[added] option to not create durable on subscribe

Signed-off-by: Matthias Hanel <mh@synadia.com>

js.go

@@ -183,21 +183,24 @@ func (nc *Conn) JetStream(opts ...JSOpt) (JetStreamContext, error) {
 	}
 
 	// If we have check recently we can avoid another account lookup here.
-	// We want these to be lighweight and created at will.
-	nc.mu.Lock()
-	now := time.Now()
-	checkAccount := now.Sub(nc.jsLastCheck) > defaultAccountCheck
-	if checkAccount {
-		nc.jsLastCheck = now
-	}
-	nc.mu.Unlock()
+	// We want these to be lightweight and created at will.
+	// The account check is permanently disabled when the api prefix is set to anything but default
+	if js.opts.pre == defaultAPIPrefix {
+		nc.mu.Lock()
+		now := time.Now()
+		checkAccount := now.Sub(nc.jsLastCheck) > defaultAccountCheck
+		if checkAccount {
+			nc.jsLastCheck = now
+		}
+		nc.mu.Unlock()
 
-	if checkAccount {
-		if _, err := js.AccountInfo(); err != nil {
-			if err == ErrNoResponders {
-				err = ErrJetStreamNotEnabled
+		if checkAccount {
+			if _, err := js.AccountInfo(); err != nil {
+				if err == ErrNoResponders {
+					err = ErrJetStreamNotEnabled
+				}
+				return nil, err
 			}
-			return nil, err
 		}
 	}
 
@@ -1041,15 +1044,20 @@ func (js *js) subscribe(subj, queue string, cb MsgHandler, ch chan *Msg, isSync
 	// With an explicit durable name, then can lookup
 	// the consumer to which it should be attaching to.
 	consumer = o.cfg.Durable
-	if consumer != _EMPTY_ {
+	if consumer == _EMPTY_ && o.noDurCreate {
+		return nil, fmt.Errorf("nats: NoCreate requires a durable name")
+	} else if consumer != _EMPTY_ && !o.noDurCreate {
 		// Only create in case there is no consumer already.
 		info, err = js.ConsumerInfo(stream, consumer)
 		if err != nil && err.Error() != "nats: consumer not found" {
 			return nil, err
 		}
 	}
 
-	if info != nil {
+	if o.noDurCreate {
+		attached = true
+		deliver = subj
+	} else if info != nil {
 		// Attach using the found consumer config.
 		ccfg = &info.Config
 		attached = true
@@ -1351,6 +1359,8 @@ type subOpts struct {
 	mack bool
 	// For creating or updating.
 	cfg *ConsumerConfig
+	// Assert consumer exists and do not create it
+	noDurCreate bool
 }
 
 // ManualAck disables auto ack functionality for async subscriptions.
@@ -1376,6 +1386,15 @@ func Durable(name string) SubOpt {
 	})
 }
 
+// NoDurableCreate instructs the API to not create a durable.
+// If no durable name was provided, an error will be raised.
+func NoDurableCreate() SubOpt {
+	return subOptFn(func(opts *subOpts) error {
+		opts.noDurCreate = true
+		return nil
+	})
+}
+
 // DeliverAll will configure a Consumer to receive all the
 // messages from a Stream.
 func DeliverAll() SubOpt {