1 files changed, 86 insertions, 0 deletions
diff --git a/src/runtime/metrics_test.go b/src/runtime/metrics_test.go
index 5d32ef469c..4bd1408dbe 100644
--- a/src/runtime/metrics_test.go
+++ b/src/runtime/metrics_test.go
@@ -9,6 +9,7 @@ import (
 	"runtime/metrics"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 	"unsafe"
@@ -319,3 +320,88 @@ func BenchmarkReadMetricsLatency(b *testing.B) {
 	b.ReportMetric(float64(latencies[len(latencies)*90/100]), "p90-ns")
 	b.ReportMetric(float64(latencies[len(latencies)*99/100]), "p99-ns")
 }
+
+var readMetricsSink [1024]interface{}
+
+func TestReadMetricsCumulative(t *testing.T) {
+	// Set up the set of metrics marked cumulative.
+	descs := metrics.All()
+	var samples [2][]metrics.Sample
+	samples[0] = make([]metrics.Sample, len(descs))
+	samples[1] = make([]metrics.Sample, len(descs))
+	total := 0
+	for i := range samples[0] {
+		if !descs[i].Cumulative {
+			continue
+		}
+		samples[0][total].Name = descs[i].Name
+		total++
+	}
+	samples[0] = samples[0][:total]
+	samples[1] = samples[1][:total]
+	copy(samples[1], samples[0])
+
+	// Start some noise in the background.
+	var wg sync.WaitGroup
+	wg.Add(1)
+	done := make(chan struct{})
+	go func() {
+		defer wg.Done()
+		for {
+			// Add more things here that could influence metrics.
+			for i := 0; i < len(readMetricsSink); i++ {
+				readMetricsSink[i] = make([]byte, 1024)
+				select {
+				case <-done:
+					return
+				default:
+				}
+			}
+			runtime.GC()
+		}
+	}()
+
+	sum := func(us []uint64) uint64 {
+		total := uint64(0)
+		for _, u := range us {
+			total += u
+		}
+		return total
+	}
+
+	// Populate the first generation.
+	metrics.Read(samples[0])
+
+	// Check to make sure that these metrics only grow monotonically.
+	for gen := 1; gen < 10; gen++ {
+		metrics.Read(samples[gen%2])
+		for i := range samples[gen%2] {
+			name := samples[gen%2][i].Name
+			vNew, vOld := samples[gen%2][i].Value, samples[1-(gen%2)][i].Value
+
+			switch vNew.Kind() {
+			case metrics.KindUint64:
+				new := vNew.Uint64()
+				old := vOld.Uint64()
+				if new < old {
+					t.Errorf("%s decreased: %d < %d", name, new, old)
+				}
+			case metrics.KindFloat64:
+				new := vNew.Float64()
+				old := vOld.Float64()
+				if new < old {
+					t.Errorf("%s decreased: %f < %f", name, new, old)
+				}
+			case metrics.KindFloat64Histogram:
+				new := sum(vNew.Float64Histogram().Counts)
+				old := sum(vOld.Float64Histogram().Counts)
+				if new < old {
+					t.Errorf("%s counts decreased: %d < %d", name, new, old)
+				}
+			}
+		}
+	}
+	close(done)
+
+	wg.Wait()
+}