Add raft_voter_contact()

This returns the number of voting nodes that are recently in contact
with the leader, to allow determining if the cluster is currently in a
degraded / at risk state.

include/raft.h

@@ -783,7 +783,9 @@ struct raft
             raft_index round_index;         /* Target of the current round. */
             raft_time round_start;          /* Start of current round. */
             void *requests[2];              /* Outstanding client requests. */
-            uint64_t reserved[8];           /* Future use */
+            uint32_t voter_contacts;        /* Current number of voting nodes we are in contact with */
+            uint32_t reserved2;             /* Future use */
+            uint64_t reserved[7];           /* Future use */
         } leader_state;
     };
 
@@ -981,6 +983,18 @@ RAFT_API raft_index raft_last_index(struct raft *r);
  */
 RAFT_API raft_index raft_last_applied(struct raft *r);
 
+/**
+ * Return the number of servers that the leader has recently been in contact
+ * with.
+ *
+ * Valid values will always be >= 1, because a leader is always in contact with
+ * itself.
+ * Returns 0 on a leader that has not yet had chance to calculate a valid
+ * value.
+ * Returns -1 if called on a follower.
+ */
+RAFT_API int raft_voter_contacts(struct raft *r);
+
 /**
  * Common fields across client request types.
  * `req_id`, `client_id` and `unique_id` are currently unused.

src/raft.c

@@ -191,6 +191,15 @@ const char *raft_errmsg(struct raft *r)
     return r->errmsg;
 }
 
+int raft_voter_contacts(struct raft *r)
+{
+    if(r->state == RAFT_LEADER){
+        return (int)r->leader_state.voter_contacts;
+    }else{
+        return -1;
+    }
+}
+
 int raft_bootstrap(struct raft *r, const struct raft_configuration *conf)
 {
     int rv;

src/tick.c

@@ -107,6 +107,7 @@ static bool checkContactQuorum(struct raft *r)
             contacts++;
         }
     }
+    r->leader_state.voter_contacts = contacts;
 
     return contacts > configurationVoterCount(&r->configuration) / 2;
 }