[master] fix updateRTT for fast responses

- updateRTT now accepts 0 (and will change it internally to 1 as it needs a value >0 for division)
- improved check for negative values due to clock changes in the caller

src/lib/nsas/nameserver_entry.cc

@@ -174,6 +174,9 @@ NameserverEntry::updateAddressRTTAtIndex(uint32_t rtt, size_t index,
     uint32_t old_rtt = addresses_[family][index].getRTT();
     uint32_t new_rtt = (uint32_t)(old_rtt * UPDATE_RTT_ALPHA + rtt *
         (1 - UPDATE_RTT_ALPHA));
+    if (new_rtt == 0) {
+        new_rtt = 1;
+    }
     addresses_[family][index].setRTT(new_rtt);
 }
 

src/lib/resolve/recursive_query.cc

@@ -608,18 +608,19 @@ public:
             // Update the NSAS with the time it took
             struct timeval cur_time;
             gettimeofday(&cur_time, NULL);
-            uint32_t rtt;
-            if (cur_time.tv_sec >= current_ns_qsent_time.tv_sec ||
-                cur_time.tv_usec > current_ns_qsent_time.tv_usec) {
+            uint32_t rtt = 0;
+
+            // Only calculate RTT if it is positive
+            if (cur_time.tv_sec > current_ns_qsent_time.tv_sec ||
+                (cur_time.tv_sec == current_ns_qsent_time.tv_sec &&
+                 cur_time.tv_usec > current_ns_qsent_time.tv_usec)) {
                 rtt = 1000 * (cur_time.tv_sec - current_ns_qsent_time.tv_sec);
                 rtt += (cur_time.tv_usec - current_ns_qsent_time.tv_usec) / 1000;
-            } else {
-                rtt = 1;
             }
 
             dlog("RTT: " + boost::lexical_cast<std::string>(rtt));
             current_ns_address.updateRTT(rtt);
-            
+
             try {
                 Message incoming(Message::PARSE);
                 InputBuffer ibuf(buffer_->getData(), buffer_->getLength());