Account for ntp time adjustment in wdt stats

Summary:
The wdt progress reporting path checks that time spent is non-negative for a given step but the stats collection part doesn't account for it.

The source of the problem might be some other path or issue but checking for time underflow is safe to add in any case.

Differential Revision: D15602928

fbshipit-source-id: 85f283fbd7d197713f14ec955d2b6501ef8c8af7

util/CommonImpl.h

@@ -104,7 +104,10 @@ class PerfStatCollector {
   ~PerfStatCollector() {
     if (threadCtx_.getOptions().enable_perf_stat_collection) {
       int64_t duration = durationMicros(Clock::now() - startTime_);
-      threadCtx_.getPerfReport().addPerfStat(statType_, duration);
+      if (duration >= 0) {
+        // If time goes back due to clock adjustment, ignore the sample.
+        threadCtx_.getPerfReport().addPerfStat(statType_, duration);
+      }
     }
   }