Send protocol error on topic alias not found.

The error topic-alias-invalid was being sent if an MQTT v5 client published
a message with empty topic and topic alias set, but the topic alias hadn't
already been configured on the broker. This has been fixed to send a
protocol error, as per section 3.3.4 of the specification.

ChangeLog.txt

@@ -10,6 +10,10 @@ Broker:
 - Fix reloading of listeners where multiple listeners have been defined with
   the same port but different bind addresses. Closes #2029.
 - Fix `message_size_limit` not applying to the Will payload. Closes #2022.
+- The error topic-alias-invalid was being sent if an MQTT v5 client published
+  a message with empty topic and topic alias set, but the topic alias hadn't
+  already been configured on the broker. This has been fixed to send a
+  protocol error, as per section 3.3.4 of the specification.
 
 Apps:
 - Allow command line arguments to override config file options in

src/handle_publish.c

@@ -186,7 +186,7 @@ int handle__publish(struct mosquitto *context)
 			rc = alias__find(context, &msg->topic, (uint16_t)topic_alias);
 			if(rc){
 				db__msg_store_free(msg);
-				return MOSQ_ERR_TOPIC_ALIAS_INVALID;
+				return MOSQ_ERR_PROTOCOL;
 			}
 		}
 	}

test/broker/02-subpub-qos0-topic-alias-unknown.py

@@ -14,7 +14,7 @@ def do_test():
     props = mqtt5_props.gen_uint16_prop(mqtt5_props.PROP_TOPIC_ALIAS, 3)
     publish1_packet = mosq_test.gen_publish("", qos=0, payload="message", proto_ver=5, properties=props)
 
-    disconnect_packet = mosq_test.gen_disconnect(reason_code=148, proto_ver=5)
+    disconnect_packet = mosq_test.gen_disconnect(reason_code=mqtt5_rc.MQTT_RC_PROTOCOL_ERROR, proto_ver=5)
 
     port = mosq_test.get_port()
     broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)