Friday, May 06, 2011

Arc and lcmaps

Last time I was talking about Arc, I mentioned that there was an issue with LCMAPS, relating to the bitness of the available libraries.

And that once a 64 bit LCMAPS library was available, that'd be it.

Well, as you might have infered from a very slight delay, there's just a teensy bit more to it than that.

64 bit libraries are now common place, and did, indeed resolve the problem we had. However, they just turned up more problems.

Cue one long, and rather frustrating search down the rabbit hole of shared library dependencies. The root problem was that nothing was defining a symbol 'getMajorVersionNumber()', or the minor or patch number versions. Finding what _should_ be doing that, and what those values aught to be was the tricky part. Perhaps that's more a symptom of my not having spent very much time debugging shared library issues, rather than a sign of a genuinely hard problem.

In the end, it's a known problem with the VOMS libraries, and it's not hard to correct for it in the small scale, by adding stub methods that return 0 in the application code, and compiling with -rdynamic.

However, translating that into something that works for ARC is non-trival. Recompling all of AREX to export functions to shared libraries is asking for trouble, given the size of the thing. It's also debatable whether it's the right thing to do to work around what's really a bug in the libraries themselves.

Fortunately, there is another option. Arc can call plugins to do pool account mapping, and these are small external programs. So writing a short wrapper around LCMAPS is straight forward, and then Arc delegates responsability to this plugin, which is a nice, self contained place to have the workarounds.

My version of such a plugin is here, and should be identified in the arc.conf as
unixgroup=mapplugin 5 arc-lcmap %D %P

This now lets us use the same pool account mapping and authorisation infrastructure with both gLite and Arc. In particular, this lets us open up the Arc CE to any of our normally supported VO's; as a option for them to explore. That's a topic I'll be working with some VO's on over the summer.

For the moment though, I need to dismantle the layer of auth systems hacks we were using for Arc.

1 comment:

Leif Nixon said...

Ow. That's heroic bug hunting. Have you reported it to the ARC team? Even if not an ARC issue per se, it might be good for them to know about it.