Unable to get NanoMQ implementation of Retained Messages to function correctly

[codetronics]

Describe the bug
Cannot get the Retained Messages feature to work correctly with nanomq:0.21.9-full docker image over TLS. Once a message with retain flag is set and published, the same client with the intended topic already subscribed will cause a failure on reconnect. We are using the following tutorial to get the 'Retained Messages' feature to function.

https://nanomq.io/docs/en/latest/tutorial/retain-msg-persistence.html

Expected behavior
Supposed to work correctly like with EMQX broker.

Actual Behavior
Client is unable to establish connection with the broker if the subscribed topic has a retained message.

To Reproduce
We are testing the docker version of nanomq.

Clear /tmp content, restart nanomq app and then try to send a retain message to topic A. Subscribe to the topic A with the same client and then try to reconnect. Message should persist.

Environment Details

• NanoMQ version = 0.21.9-full
• Operating system and version = Ubuntu 20.04 LTS (ARM 32-bit Allwinner A20 SoC)
• Testing scenario = Docker Environment

Additional context
Below is the log from NanoMQ when the failure occurs.

nanomq.log

Below is the log from MQTTX client on failure to reconnect :-

[2024-05-02 17:20:21] [INFO] Resubscription in progress for topic: test/topic
[2024-05-02 17:20:21] [ERROR] Error subscribing to topic: Error: Connection closed
[2024-05-02 17:20:21] [ERROR] Connection for 192.168.20.101 failed, MQTT.js onError trigger, Error: Error: Invalid variable byte integer

[JaylinYu]

Thanks for reporting issue
0.21.9 is a pre-release ver, could u confirm with 0.21.8 as well?

[codetronics]

We've tried as old as 0.21.5-full and the problem is still there. We observed a weird behavior with the non-TLS connection, though. Somehow, the retain feature works fine over non-TLS connection. Upon multiple manual connect/disconnect, we are able to get the retained message with MQTTX.

If we publish a retained message over non-TLS connection, and try to connect over TLS connection with the same subscription, then we can actually receive the retained message without any issue. However, once we publish the same retained message over TLS, then the issue comes back.

We have even tested with MQTT Version 3.1, and the issue is still there.

Hope that helps. Thank you.

[JaylinYu]

A great catch!Thank you! @codetronics
Your issue report is the most detailed one I have ever seen!

[codetronics]

We are happy to be of great help, and glad that the issue is fixed. Thank you!

[JaylinYu]

Will publish a 0.21.10 release soon. happy to get any feedback from you.

[codetronics]

Tested the latest 0.21.10-full docker image and the reported issue is resolved. Thank you very much! @JaylinYu