Avoid NaN values causing problems in metrics output

metrics/metrics/src/main/java/io/helidon/metrics/ExponentiallyDecayingReservoir.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -185,6 +185,9 @@ private void rescale(long now, long next) {
                         final WeightedSnapshot.WeightedSample sample = values.remove(key);
                         final WeightedSnapshot.WeightedSample newSample = new WeightedSnapshot.WeightedSample(sample.getValue(),
                                                                                                               sample.getWeight() * scalingFactor);
+                        if (Double.compare(newSample.getWeight(), 0) == 0) {
+                            continue;
+                        }
                         values.put(key * scalingFactor, newSample);
                     }
                 }

metrics/metrics/src/main/java/io/helidon/metrics/HelidonTimer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -287,7 +287,7 @@ private static class TimerImpl implements Timer {
             this.histogram = HelidonHistogram.create(repoType, Metadata.builder()
                     .withName(name)
                     .withType(MetricType.HISTOGRAM)
-                    .build());
+                    .build(), clock);
             this.clock = clock;
         }
 

metrics/metrics/src/main/java/io/helidon/metrics/WeightedSnapshot.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -65,7 +65,11 @@ class WeightedSnapshot extends Snapshot {
 
         for (int i = 0; i < copy.length; i++) {
             this.values[i] = copy[i].value;
-            this.normWeights[i] = copy[i].weight / sumWeight;
+            /*
+             * A zero denominator could cause the resulting double to be infinity or, if the numerator is also 0,
+             * NaN. Either causes problems when formatting for JSON output. Just use 0 instead.
+             */
+            this.normWeights[i] = sumWeight != 0 ? copy[i].weight / sumWeight : 0;
         }
 
         for (int i = 1; i < copy.length; i++) {

metrics/metrics/src/test/java/io/helidon/metrics/HelidonHistogramTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020 Oracle and/or its affiliates.
+ * Copyright (c) 2018, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,7 @@
 import javax.json.JsonObject;
 import javax.json.JsonObjectBuilder;
 
+import org.eclipse.microprofile.metrics.Histogram;
 import org.eclipse.microprofile.metrics.Metadata;
 import org.eclipse.microprofile.metrics.MetricID;
 import org.eclipse.microprofile.metrics.MetricType;
@@ -43,6 +44,8 @@
 import static io.helidon.metrics.HelidonMetricsMatcher.withinTolerance;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.notNullValue;
 import static org.junit.jupiter.api.Assertions.assertAll;
 import static org.junit.jupiter.api.Assertions.fail;
@@ -227,6 +230,27 @@ void testStatisticalValues() {
         testSnapshot(10, "delegating longs", delegatingHistoLong.getSnapshot(), 506.3, 294.389);
     }
 
+    @Test
+    void testNaNAvoidance() {
+        Metadata metadata = Metadata.builder()
+                .withName("long_idle_test")
+                .withDisplayName("theDisplayName")
+                .withDescription("Simulates a long idle period")
+                .withType(MetricType.HISTOGRAM)
+                .withUnit(MetricUnits.SECONDS)
+                .build();
+        TestClock testClock = TestClock.create();
+        Histogram idleHistogram = HelidonHistogram.create("application", metadata, testClock);
+
+        idleHistogram.update(100);
+
+        for (int i = 1; i < 48; i++) {
+            testClock.add(1, TimeUnit.HOURS);
+            assertThat("Idle histogram failed after simulating " + i + " hours", idleHistogram.getSnapshot().getMean(),
+                    not(equalTo(Double.NaN)));
+        }
+    }
+
     private void testSnapshot(int factor, String description, Snapshot snapshot, double mean, double stddev) {
         assertAll("Testing statistical values for " + description,
                   () -> assertThat("median", snapshot.getMedian(), is(withinTolerance(factor * 48))),

metrics/metrics/src/test/java/io/helidon/metrics/HelidonTimerTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2020 Oracle and/or its affiliates.
+ * Copyright (c) 2018, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -221,4 +221,23 @@ void testPrometheus() {
                                                           + "/index.html\n"
                                                           + "application_response_time_seconds_count 200"));
     }
+
+    @Test
+    void testNaNAvoidance() {
+        TestClock testClock = TestClock.create();
+        HelidonTimer helidonTimer = HelidonTimer.create("application", meta, testClock);
+        MetricID metricID = new MetricID("idleTimer");
+
+        JsonObjectBuilder builder = MetricImpl.JSON.createObjectBuilder();
+        helidonTimer.update(1L, TimeUnit.MILLISECONDS);
+
+        for (int i = 1; i < 48; i++) {
+            testClock.add(1L, TimeUnit.HOURS);
+            try {
+                helidonTimer.jsonData(builder, metricID);
+            } catch (Throwable t) {
+                fail("Failed after simulating " + i + " hours");
+            }
+        }
+    }
 }