Fix bug for messages of length 0x100

[martinkirsche]

If the parameter length is 255 bytes the result of the previous MQTTSNPacket_len implementation was 256 bytes. But such a message actually needs a 3-octet long Length field, so it should return 258 instead.

Signed-off-by: Martin Kirsche martin.kirsche@gmail.com

[ty4tw]

Hi,
Thank you for the PR.
A current implementation goes with the spec, I think.

[martinkirsche]

This issue has nothing to do with the spec. The problem is that the output of the function MQTTSNPacket_len is fet into MQTTSNPacket_encode. So if I put 255 as the length of the packet without the length field into MQTTSNPacket_len it will currently return 256. But MQTTSNPacket_encode will use a 3-octed length field for such a message. The result is a MQTT-SN message that is longer than its length field specifies.

Here is a test that demonstrates this:

import socket

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(("127.0.0.1", 12001))


def connect():
    return bytearray([
        0x07, # Length(0)
        0x04, # MsgType(1)
        0x04, # Flags(2)
        0x01, # ProtocolId(3)
        0x00, # Duration(4)
        0x3c, # Duration(5)
        0x47  # ClientId(6)
    ])

def sub():
    return bytearray([
        0x07, # Length(0)
        0x12, # MsgType(1)
        0x02, # Flags(2)
        0x00, # MsgId (3)
        0x01, # MsgId (4)
        0x41, # TopicName (5)
        0x41  # TopicName (6)
    ])

def pub():
    return bytearray([
        0x01, # Length(0)
        0x01,
        0x02,
        0x0C, # MsgType(1)
        0x02, # Flags(2)
        0x41, # TopicName (3)
        0x41, # TopicName (4)
        0x00, # MsgId (5)
        0x02, # MsgId (6)
    ]) + (bytearray((0x41,)) * (0x102-9))

sock.sendto(connect(), ("127.0.0.1", 10000))
print(sock.recvfrom(2))

sock.sendto(sub(), ("127.0.0.1", 10000))
print(sock.recvfrom(8))

sock.sendto(pub(), ("127.0.0.1", 10000))
print(sock.recvfrom(0x102))

Here is the MQTT-SN-Gateway log:

20200323 221723.906   CONNECT           <---  G                                   07 04 04 01 00 3C 47
20200323 221723.983   CONNACK           --->  GatewayTestClient                   03 05 00

20200323 221723.983   SUBSCRIBE   0001  <---  GatewayTestClient                   07 12 02 00 01 41 41
20200323 221723.983   SUBACK      0001  --->  GatewayTestClient                   08 13 00 00 00 00 01 00
20200323 221723.983   PUBLISH     0002  <---  GatewayTestClient                   01 01 02 0C 02 41 41 00 02 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41
20200323 221723.984   PUBLISH           --->  GatewayTestClient                   01 01 00 0C 02 41 41 00 00 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41

[ty4tw]

Thank you,
I could understand your PR and merged it.