I added support for the full search algorithm to the RBT database
  code.  The database has some special needs in searching, in particular
  certain nodes need to be examined before the DOWN pointer is followed.

  I solved this problem by adding a 'callback' bit to the node structure.
  When findnode is called, the caller can supply a callback.  Before we go
  DOWN at a node, we call the callback if the callback bit is set at the
  node.  If the callback returns DNS_R_CONTINUE, we proceed.  Otherwise, we
  will stop right there and return what the caller said to return.  I
  added an 'empty_data_ok' flag to findnode as well, so that nodes with
  empty data can be made candidates for partial match status.

  I also wanted to make dns_rbtnodechain_t public, so that a chain could
  be allocated on the stack by the caller.  I added two routines,
  dns_rbtnodechain_init() and dns_rbtnodechain_reset() to work with them.
  The latter routine frees any dynamically allocated memory associated with
  the chain.  The chain structure now contains the memory context.  I also
  moved get_ancestor_mem() and put_ancestor_mem() up in the file, so that
  inlining could be more effective.

  In the nits department, you wrote

                  *node = result == DNS_R_SUCCESS ? current : NULL;

  In the future, please write this instead (the patch has this change in it):

                  if (result == DNS_R_SUCCESS)
                          *node = current;
                  else
                          *node = NULL;