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Re: controls and dynamic support

Pierangelo Masarati wrote:

Howard Chu wrote:

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

.. or void *o_controls[NID_MAX]; ?

I guess that would be OK. Any reason you prefer that over dynamically sizing the array? (I prefer to avoid hard limits, and I don't see a compelling need to limit this one.)

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):

do you mean something like

#define o_managedsait o_controls[slap_control.sc_id_manageDSAIT]
#define get_manageDSAit(op) ((int)(op)->o_managedsait == CONTROL_PRESENT)


Yes, something like that. Except for presence tests I would just use #define get_manageDSAit(op) ((op)->o_managedsait != CONTROL_ABSENT)

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.

No objection; except, to avoid some of the problems you highlighted with, e.g. pagedResults, we could use the overlay approach only where the undelying backend does not provide a more effective mechanism, e.g. for back-bdb (by saving the last sent ID) or back-ldap (where the control is simply propagated to the proxied DSA). Or, by enabling/disabling the overlay one can select to use the control or leave it to the backend.


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