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controls and dynamic support

I was thinking about how both pagedresults and server-side sorting could be accomplished using overlays, and the idea is simple enough. But one of the more awkward aspects of adding new controls is plugging them into the Operation structure so they can be easily referenced by other code. I recall how OpenSSL handles various OID-based entities - it has its own internal registry and assigns small integers (NIDs) for each one. I think we need a similar mechanism for slapd.

Specifically, the register_supported_control function should return the index of the control in slapd's internal table. The Operation structure should have an array
void **o_controls
that is preallocated to a size equal to the number of currently registered controls. Each element of the array will be set by the control's parser to point to a control-specific structure to carry whatever state that control needs. (Only controls that were actually received will have non-NULL entries in the array.)

Certain controls that we decide to leave inside the core can have well-known variables carrying their assigned control IDs (CIDs):

Others should be migrated out to private files specific to their implementation. (I of course want to migrate all the syncrepl state info out as well.)

So a pagedResults overlay would register its control and save the returned index. Its parser function would allocate a private structure and assign its address to op->o_controls[pagedresults_cid] on an incoming operation, and all of the overlay code that needs to reference the control parameters can find it there.

We can also export a find_ctrl function that returns the CID for a given OID, etc.

As for the actual controls...

For server-side sorting all you need is an overlay that intercepts search responses and builds a sorted list before actually returning results to a client. The biggest issue here is configuring how much memory it's allowed to consume per search.

For paged results you could be really gross and actually suspend the current thread at the end of a page. (Of course, this means tying up threads for some amount of time. Not so friendly.) That is, the overlay's search response callback just waits on a condition variable when it gets to the page limit. The overlay's search handler checks for any currently suspended operations when it receives a continuing paged results request, and signals the condition if it finds the match. There can be an idletimeout runtask that abandons operations that have been suspended for too long. It's a rather coarse-grained approach to saving search state, but it would work for all backends without needing to redesign them all.

For that matter, the noop control could be entirely processed as an overlay. It can call the underlying backend to retrieve entry info, perform ACL checks, and then return a result without performing any write operations on the actual backend.

For the syncrepl provider there's bits scattered across the Operation and BackendDB structures that I plan to migrate into private structures. Currently I'm testing with back-ldbm; when I get through all of the existing syncrepl test scripts I will start deleting the old definitions from the various places.

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