Publisher can get stuck dropping all messages with CongestionControl::Drop

[oteffahi]

Describe the bug

This bug was reported by a user on the official Zenoh Discord server. Their use-case has 4 publishers put 5Mb messages at 30Hz each, and one session declares a subscriber for each topic.
In this case, one or multiple publishers eventually get stuck in a dropping loop, where all PUT messages are dropped by congestion control and the subscriber does not receive any sample on the respective topics.

To reproduce

1. Create 4 sessions that declare one publisher each, 30Hz, 5Mb per message.
2. Create one session that declares 4 subscribers, one for each topic, and recv on each subscriber with a 5 second timeout.
3. Depending on the machine's specs, you may need to artificially increase its CPU load.
4. Eventually one or multiple publishers will drop all of their messages, and their respective subscribers will timeout every 5 seconds.

System info

• Ubuntu 24.04
• Tested on both x86 and ARM CPUs.
• Zenoh 1.1.1

[wyfo]

Fixed by #1740