Major Description
NetHandler has a method: _close_vc ， It is called by InactivityCop.
first, create a dummy Event in stack,
then call UnixNetVConnection::mainEvent by vc->handleEvent(EVENT_IMMEDIATE, &event);
the handleEvent is mainEvent here.
In the UnixNetVConnection::mainEvent code:
```
int
UnixNetVConnection::mainEvent(int event, Event *e)
{
ink_assert(event == EVENT_IMMEDIATE || event == EVENT_INTERVAL);
ink_assert(thread == this_ethread());
MUTEX_TRY_LOCK(hlock, get_NetHandler(thread)>mutex, e>ethread);
MUTEX_TRY_LOCK(rlock, read.vio.mutex ? read.vio.mutex : e->ethread->mutex, e->ethread);
MUTEX_TRY_LOCK(wlock, write.vio.mutex ? write.vio.mutex : e->ethread->mutex, e->ethread);
if (!hlock.is_locked() || !rlock.is_locked() || !wlock.is_locked() ||
(read.vio.mutex && rlock.get_mutex() != read.vio.mutex.get()) ||
(write.vio.mutex && wlock.get_mutex() != write.vio.mutex.get()))
```
the dummy Event would be schedule_in into Event System by e->schedule_in(HRTIME_MSECONDS(net_retry_delay));
I think we should move the schedule_in into the INACTIVITY_TIMEOUT macro.
```
#ifdef INACTIVITY_TIMEOUT
if (e == active_timeout)
e->schedule_in(HRTIME_MSECONDS(net_retry_delay));
#endif
```
I'm try to allocate a Event instead dummy Event, but meet Event System callback on a deallocated UnixNetVConnection.
due to NetHandler called close_UnixNetVConnection before Event System callback the Event by schedule_in.
In NetHandler::_close_vc, depend the return value (EVENT_DONE or EVENT_CONT) from UnixNetVConnection::mainEvent, to do ++handle_event; or not.
```
if (vc->handleEvent(EVENT_IMMEDIATE, &event) == EVENT_DONE)
++handle_event;
```
the 3 MUTEX_TRY_LOCK not always success on InactivityCop callback,
due to the mutex of ServerSessionVC may different from ClientSessionVC.
Only mutex of ServerSession is set to HttpSM when HttpSM pick up a Server Session from SessionPool. ServerSessionVC still keep the old mutex.
