After further deploying builds with the asserts in handleEvent we have seen
several variants of the following stack. In the cases I've dug into the
continuation called from reply_to_cont is a HttpSM. And indeed it is unlocked.
Grabbing the lock at the top of probeEvent and rescheduling if either lock
cannot be obtained should solve the problem. To simply the lock scope problem,
we get the action.mutex lock again if the continuation lock is not present.
Should only be a compare and increment overhead.
```
0 ] libc-2.17.so __GI_raise
( :undefined )
[ 1 ] libc-2.17.so __GI_abort
( :undefined )
[ 2 ] libtsutil.so.7 ink_abort
( ink_error.cc:99 )
[ 3 ] libtsutil.so.7 _ink_assert
( ink_assert.cc:37 )
[ 4 ] traffic_server Continuation::handleEvent(int, void*)
( Continuation.cc:32 )
[ 5 ] traffic_server reply_to_cont
( HostDB.cc:491 )
[ 6 ] traffic_server HostDBContinuation::probeEvent(int, Event*)
( HostDB.cc:1736 )
[ 7 ] traffic_server HostDBContinuation::dnsPendingEvent(int, Event*)
( HostDB.cc:1193 )
[ 8 ] traffic_server Continuation::handleEvent(int, void*)
( Continuation.cc:33 )
[ 9 ] traffic_server HostDBContinuation::remove_trigger_pending_dns()
( HostDB.cc:1793 )
[ 10 ] traffic_server HostDBContinuation::dnsEvent(int, HostEnt*)
( HostDB.cc:1487 )
[ 11 ] traffic_server Continuation::handleEvent(int, void*)
( Continuation.cc:33 )
[ 12 ] traffic_server DNSEntry::postEvent(int, Event*)
( DNS.cc:1282 )
[ 13 ] traffic_server Continuation::handleEvent(int, void*)
( Continuation.cc:33 )
[ 14 ] traffic_server EThread::process_event(Event*, int)
( UnixEThread.cc:132 )
[ 15 ] traffic_server EThread::process_queue(Queue<Event,
Event::Link_link>*, i... ( UnixEThread.cc:171 )
[ 16 ] traffic_server EThread::execute_regular()
( UnixEThread.cc:231 )
[ 17 ] traffic_server EThread::execute()
( UnixEThread.cc:326 )
[ 18 ] traffic_server spawn_thread_internal
( Thread.cc:85 )
[ 19 ] libpthread-2.17.so start_thread
( :undefined )
```
[ Full content available at: https://github.com/apache/trafficserver/pull/4142 ]
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