Following example demonstrates that TCP connections may stuck for long time (I waited for 30+ min); connect_timeout was set to 60 just in case for the 'Read-Write Service'.
This means that socket or open file limit exhaust may happen with unstable network or unstable client programs, etc.
I tried to configure tcp keepalive just in case - but it doesn't seem to have any effect here.
I also tried the same 'attack' directly towards MariaDB Server :
And 'show processlist' doesn't show any of those connection hanging around.