Add percentiles to the desc stats reporting

To get a better idea what's going on on Tonga, add some code to report
how often the most and least frequently fetched descriptor was fetched,
as well as 25, 50, 75 percentile.

Also ensure we only count bridge descriptors here.

changes/ticket4294

@@ -0,0 +1,8 @@
+(NOTE: THIS SHOULD REPLACE THE TICKET 4200 ENTRY IN THE CHANGELOG)
+  o Minor features:
+    - The Bridge Authority now writes statistics on how many bridge
+      descriptors it gave out in total, and how many unique descriptors it
+      gave out. It also lists how often the most and least commonly fetched
+      descriptors were given out, as well as the median and 25/75 percentile.
+      Implements tickets 4200 and 4294.
+

src/or/dirserv.c

@@ -3488,6 +3488,8 @@ connection_dirserv_add_servers_to_outbuf(dir_connection_t *conn)
                conn->dir_spool_src == DIR_SPOOL_EXTRA_BY_DIGEST);
   time_t publish_cutoff = time(NULL)-ROUTER_MAX_AGE_TO_PUBLISH;
 
+  const or_options_t *options = get_options();
+
   while (smartlist_len(conn->fingerprint_stack) &&
          connection_get_outbuf_len(TO_CONN(conn)) < DIRSERV_BUFFER_MIN) {
     const char *body;
@@ -3509,7 +3511,16 @@ connection_dirserv_add_servers_to_outbuf(dir_connection_t *conn)
        * unknown bridge descriptor has shown up between then and now. */
       continue;
     }
+
+    /** If we are the bridge authority and the descriptor is a bridge
+     * descriptor, remember that we served this descriptor for desc stats. */
+    if (options->BridgeAuthoritativeDir && by_fp) {
+      const routerinfo_t *router =
+          router_get_by_id_digest(sd->identity_digest);
+      tor_assert(router);
+      if (router->purpose == ROUTER_PURPOSE_BRIDGE)
         rep_hist_note_desc_served(sd->identity_digest);
+    }
     body = signed_descriptor_get_body(sd);
     if (conn->zlib_state) {
       /* XXXX022 This 'last' business should actually happen on the last

src/or/rephist.c

@@ -2643,7 +2643,7 @@ rep_hist_reset_desc_stats(time_t now)
 void
 rep_hist_desc_stats_term(void)
 {
-  digestmap_free(served_descs, NULL);
+  digestmap_free(served_descs, _tor_free);
   served_descs = NULL;
   start_of_served_descs_stats_interval = 0;
   total_descriptor_downloads = 0;
@@ -2659,18 +2659,43 @@ rep_hist_format_desc_stats(time_t now)
   char t[ISO_TIME_LEN+1];
   char *result;
 
+  digestmap_iter_t *iter;
+  const char *key;
+  void *val;
+  unsigned size;
+  int *vals;
+  int n = 0, *count;
+
   if (!start_of_served_descs_stats_interval)
     return NULL;
 
+  size =  digestmap_size(served_descs);
+  vals = tor_malloc(size * sizeof(int));
+
+  for (iter = digestmap_iter_init(served_descs); !digestmap_iter_done(iter);
+      iter = digestmap_iter_next(served_descs, iter) ) {
+    digestmap_iter_get(iter, &key, &val);
+    count = val;
+    vals[n++] = *count;
+      (void)key;
+  }
+
   format_iso_time(t, now);
 
   tor_asprintf(&result,
-               "served-descs-stats-end %s (%d s) total=%lu unique=%u\n",
+               "served-descs-stats-end %s (%d s) total=%lu unique=%u "
+               "max=%d q3=%d md=%d q1=%d min=%d\n",
                t,
                (unsigned) (now - start_of_served_descs_stats_interval),
                total_descriptor_downloads,
-               digestmap_size(served_descs));
+               size,
+               find_nth_int(vals, size, size-1),
+               find_nth_int(vals, size, (3*size-1)/4),
+               find_nth_int(vals, size, (size-1)/2),
+               find_nth_int(vals, size, (size-1)/4),
+               find_nth_int(vals, size, 0));
 
+  tor_free(vals);
   return result;
 }
 
@@ -2712,9 +2737,17 @@ rep_hist_desc_stats_write(time_t now)
 void
 rep_hist_note_desc_served(const char * desc)
 {
+  int *val;
   if (!served_descs)
     return; // We're not collecting stats
-  digestmap_set(served_descs, desc, (void *)1);
+  val = digestmap_get(served_descs, desc);
+  if (!val) {
+    val = tor_malloc(sizeof(int));
+    *val = 1;
+    digestmap_set(served_descs, desc, val);
+  } else {
+    (*val)++;
+  }
   total_descriptor_downloads++;
 }