stun_reachability: connection_already_closed in network_events

[bassosimone]

Describe the bug

I have noticed that stun_reachability measurements include several connection_already_closed inside the network_events at the end of the measurement (see, e.g., 20230911101025.015985_US_stunreachability_903c9e8d6089c755.

To Reproduce

Just run stun_reachability.

Expected behavior

There should not be attempts to use the connection once it has been closed.

Screenshots

N/A

System information (please complete the following information):

• Device: N/A
• OS: N/A
• OONI Probe version: N/A

Additional context

This error probably means that the shutdown sequence of the stun client we're using could be improved.

[bassosimone]

This issue is more complex to solve than expected, because pion/stun does not check for read errors in its read loop, therefore the read loop does not return immediately when the client is closed (and hence the con is closed) but runs for a few more loops. So, with the need to release soon, I'll postpone this change to the future.