From 1f39d744dd20263711077c55be6fc716ffb8e5b4 Mon Sep 17 00:00:00 2001
From: Derek Collison <derek@nats.io>
Date: Wed, 12 Jul 2023 14:46:06 -0700
Subject: [PATCH] Only discard messages from MQTT QoS0 from internal jetstream
 clients if really a QoS1 jetstream publish.

Signed-off-by: Derek Collison <derek@nats.io>
---
 server/mqtt.go      |  6 +++---
 server/mqtt_test.go | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/server/mqtt.go b/server/mqtt.go
index 2d214e01..8680e8dd 100644
--- a/server/mqtt.go
+++ b/server/mqtt.go
@@ -1,4 +1,4 @@
-// Copyright 2020-2021 The NATS Authors
+// Copyright 2020-2023 The NATS Authors
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -3407,8 +3407,8 @@ func mqttSubscribeTrace(pi uint16, filters []*mqttFilter) string {
 // message and this is the callback for a QoS1 subscription because in
 // that case, it will be handled by the other callback. This avoid getting
 // duplicate deliveries.
-func mqttDeliverMsgCbQos0(sub *subscription, pc *client, _ *Account, subject, _ string, rmsg []byte) {
-	if pc.kind == JETSTREAM {
+func mqttDeliverMsgCbQos0(sub *subscription, pc *client, _ *Account, subject, reply string, rmsg []byte) {
+	if pc.kind == JETSTREAM && len(reply) > 0 && strings.HasPrefix(reply, jsAckPre) {
 		return
 	}
 
diff --git a/server/mqtt_test.go b/server/mqtt_test.go
index a5068ac7..625c206b 100644
--- a/server/mqtt_test.go
+++ b/server/mqtt_test.go
@@ -1,4 +1,4 @@
-// Copyright 2020 The NATS Authors
+// Copyright 2020-2023 The NATS Authors
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -6367,6 +6367,49 @@ func TestMQTTSubjectWildcardStart(t *testing.T) {
 	require_True(t, si.State.Msgs == 0)
 }
 
+// Issue https://github.com/nats-io/nats-server/issues/4291
+func TestMQTTJetStreamRepublishAndQoS0Subscribers(t *testing.T) {
+	conf := createConfFile(t, []byte(`
+		listen: 127.0.0.1:-1
+		server_name: mqtt
+		jetstream: enabled
+		mqtt {
+			listen: 127.0.0.1:-1
+		}
+	`))
+	s, o := RunServerWithConfig(conf)
+	defer testMQTTShutdownServer(s)
+
+	nc, js := jsClientConnect(t, s)
+	defer nc.Close()
+
+	// Setup stream with republish on it.
+	_, err := js.AddStream(&nats.StreamConfig{
+		Name:     "TEST",
+		Subjects: []string{"foo"},
+		RePublish: &nats.RePublish{
+			Source:      "foo",
+			Destination: "mqtt.foo",
+		},
+	})
+	require_NoError(t, err)
+
+	// Create QoS0 subscriber to catch re-publishes.
+	mc, r := testMQTTConnect(t, &mqttConnInfo{cleanSess: true}, o.MQTT.Host, o.MQTT.Port)
+	defer mc.Close()
+	testMQTTCheckConnAck(t, r, mqttConnAckRCConnectionAccepted, false)
+
+	testMQTTSub(t, 1, mc, r, []*mqttFilter{{filter: "mqtt/foo", qos: 0}}, []byte{0})
+	testMQTTFlush(t, mc, nil, r)
+
+	msg := []byte("HELLO WORLD")
+	_, err = js.Publish("foo", msg)
+	require_NoError(t, err)
+
+	testMQTTCheckPubMsg(t, mc, r, "mqtt/foo", 0, msg)
+	testMQTTExpectNothing(t, r)
+}
+
 //////////////////////////////////////////////////////////////////////////
 //
 // Benchmarks