fix: send Malformed Packet CONNACK for invalid Will QoS=3 in MQTT 5.0

[liooooo29]

Problem

When a MQTT 5.0 CONNECT packet contains Will QoS=3 (an invalid value per [MQTT-3.1.2-7]), Mosquitto silently closes the connection without sending a CONNACK packet.

Per the MQTT 5.0 OASIS specification §3.1.2.5, Will QoS is a 2-bit field with valid values 0, 1, 2. QoS=3 is reserved and must be rejected.

Current Behavior

// verify_will_options() in handle_connect.c
if(will_qos == 3){
    log__printf(NULL, MOSQ_LOG_INFO, "Protocol error ...");
    return MOSQ_ERR_PROTOCOL;  // Connection closed without CONNACK
}

Expected Behavior

Per [MQTT-3.1.2-7] and §4.13, the server should send a CONNACK with Reason Code 0x81 (Malformed Packet) before closing the connection.

Fix

if(will_qos == 3){
    log__printf(NULL, MOSQ_LOG_INFO, "Protocol error ...");
    if(protocol_version == mosq_p_mqtt5){
        send__connack(context, 0, MQTT_RC_MALFORMED_PACKET, NULL);
    }
    return MOSQ_ERR_MALFORMED_PACKET;
}

Test Case

Verified with an automated MQTT 5.0 conformance test (T-64) that sends CONNECT with Will QoS=3 and expects a CONNACK with Reason Code 0x81.

References

• MQTT 5.0 OASIS Standard (2019-03-07), §3.1.2.5: Will QoS
• [MQTT-3.1.2-7]: If Will Flag is set to 1, the value of Will QoS can be 0 (0x00), 1 (0x01), or 2 (0x10)