On Wed, Feb 27, 2019 at 3:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Thomas Munro <thomas.munro@gmail.com> writes:
> > Question
> > for the list: other stuff in the server needs libpthread (SSL, LLVM,
> > ...), so why are we insisting on using non-MT LDAP?
>
> The traditional reason for avoiding that is the risk of a server
> process becoming multi-threaded. There are live bugs of that ilk
> on Darwin, and we actually have cross-checks for the case in our
> code (see HAVE_PTHREAD_IS_THREADED_NP stanzas).
>
> If pthread_is_threaded_np(), or something equivalent, is widely available
> then it might be all right to try solving this going forward by switching
> to libldap_r and seeing if anyone hits those cross-checks. I'd be afraid
> to risk it in the back branches though ...
Hmm. Well here is a new data point: it looks like the Red Hat family
of distributions is in the process of making the same decision as
Debian (namely: to expunge the non-MT variant, because it bites
various projects in the same way that it bites us), but they haven't
quite hasn't pulled the trigger yet:
https://fedoraproject.org/wiki/Changes/OpenLDAPwithoutNonthreadedLibraries
So if we do nothing at all, it seems likely that this problem will
eventually go away by itself on practically all Linux systems, leaving
this unfixed LDAP vs postgres_fdw bug to trip up the other Unix
systems. Bleugh.
I don't see pthread_is_threaded_np() on any non-Apple systems in my
lab. Clearly libdap_r is *capable* of creating threads: it contains a
function ldap_pvt_thread_create(), and we can see that slapd and other
OpenLDAP things use that, but AFAICT that's a private facility not
intended for end users to call, so there's no danger if you just use
the documented LDAP client API. Since pthread_is_threaded_np() is a
Mac thing, note also that Macs aren't directly exposed to this
particular choice anyway because (at least if you use system-provided
libraries rather than MacPorts et al) libldap.dylib and
libldap_r.dylib are already symlinks to the same Apple voodoo
"/System/Library/Frameworks/LDAP.framework/Versions/A/LDAP".
--
Thomas Munro
https://enterprisedb.com
