Possible bug in mqttsn_packet_sender.c

[MarcoLG]

Hello,

I noticed that publishing mqttsn messages with a payload larger than ~255 bytes caused an error in the mqttsn library. I think the problem comes from this definition in mqttsn_packet_sender.c:

#define MQTTSN_PACKET_PUBLISH_LENGTH 7

This definition is used in mqttsn_packet_sender_publish() to allocate a buffer for the encoded packet of size payload_len + MQTTSN_PACKET_PUBLISH_LENGTH.
However, in the subsequent call to MQTTSNSerialize_publish() the following check is made:

if ((len = MQTTSNPacket_len(MQTTSNSerialize_publishLength(payloadlen, topic, qos))) > buflen)
{
    rc = MQTTSNPACKET_BUFFER_TOO_SHORT;
    goto exit;
}

In nominal conditions, MQTTSNSerialize_publishLength() returns payloadlen + 6, and MQTTSNPacket_len(x) returns x > 255 ? x + 3 : x + 1.
So, if payloadLen > 249 bytes, MQTTSNPacket_len will always return a value > buflen, causing the problem i've experienced.

I think the correct value for MQTTSN_PACKET_PUBLISH_LENGTH should be 9 instead of 7 to account for packets with size > 249 bytes.
I suspect the same problem affects other aspects of the library, but i've not looked for it yet.

Can you please confirm that this is an issue with nordic implementation of the mqtt-sn client and that my change does make sense?

Thanks

[McGregor]

Hello,

Did you faced any issue while subscribing to a topic.Your above answer helped in publishing higher payload size

[MarcoLG]

Hi,

uhm no, i don't remember having any issue when subscribing.