Add test to expose #275

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/histogram_test.go

@@ -17,6 +17,7 @@ import (
 	"math"
 	"math/rand"
 	"reflect"
+	"runtime"
 	"sort"
 	"sync"
 	"testing"
@@ -346,3 +347,41 @@ func TestBuckets(t *testing.T) {
 		t.Errorf("linear buckets: got %v, want %v", got, want)
 	}
 }
+
+func TestHistogramAtomicObserve(t *testing.T) {
+	var (
+		quit = make(chan struct{})
+		his  = NewHistogram(HistogramOpts{
+			Buckets: []float64{0.5, 10, 20},
+		})
+	)
+
+	defer func() { close(quit) }()
+
+	go func() {
+		for {
+			select {
+			case <-quit:
+				return
+			default:
+				his.Observe(1)
+			}
+		}
+	}()
+
+	for i := 0; i < 100; i++ {
+		m := &dto.Metric{}
+		if err := his.Write(m); err != nil {
+			t.Fatal("unexpected error writing histogram:", err)
+		}
+		h := m.GetHistogram()
+		if h.GetSampleCount() != uint64(h.GetSampleSum()) ||
+			h.GetSampleCount() != h.GetBucket()[1].GetCumulativeCount() {
+			t.Fatalf(
+				"inconsistent counts in histogram: count=%d sum=%f bucket=%d",
+				h.GetSampleCount(), h.GetSampleSum(), h.GetBucket()[1].GetCumulativeCount(),
+			)
+		}
+		runtime.Gosched()
+	}
+}