src: clamp idle time to avoid underflow in metrics

Clamp idle time values before subtracting from loop duration to
avoid unsigned underflow when clock skew causes idle time to be
slightly larger than total loop time. This manifests on Windows
when the loop is nearly 100% idle.

Applied in two locations:
- uv_metrics_cb_: prevents bogus rolling_est_lag_ values
- get_event_loop_stats_: prevents bogus loop_utilization values

Fixes: nodesource/nsolid-private#15
PR-URL: #372
Reviewed-By: Juan José Arboleda <[email protected]>
Reviewed-By: Rafael Gonzaga <[email protected]>

Author: santigimeno

src/nsolid/nsolid_api.cc

@@ -611,6 +611,10 @@ void EnvInst::uv_metrics_cb_(nsuv::ns_prepare* handle, EnvInst* envinst) {
   // Use the previous values to calculate the values for this loop iteration.
   loop_duration = metrics_cb_time - envinst->prev_metrics_cb_time_;
   loop_idle_time = idle_time - envinst->prev_loop_idle_time_;
+  // Clamp loop_idle_time to avoid unsigned underflow in loop_proc_time when
+  // clock skew causes idle_time to be slightly larger than loop_duration.
+  if (loop_idle_time > loop_duration)
+    loop_idle_time = loop_duration;
   loop_proc_time = loop_duration - loop_idle_time;
   events_procd = metrics->events - envinst->prev_events_processed_;
   events_waiting = metrics->events_waiting - envinst->prev_events_waiting_;
@@ -2162,6 +2166,12 @@ void EnvInst::get_event_loop_stats_(EnvInst* envinst,
   uint64_t loop_idle_time = uv_metrics_idle_time(envinst->event_loop());
   uint64_t loop_diff = stor->current_hrtime_ - stor->prev_call_time_;
   uint64_t idle_diff = loop_idle_time - stor->prev_idle_time_;
+  // On some platforms the clocks backing loop_diff and idle_diff can differ by
+  // a few hundred nanoseconds when the loop is nearly 100% idle, making
+  // idle_diff slightly larger than loop_diff. Clamp to avoid unsigned
+  // underflow in proc_diff and bogus utilization values.
+  if (idle_diff > loop_diff)
+    idle_diff = loop_diff;
   uint64_t proc_diff = loop_diff - idle_diff;
   uint64_t count_diff = metrics->loop_count - stor->loop_iterations;
   auto entry_exit = envinst->provider_times();