Fixed in trunk, r75.

This change would actually be more interesting for mod_asn, than for 
mod_mirrorbrain. mod_asn is usually configured to be used for all requests; like 
mod_geoip, it runs early enough so it can't be known if the data looked up will 
actually be used.

mod_mirrorbrain however can be configured with various exceptions to not handle a 
requests at all, and ap_dbd_acquire() runs late (not earlier than needed) and 
might not be reached at all. Especially, that stage is not reached for those 
requests that actually serve data to clients and might persist for a while 
therefore.

Those functions are probably much less used than ap_dbd_acquire(), and less 
tested. If they work, it'd be nice; it would be important to make sure that no 
close() is forgetten in one of the various "quick exit" code paths.

from the old TODO file:

* check whether usage of explicit ap_dbd_open/ap_dbd_close, instead of the
  ap_dbd_acquire wrapping function, results in better utilization of the database
  connection pool
  but apr_reslist is undergoing some fixes/changes currently (1.2.11 time), so 
let's wait