Tweak consensus method 17 based on arma's comments

Instead of capping whenever a router has fewer than 3 measurements,
we cap whenever a router has fewer than 3 measurements *AND* there
are at least 3 authorities publishing measured bandwidths.

We also generate bandwidth lines with a new "Unmeasured=1" flag,
meaning that we didn't have enough observations for a node to use
measured bandwidth values in the authority's input, whether we capped
it or not.

changes/bug2286

@@ -0,0 +1,5 @@
+  o Major features (directory authority):
+    - Directory authorities now support a new consensus method (17)
+      where they cap the published bandwidth of servers for which
+      insufficient bandwidth measurements exist. Fixes part of bug
+      2286.

src/or/dirvote.c

@@ -1638,6 +1638,7 @@ networkstatus_compute_consensus(smartlist_t *votes,
     int *named_flag; /* Index of the flag "Named" for votes[j] */
     int *unnamed_flag; /* Index of the flag "Unnamed" for votes[j] */
     int chosen_named_idx;
+    int n_authorities_measuring_bandwidth;
 
     strmap_t *name_to_id_map = strmap_new();
     char conflict[DIGEST_LEN];
@@ -1726,6 +1727,14 @@ networkstatus_compute_consensus(smartlist_t *votes,
       } SMARTLIST_FOREACH_END(v);
     }
 
+    /* We need to know how many votes measure bandwidth. */
+    n_authorities_measuring_bandwidth = 0;
+    SMARTLIST_FOREACH(votes, networkstatus_t *, v,
+       if (v->has_measured_bws) {
+         ++n_authorities_measuring_bandwidth;
+       }
+    );
+
     /* Now go through all the votes */
     flag_counts = tor_malloc(sizeof(int) * smartlist_len(flags));
     while (1) {
@@ -1889,14 +1898,17 @@ networkstatus_compute_consensus(smartlist_t *votes,
       /* Pick a bandwidth */
       if (consensus_method >= 6 && num_mbws > 2) {
         rs_out.has_bandwidth = 1;
+        rs_out.has_measured_bw = 1;
         rs_out.bandwidth = median_uint32(measured_bws, num_mbws);
       } else if (consensus_method >= 5 && num_bandwidths > 0) {
         rs_out.has_bandwidth = 1;
         rs_out.bandwidth = median_uint32(bandwidths, num_bandwidths);
-        if (consensus_method >= MIN_METHOD_TO_CLIP_UNMEASURED_BW) {
-          /* Cap non-measured bandwidths to 20k. */
-          if (rs_out.bandwidth > max_unmeasured_bw)
+        if (consensus_method >= MIN_METHOD_TO_CLIP_UNMEASURED_BW &&
+            n_authorities_measuring_bandwidth > 2) {
+          /* Cap non-measured bandwidths. */
+          if (rs_out.bandwidth > max_unmeasured_bw) {
             rs_out.bandwidth = max_unmeasured_bw;
+          }
         }
       }
 
@@ -2039,7 +2051,10 @@ networkstatus_compute_consensus(smartlist_t *votes,
       smartlist_add(chunks, tor_strdup("\n"));
       /*     Now the weight line. */
       if (rs_out.has_bandwidth) {
-        smartlist_add_asprintf(chunks, "w Bandwidth=%d\n", rs_out.bandwidth);
+        int unmeasured = ! rs_out.has_measured_bw &&
+          consensus_method >= MIN_METHOD_TO_CLIP_UNMEASURED_BW;
+        smartlist_add_asprintf(chunks, "w Bandwidth=%d%s\n", rs_out.bandwidth,
+                               unmeasured?" Unmeasured=1":"");
       }
 
       /*     Now the exitpolicy summary line. */

src/or/or.h

@@ -2383,6 +2383,9 @@ typedef enum {
 typedef struct networkstatus_t {
   ENUM_BF(networkstatus_type_t) type : 8; /**< Vote, consensus, or opinion? */
   ENUM_BF(consensus_flavor_t) flavor : 8; /**< If a consensus, what kind? */
+  unsigned int has_measured_bws : 1;/**< True iff this networkstatus contains
+                                     * measured= bandwidth values. */
+
   time_t published; /**< Vote only: Time when vote was written. */
   time_t valid_after; /**< Time after which this vote or consensus applies. */
   time_t fresh_until; /**< Time before which this is the most recent vote or

src/or/routerparse.c

@@ -1985,6 +1985,7 @@ routerstatus_parse_entry_from_string(memarea_t *area,
           goto err;
         }
         rs->has_measured_bw = 1;
+        vote->has_measured_bws = 1;
       }
     }
   }