From 7470975907e1267a6e091f333a5c21e655b31c8f Mon Sep 17 00:00:00 2001
From: Luis Gustavo <luisgustavomacedo13@gmail.com>
Date: Wed, 20 Nov 2024 12:36:24 -0300
Subject: [PATCH] fix check of the backoff and max delivery configs

Match the check of the backoff and maxDeliver configs with the official documentation https://docs.nats.io/nats-concepts/jetstream/consumers#:~:text=The%20sequence%20length,MaxDelivery
---
 server/consumer.go                |  4 +--
 server/jetstream_consumer_test.go | 57 +++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/server/consumer.go b/server/consumer.go
index b1d316a6d90..f4e7bfbd0ab 100644
--- a/server/consumer.go
+++ b/server/consumer.go
@@ -623,7 +623,7 @@ func checkConsumerCfg(
 	}
 
 	// Check if we have a BackOff defined that MaxDeliver is within range etc.
-	if lbo := len(config.BackOff); lbo > 0 && config.MaxDeliver != -1 && config.MaxDeliver <= lbo {
+	if lbo := len(config.BackOff); lbo > 0 && config.MaxDeliver != -1 && lbo > config.MaxDeliver {
 		return NewJSConsumerMaxDeliverBackoffError()
 	}
 
@@ -2091,7 +2091,7 @@ func (acc *Account) checkNewConsumerConfig(cfg, ncfg *ConsumerConfig) error {
 	}
 
 	// Check if BackOff is defined, MaxDeliver is within range.
-	if lbo := len(ncfg.BackOff); lbo > 0 && ncfg.MaxDeliver != -1 && ncfg.MaxDeliver <= lbo {
+	if lbo := len(ncfg.BackOff); lbo > 0 && ncfg.MaxDeliver != -1 && lbo > ncfg.MaxDeliver {
 		return NewJSConsumerMaxDeliverBackoffError()
 	}
 
diff --git a/server/jetstream_consumer_test.go b/server/jetstream_consumer_test.go
index a566908ad8b..2fd1aa45572 100644
--- a/server/jetstream_consumer_test.go
+++ b/server/jetstream_consumer_test.go
@@ -959,6 +959,22 @@ func TestJetStreamConsumerBackOff(t *testing.T) {
 			},
 			shouldErr: false,
 		},
+		{
+			name: "backoff_with_max_deliver_equal",
+			config: nats.ConsumerConfig{
+				MaxDeliver: 3,
+				BackOff:    []time.Duration{time.Second, time.Minute, time.Hour},
+			},
+			shouldErr: false,
+		},
+		{
+			name: "backoff_with_max_deliver_equal_to_zero",
+			config: nats.ConsumerConfig{
+				MaxDeliver: 0,
+				BackOff:    []time.Duration{},
+			},
+			shouldErr: false,
+		},
 		{
 			name: "backoff_with_max_deliver_smaller",
 			config: nats.ConsumerConfig{
@@ -2476,3 +2492,44 @@ func TestJetStreamConsumerBackoffNotRespectedWithMultipleInflightRedeliveries(t
 		}
 	}
 }
+
+func TestJetStreamConsumerBackoffWhenBackoffLengthIsEqualToMaxDeliverConfig(t *testing.T) {
+	s := RunBasicJetStreamServer(t)
+	defer s.Shutdown()
+
+	nc, js := jsClientConnect(t, s)
+	defer nc.Close()
+
+	_, err := js.AddStream(&nats.StreamConfig{
+		Name:     "TEST",
+		Subjects: []string{"events.>"},
+	})
+	require_NoError(t, err)
+
+	maxDeliver := 3
+	backoff := []time.Duration{time.Second, 2 * time.Second, 3 * time.Second}
+	sub, err := js.SubscribeSync(
+		"events.>",
+		nats.MaxDeliver(maxDeliver),
+		nats.BackOff(backoff),
+		nats.AckExplicit(),
+	)
+	require_NoError(t, err)
+
+	calculateExpectedBackoff := func(numDelivered int) time.Duration {
+		return backoff[numDelivered-1] + 50*time.Millisecond // 50ms of margin to system overhead
+	}
+
+	// message to be redelivered using backoff duration.
+	firstMsgSent := time.Now()
+	sendStreamMsg(t, nc, "events.first", "msg-1")
+	_, err = sub.NextMsg(time.Second)
+	require_NoError(t, err)
+	require_LessThan(t, time.Since(firstMsgSent), calculateExpectedBackoff(1))
+	_, err = sub.NextMsg(2 * time.Second)
+	require_NoError(t, err)
+	require_LessThan(t, time.Since(firstMsgSent), calculateExpectedBackoff(2))
+	_, err = sub.NextMsg(3 * time.Second)
+	require_NoError(t, err)
+	require_LessThan(t, time.Since(firstMsgSent), calculateExpectedBackoff(3))
+}