[FIXED] JetStream: BackOff redeliveries would always use first in list

If the consumer's sequence was not the same than the stream's sequence,
then the redelivery would always use the first duration from the
BackOff list.

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

server/consumer.go

@@ -3130,7 +3130,7 @@ func (o *consumer) checkPending() {
 		}
 		elapsed, deadline := now-p.Timestamp, ttl
 		if len(o.cfg.BackOff) > 0 && o.rdc != nil {
-			dc := int(o.rdc[p.Sequence])
+			dc := int(o.rdc[seq])
 			if dc >= len(o.cfg.BackOff) {
 				dc = len(o.cfg.BackOff) - 1
 			}

server/jetstream_cluster_test.go

@@ -10345,16 +10345,23 @@ func TestJetStreamClusterRedeliverBackoffs(t *testing.T) {
 	_, err := js.AddStream(&nats.StreamConfig{
 		Name:     "TEST",
 		Replicas: 2,
-		Subjects: []string{"foo"},
+		Subjects: []string{"foo", "bar"},
 	})
 	require_NoError(t, err)
 
+	// Produce some messages on bar so that when we create the consumer
+	// on "foo", we don't have a 1:1 between consumer/stream sequence.
+	for i := 0; i < 10; i++ {
+		js.Publish("bar", []byte("msg"))
+	}
+
 	// Test when BackOff is configured and AckWait and MaxDeliver are as well.
 	// Currently the BackOff will override AckWait, but we want MaxDeliver to be set to be at least len(BackOff)+1.
 	ccReq := &CreateConsumerRequest{
 		Stream: "TEST",
 		Config: ConsumerConfig{
 			Durable:        "dlc",
+			FilterSubject:  "foo",
 			DeliverSubject: "x",
 			AckPolicy:      AckExplicit,
 			AckWait:        30 * time.Second,