back-bdb online indexing

[Howard Chu <hyc@symas.com>]

While back-config now allows you to alter back-bdb's index configuration 
on the fly, there's some missing functionality here (so adding an index 
directive is a Bad Idea for now).

The idea so far - when new indexing is configured, submit a runqueue 
task that will do the necessary slapindex operations. The problem is 
that this task will have to iterate over every entry in the database, 
and we can't use the indexes to satisfy any searches until the task 
completes. That means, we cannot allow the rest of the backend to see 
the new index configuration while performing this task. We may also have 
to prevent any other updates from occurring while this goes on. I.e., if 
we just use a database cursor to walk thru the id2entry database, we 
don't want anybody modifying any entries we've already done (because the 
modifications may not know to update the in-progress indices. I suppose 
we could arrange for modifications to work...)

Given these constraints, is it worthwhile to allow this operation, or 
should we just disallow new indexing to be configured at runtime?

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