backend glue overlay

[Howard Chu <hyc@symas.com>]

Well, the glue overlay basically works, but since the current overlay 
framework doesn't intercept the tool entry points, the glue code still 
intercepts those directly. I suppose for completeness' sake we should 
have the overlay framework intercept everything. Yes?

Meanwhile, it still seems that you need separate syncrepl providers (and 
consumers) for each database in a glued tree. I guess that's OK; 
presumably if you split the tree due to size constraints in the first 
place, the same constraints would exist on the consumers anyway. In 
order to unify everything under a single provider, the glue overlay 
would also need to intercept all write operations. Right now it only 
intercepts searches. I guess it wouldn't be a big deal to also intercept 
the writes.

I'm also wondering about whether certain overlays should always be 
enabled by default. Since syncrepl support was hardcoded into back-bdb 
before I've left its default to "yes" in configure. Will probably do the 
same for the glue overlay.

By the way, I noticed a mistake in the existing backglue's search order; 
the intent was for it to always execute searches top-down but in fact it 
searches the top/root context first, and then does the other contexts 
bottom-up. The glue overlay does it "right" but the test data sets were 
created for the "wrong" order so test011/test012 using the overlay 
report failures/differences.

--
 -- Howard Chu
 Chief Architect, Symas Corp.       Director, Highland Sun
 http://www.symas.com               http://highlandsun.com/hyc
 Symas: Premier OpenSource Development and Support