fix off-by-one error causing Infinity in 100% EIL

Author: brendankenny

lighthouse-core/lib/traces/tracing-processor.js

@@ -165,7 +165,7 @@ class TraceProcessor {
       // Loop over durations, calculating a CDF value for each until it is above
       // the target percentile.
       const percentileTime = percentile * totalTime;
-      while (cdfTime < percentileTime && durationIndex < durations.length) {
+      while (cdfTime < percentileTime && durationIndex < durations.length - 1) {
         completedTime += duration;
         remainingCount -= (duration < 0 ? -1 : 1);
 

lighthouse-core/test/lib/traces/tracing-processor.js

@@ -140,5 +140,17 @@ describe('TracingProcessor lib', () => {
           [16, 16, 16, 31, 46, 55, 56]);
       assert.deepEqual(results, expected);
     });
+
+    it('does not divide by zero when duration sum is less than whole', () => {
+      // Durations chosen such that, due to floating point error:
+      //   const idleTime = totalTime - (duration1 + duration2);
+      //   (idleTime + duration1 + duration2) < totalTime
+      const duration1 = 67 / 107;
+      const duration2 = 67 / 53;
+      const totalTime = 10;
+      const results = TracingProcessor._riskPercentiles([duration1, duration2], totalTime, [1], 0);
+      const expected = createRiskPercentiles([1], [16 + duration2]);
+      assert.deepEqual(results, expected);
+    });
   });
 });