SLAP_LIGHTWEIGHT_LISTENER, using lazy_sem is a bad design

[Howard Chu <hyc@symas.com>]

This solution will deadlock if all worker threads are stuck in a write 
wait. Since the semaphore completely blocks the listener thread, there 
will be no way to wake up the waiting writers and free up more threads. 
Also, blocking the listener thread like this prevents the idletimeout 
checker from working. I.e., you have managed to disable two key 
mechanisms for returning server resources to the pool, precisely when 
they are needed the most.

The listener thread must never block, period.

It would be better to simply have ldap_pvt_thread_pool_submit return a 
result code (e.g. LDAP_BUSY if the submitted op will be queued because 
there are no available workers, LDAP_SUCCESS otherwise) that is passed 
back to the listener thread. When the listener thread gets this result 
it should drop all read descriptors from the event set, but keep 
monitoring the wake_sds and the write events.

Alternatively we should stop monitoring write events and just let the 
writers unblock themselves. Is there a particular reason why we monitor 
write events? I don't see any benefit. Eliminating one set of event 
sources would reduce our kernel load by half.

Aside from this issue there is definitely a bug in the current 
implementation; I see the same event being submitted multiple times in 
rapid succession. The CPU usage goes to 100% and there does not appear 
to be any end condition that disables the event. This occurs most often 
in test033, but should occur in any test that uses syncrepl (or 
listener-managed client tasks like syncrepl). After the syncrepl task 
has sent a search request to the provider and the first reply arrives, 
marking the socket readable. It appears this readable state is not 
getting reset.

--
 -- Howard Chu
 Chief Architect, Symas Corp.  http://www.symas.com
 Director, Highland Sun        http://highlandsun.com/hyc
 OpenLDAP Core Team            http://www.openldap.org/project/