subscribe_many with empty topic list emits malformed packet

[KisImre]

Description

Calling Client::subscribe_many with an empty topic list causes it to send a subscribe packet without any topic filter. According to [MQTT-3.8.3-3] "The payload of a SUBSCRIBE packet MUST contain at least one Topic Filter / QoS pair. A SUBSCRIBE packet with no payload is a protocol violation". v5 has the same requirement. When the broker gets this message is closes the connection due to a malformed packet.

• This affects try_subscribe_many as well
• Client::subscribe_many should return an error when called with an empty topic list
• I haven't looked into rumqttd but it shouldn't accept these empty subscribe messages

Test setup

rumqttc v0.12.0
mosquitto version 2.0.14

Test code

use rumqttc::{Client, MqttOptions};

fn main() {
    let mqttoptions = MqttOptions::new("rumqtt-sync", "localhost", 1883);

    let (mut client, mut connection) = Client::new(mqttoptions, 10);

    client.subscribe_many(Vec::new()).unwrap();

    for notification in connection.iter() {
        println!("Notification = {:?}", notification);
    }
}

Output

Notification = Ok(Incoming(ConnAck(ConnAck { session_present: false, code: Success })))
Notification = Ok(Outgoing(Subscribe(1)))
Notification = Err(MqttState(Io(Custom { kind: ConnectionAborted, error: "connection closed by peer" })))

Broker log

mosquitto[2575]: 1650909678: New connection from ::1:39042 on port 1883.
mosquitto[2575]: 1650909678: New client connected from ::1:39042 as rumqtt-sync (p2, c1, k60).
mosquitto[2575]: 1650909678: Client rumqtt-sync disconnected due to malformed packet.

Packet

[de-sh]

Thank you for pointing this out @KisImre. I have opened a branch to work on a possible solution to this. Do let me know your thoughts on the included changes.

[KisImre]

Thank you for the fix, it looks good to me. Maybe it would be worth adding a check to Subscribe::read as well to prevent future issues.