THREAD POOLING STRESS TEST
PROBLEM:
Connection stress tests which consists of concurrent
kill connections interleaved with mysql ping queries
cause the mysqld server which uses thread pool scheduler
to crash.
FIX:
Killing a connection involves shutdown and close of client
socket and this can cause EPOLLHUP(or EPOLLERR) events to be
to be queued and handled after disarming and cleanup of 
of the connection object (THD) is being done.We disarm the 
the connection by modifying the epoll mask to zero which
ensure no events come and release the ownership of waiting 
thread that collect events and then do the cleanup of THD.
object.As per the linux kernel epoll source code (               
http://lxr.linux.no/linux+*/fs/eventpoll.c#L1771), EPOLLHUP
(or EPOLLERR) can't be masked even if we set EPOLL mask
to zero. So we disarm the connection and thus prevent 
execution of any query processing handler/queueing to 
client ctx. queue by removing the client fd from the epoll        
set via EPOLL_CTL_DEL. Also there is a race condition which
involve the following threads:
1) Thread X executing KILL CONNECTION Y and is in THD::awake
and using mysys_var (holding LOCK_thd_data).
2) Thread Y in tp_process_event executing and is being killed.
3) Thread Z receives KILL flag internally and possible call
the tp_thd_cleanup function which set thread session variable
and changing mysys_var.
The fix for the above race is to set thread session variable
under LOCK_thd_data.
We also do not call THD::awake if we found the thread in the
thread list that is to be killed but it's KILL_CONNECTION flag
set thus avoiding any possible concurrent cleanup. This patch
is approved by Mikael Ronstrom via email review.