Document and make more conservative the logic for picking the size of the...

  int caches = directory_caches_dir_info(get_options());

  const networkstatus_t *consensus = networkstatus_get_latest_consensus();

  const smartlist_t *networkstatus_v2_list = networkstatus_get_v2_list();

  int n_expected_retain = 0;

  trusted_dirs_remove_old_certs();

  // routerlist_assert_ok(routerlist);

  n_expected_retain = smartlist_len(consensus->routerstatus_list);

  if (caches &&

      networkstatus_v2_list && smartlist_len(networkstatus_v2_list)) {

  /* We need to guess how many router descriptors we will wind up wanting to

     retain, so that we can be sure to allocate a large enough Bloom filter

     to hold the digest set.  Overestimating is fine; underestimating is bad.

  */

  {

    /* We'll probably retain everything in the consensus. */

    int n_max_retain = smartlist_len(consensus->routerstatus_list);

    if (caches && networkstatus_v2_list) {

      /* If we care about v2 statuses, we'll retain at most as many as are

         listed any of the v2 statues.  This will be at least the length of

         the largest v2 networstatus, and in the worst case, this set will be

         equal to the sum of the lengths of all v2 consensuses.  Take the

         worst case.

      */

      SMARTLIST_FOREACH(networkstatus_v2_list, networkstatus_v2_t *, ns,

                      n_expected_retain += smartlist_len(ns->entries));

    /* DOCDOC XXX021 too much magic. */

    n_expected_retain /= (smartlist_len(networkstatus_v2_list)/2+1);

                        n_max_retain += smartlist_len(ns->entries));

    }

    retain = digestset_new(n_max_retain);

  }

  //log_notice(LD_DIR,"n_expected_retain=%d",n_expected_retain);

  retain = digestset_new(n_expected_retain);

  cutoff = now - OLD_ROUTER_DESC_MAX_AGE;

  /* Build a list of all the descriptors that _anybody_ lists. */