HADOOP-18502. MutableStat should return 0 when there is no change (#5058)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/lib/MutableStat.java

@@ -140,14 +140,14 @@ public class MutableStat extends MutableMetric {
     if (all || changed()) {
       numSamples += intervalStat.numSamples();
       builder.addCounter(numInfo, numSamples)
-             .addGauge(avgInfo, lastStat().mean());
+             .addGauge(avgInfo, intervalStat.mean());
       if (extended) {
-        builder.addGauge(stdevInfo, lastStat().stddev())
-               .addGauge(iMinInfo, lastStat().min())
-               .addGauge(iMaxInfo, lastStat().max())
+        builder.addGauge(stdevInfo, intervalStat.stddev())
+               .addGauge(iMinInfo, intervalStat.min())
+               .addGauge(iMaxInfo, intervalStat.max())
                .addGauge(minInfo, minMax.min())
                .addGauge(maxInfo, minMax.max())
-               .addGauge(iNumInfo, lastStat().numSamples());
+               .addGauge(iNumInfo, intervalStat.numSamples());
       }
       if (changed()) {
         if (numSamples > 0) {

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/metrics2/lib/TestMutableMetrics.java

@@ -290,6 +290,27 @@ public class TestMutableMetrics {
     }
   }
 
+  /**
+   * MutableStat should output 0 instead of the previous state when there is no change.
+   */
+  @Test public void testMutableWithoutChanged() {
+    MetricsRecordBuilder builderWithChange = mockMetricsRecordBuilder();
+    MetricsRecordBuilder builderWithoutChange = mockMetricsRecordBuilder();
+    MetricsRegistry registry = new MetricsRegistry("test");
+    MutableStat stat = registry.newStat("Test", "Test", "Ops", "Val", true);
+    stat.add(1000, 1000);
+    stat.add(1000, 2000);
+    registry.snapshot(builderWithChange, true);
+
+    assertCounter("TestNumOps", 2000L, builderWithChange);
+    assertGauge("TestINumOps", 2000L, builderWithChange);
+    assertGauge("TestAvgVal", 1.5, builderWithChange);
+
+    registry.snapshot(builderWithoutChange, true);
+    assertGauge("TestINumOps", 0L, builderWithoutChange);
+    assertGauge("TestAvgVal", 0.0, builderWithoutChange);
+  }
+
   @Test
   public void testDuplicateMetrics() {
     MutableRatesWithAggregation rates = new MutableRatesWithAggregation();