Re: libldap_r

[Philip Guenther <guenther+ldapdev@sendmail.com>]

On Thu, 24 Sep 2009, Howard Chu wrote:
> It seems the dichotomy between libldap and libldap_r is a relic from the 
> bad old days of dcethreads / cmathreads when linking a threaded library 
> into an otherwise non-threaded program would cause all sorts of strange 
> and wonderful failures.

Hmm, my impression from reading the source was that it was more about use 
models, where libldap is appropriate when any given LDAP handle would not 
be accessed concurrently by multiple threads and libldap_r is appropriate 
when a given LDAP handle may be serviced/used concurrently by multiple 
threads.

Indeed, my impression from reading the ldap_r bits was that processing of 
replies wasn't ideal when threads were asking for specific replies, as 
asking for a given reply blocked *all* other ldap_result() calls on that 
handle until that specific reply was received.

(To hopefully make clear:
		thread1			thread2
		ldap_result(h, msg_A)
					ldap_result(h, msg_B)
receive msg_B
					<still blocked...>
receive msg_A
		return from ldap_result
					return from ldap_result
)


For an application design where that would be a problem, it's better off 
with independent handles or its own demultiplexor...at which point all 
those locks inside the LDAP handle are just overhead.  At Sendmail, 
several of our commercial programs use a library to manage reconnections, 
resubmitting queries after failover between servers, etc.  They all just 
link against libldap** and not libldap_r, despite having multiple threads 
concurrently in the LDAP code, because those extra locks would be wasted 
given the design.

** compiled with whatever it takes to get a thread-safe errno, etc

..
> libldap_r is still missing some thread-specific features though - we should
> wrap all library initialization in a pthread_once() call, and we should be
> using thread-specific data for the LDAP* errno value.

Without knowing what usage models this is supposed to address or provide, 
I don't see the benefit.

Philip Guenther