Proposed changes

Resubscribe to MQTT topics on reconnection when the no session is present. No session is present when mosquitto persistence has not been configured, or inaccessible (e.g. corrupt or incorrect ownership/permissions).

Previously, the reconnection logic was assuming that the only time the client needs to resubscribe to the MQTT topics is when an anonymous session is used. However if the MQTT broker persistence layer is not working, then the client should be checking if a session is present or not, and then resubscribing to the topics if no session was restored.

The most notable symptom without this change was that the tedge-agent would stop reacting to operations sent via MQTT if it would be disconnected from the local MQTT broker and if the MQTT broker persistence layer was not working or enabled. Restarting the tedge-agent service would "fix" the situation but only until the next reconnection event.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

• Discovered during the investigation of this ticket: tedge-agent should not block processing commands if one command is unacknowledged by the creator #3456

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments