fix(cpuUsageThrottle): Always queue a new timeout (#1484)

lib/plugins/cpuUsageThrottle.js

@@ -105,6 +105,7 @@ function cpuUsageThrottle (opts) {
     // use to gather CPU load averages, this allows us to cancel the timeout
     // when shutting down restify.
     self._timeout = null;
+
     // self._timeoutDelta represents the amount of time between when we _should_
     // have run updateReject and the actual time it was invoked. This allows
     // us to monitor lag caused by both the event loop and pidusage.stat
@@ -115,6 +116,10 @@ function cpuUsageThrottle (opts) {
     // usage between two invocations of updateReject.
     function updateReject() {
         pidusage.stat(process.pid, function (e, stat) {
+            // Requeue an updateReject irrespective of whether or not pidusage
+            // encountered an error
+            self._timeout = setTimeout(updateReject, self._interval);
+
             // If we were unable to get cpu usage, don't make any new decisions.
             if (!stat ||
                 typeof stat.cpu !== 'number' ||
@@ -132,7 +137,10 @@ function cpuUsageThrottle (opts) {
             // negative and we will never shed load
             self._reject =
                 (self._cpu - self._limit) / (self._max - self._limit);
-            self._timeout = setTimeout(updateReject, self._interval);
+
+            // Calculate how long it took between when our interval should have
+            // updated the _reject value and how long it actually took. This
+            // metric accounts for the misbehaviour of pidusage.stat
             var now = Date.now();
             self._timeoutDelta = now - self._timeoutStart;
             self._timeoutStart = now;