lws-minimal-mqtt-client-qos2

This is a minimal example demonstrating MQTT QoS 2 functionality with deduplication and session restoration capabilities.

The client connects to a local Mosquitto broker, subscribes to a topic (test/topic0), and publishes a QoS 2 message to that same topic. It tracks the packet_id of unacknowledged QoS 2 receives using the operations API.

Build

$ cmake . && make

Usage

This example requires a local mosquitto broker to run against.

1. Standard Testing

Run the client normally to see the full QoS 2 handshake without any interruptions:

$ ./lws-minimal-mqtt-client-qos2

The client will successfully connect, subscribe, publish a message, receive it, and log the payload, followed by the normal completion.

2. Fault Injection Mode (Session Resumption)

Run the client with the -f flag to simulate a dropped connection during the QoS 2 handshake:

$ ./lws-minimal-mqtt-client-qos2 -f

Sequence of Events:

1. The client receives the QoS 2 PUBLISH from the broker.
2. The custom my_rx_add callback fires, saving the unacknowledged packet_id to the simulated state store.
3. The client immediately drops its connection (forceful simulation).
4. The built-in retry policy triggers a reconnection.
5. On connection establishment, the client injects the stashed packet_id back into the library using lws_mqtt_client_qos2_rx_add.
6. Mosquitto resends the PUBLISH with DUP=1.
7. libwebsockets recognizes the duplicate, drops the payload to maintain the Exactly-Once guarantee, and seamlessly resumes the handshake (PUBREC -> PUBREL -> PUBCOMP).

Commandline Options

• -d <log level>: Set the logging level (e.g., -d 1039)
• -s: Use TLS / HTTPS
• -f: Simulate connection drop for testing QoS 2 restoration