broken TCP connections are not always cleaned properly

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.

$ bin/maxscale --basedir=$(pwd)

$ sudo bin/maxadmin 'list clients' | wc -l

7

$ for i in {1..100} ; do telnet 127.0.0.1 4006 & done

...

$ sudo bin/maxadmin 'list clients' | wc -l

107

$ date

Wed May 10 22:12:44 CEST 2017

$ sudo bin/maxadmin 'list clients' | wc -l

107

$ date

Wed May 10 22:45:30 CEST 2017

$ sudo bin/maxadmin 'list clients' | wc -l

107

$ netstat | grep 4006 | grep ESTAB | wc -l

200

$ netstat | grep 4006 | grep CLOSE_WAIT | wc -l

100

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 :

for i in {1..100}; do telnet 127.0.0.1 3307 & done

And 'show processlist' doesn't show any of those connection hanging around.