Failing to publish Messages if max_buffered_messages is set to zero

[KoWu]

Creating a client with .max_buffered_messages(0) fails to publish a message with a "Max buffered messages" error, even if a connection is established.

Following code:

let reponse = mqtt_client.connect(conn_options).await.unwrap();
println!("{:?}", reponse);

mqtt_client
    .publish(
        MessageBuilder::new()
            .topic("foo")
            .payload("bar")
            .qos(1)
            .finalize(),
    )
    .await
    .unwrap();

Produces the following log:

20210323 161028.130 3 testclient -> CONNECT version 5 clean: 0 (0)
20210323 161028.168 3 testclient <- CONNACK rc: 0
20210323 161028.168 Cleaning session state on connect because sessionPresent is 0
ServerResponse { rsp: Connect(ConnectResponse { server_uri: "mqtt.example.com:8883", mqtt_version: 5, session_present: false }), props: Properties { cprops: MQTTProperties { count: 2, max_count: 10, length: 6, array: ... } }, reason_code: Success }
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: PahoDescr(-12, "Max buffered messages")', src/main.rs:124:10

[fpagliughi]

I confirmed this behavior. It's happening in the underlying C library. I created an issue over there to clarify what's happening and how best to handle it:
eclipse-paho/paho.mqtt.c#1193

[fpagliughi]

OK, after testing for a while it really appears that the C client relies on this buffer limit even when the client is connected to the broker. You definitely need at least one slot to even send a message. For sanity sake, internally I'll set a lower limit to make sure you can still send messages while connected if you set this to zero.

But even better than that I expose the ability to enable/disable off-line buffering separate from the size of the buffer. Now there's a CreateOptionsBuilder::send_while_disconnected()