10 @@ func (mset *MsgSet) AddObservable(config *ObservableConfig) (*Observable, error)
 	if config.AckPolicy == AckExplicit && config.AckWait == time.Duration(0) {
 		config.AckWait = JsAckWaitDefault
 	}
+	// Setup default of -1, meaning no limit for MaxDeliver.
+	if config.MaxDeliver == 0 {
+		config.MaxDeliver = -1
+	}
 
 	// Make sure any partition subject is also a literal.
 	if config.Subject != "" {
@@ -255,7 +261,15 @@ func (mset *MsgSet) AddObservable(config *ObservableConfig) (*Observable, error)
 	}
 
 	// Set name, which will be durable name if set, otherwise we create one at random.
-	o := &Observable{mset: mset, config: *config, dsubj: config.Delivery, active: true, qch: make(chan struct{}), fch: make(chan struct{}), sfreq: int32(sampleFreq)}
+	o := &Observable{mset: mset,
+		config: *config,
+		dsubj:  config.Delivery,
+		active: true,
+		qch:    make(chan struct{}),
+		fch:    make(chan struct{}),
+		sfreq:  int32(sampleFreq),
+		maxdc:  uint64(config.MaxDeliver),
+	}
 	if isDurableObservable(config) {
 		o.name = config.Durable
 	} else {
@@ -806,6 +820,9 @@ func (o *Observable) getNextMsg() (string, []byte, uint64, uint64, error) {
 			seq = o.rdq[0]
 			o.rdq = append(o.rdq[:0], o.rdq[1:]...)
 			dcount = o.incDeliveryCount(seq)
+			if o.maxdc > 0 && dcount > o.maxdc {
+				continue
+			}
 		}
 		subj, msg, _, err := o.mset.store.LoadMsg(seq)
 		if err == nil {

test/jetstream_test.go

@@ -209,6 +209,68 @@ func TestJetStreamAddMsgSet(t *testing.T) {
 	}
 }
 
+func TestJetStreamObservableMaxDeliveries(t *testing.T) {
+	cases := []struct {
+		name    string
+		mconfig *server.MsgSetConfig
+	}{
+		{"MemoryStore", &server.MsgSetConfig{Name: "MY_WQ", Storage: server.MemoryStorage}},
+		{"FileStore", &server.MsgSetConfig{Name: "MY_WQ", Storage: server.FileStorage}},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			s := RunBasicJetStreamServer()
+			defer s.Shutdown()
+
+			mset, err := s.GlobalAccount().AddMsgSet(c.mconfig)
+			if err != nil {
+				t.Fatalf("Unexpected error adding message set: %v", err)
+			}
+			defer mset.Delete()
+
+			nc := clientConnectToServer(t, s)
+			defer nc.Close()
+
+			// Queue up our work item.
+			resp, _ := nc.Request(c.mconfig.Name, []byte("Hello World!"), 50*time.Millisecond)
+			expectOKResponse(t, resp)
+
+			sub, _ := nc.SubscribeSync(nats.NewInbox())
+			defer sub.Unsubscribe()
+			nc.Flush()
+
+			maxDeliver := 5
+			ackWait := 10 * time.Millisecond
+
+			o, err := mset.AddObservable(&server.ObservableConfig{
+				Delivery:   sub.Subject,
+				DeliverAll: true,
+				AckPolicy:  server.AckExplicit,
+				AckWait:    ackWait,
+				MaxDeliver: maxDeliver,
+			})
+			if err != nil {
+				t.Fatalf("Expected no error, got %v", err)
+			}
+			defer o.Delete()
+
+			// Wait for redeliveries to pile up.
+			checkFor(t, 250*time.Millisecond, 10*time.Millisecond, func() error {
+				if nmsgs, _, _ := sub.Pending(); err != nil || nmsgs != maxDeliver {
+					return fmt.Errorf("Did not receive correct number of messages: %d vs %d", nmsgs, maxDeliver)
+				}
+				return nil
+			})
+
+			// Now wait a bit longer and make sure we do not have more than maxDeliveries.
+			time.Sleep(2 * ackWait)
+			if nmsgs, _, _ := sub.Pending(); err != nil || nmsgs != maxDeliver {
+				t.Fatalf("Did not receive correct number of messages: %d vs %d", nmsgs, maxDeliver)
+			}
+		})
+	}
+}
+
 func TestJetStreamAddMsgSetMaxMsgSize(t *testing.T) {
 	cases := []struct {
 		name    string