Return NaN when summaries have no observations yet.

Also, properly handle decay upon Write().

This fixes https://github.com/prometheus/client_golang/issues/85 .

prometheus/summary.go

@@ -16,6 +16,7 @@ package prometheus
 import (
 	"fmt"
 	"hash/fnv"
+	"math"
 	"sort"
 	"sync"
 	"time"
@@ -277,10 +278,8 @@ func (s *summary) Write(out *dto.Metric) error {
 
 	s.bufMtx.Lock()
 	s.mtx.Lock()
-
-	if len(s.hotBuf) != 0 {
-		s.swapBufs(time.Now())
-	}
+	// Swap bufs even if hotBuf is empty to set new hotBufExpTime.
+	s.swapBufs(time.Now())
 	s.bufMtx.Unlock()
 
 	s.flushColdBuf()
@@ -288,9 +287,15 @@ func (s *summary) Write(out *dto.Metric) error {
 	sum.SampleSum = proto.Float64(s.sum)
 
 	for _, rank := range s.sortedObjectives {
+		var q float64
+		if s.headStream.Count() == 0 {
+			q = math.NaN()
+		} else {
+			q = s.headStream.Query(rank)
+		}
 		qs = append(qs, &dto.Quantile{
 			Quantile: proto.Float64(rank),
-			Value:    proto.Float64(s.headStream.Query(rank)),
+			Value:    proto.Float64(q),
 		})
 	}
 

prometheus/summary_test.go

@@ -289,6 +289,11 @@ func TestSummaryVecConcurrency(t *testing.T) {
 }
 
 func TestSummaryDecay(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping test in short mode.")
+		// More because it depends on timing than because it is particularly long...
+	}
+
 	sum := NewSummary(SummaryOpts{
 		Name:       "test_summary",
 		Help:       "helpless",
@@ -315,6 +320,12 @@ func TestSummaryDecay(t *testing.T) {
 		}
 	}
 	tick.Stop()
+	// Wait for MaxAge without observations and make sure quantiles are NaN.
+	time.Sleep(100 * time.Millisecond)
+	sum.Write(m)
+	if got := *m.Summary.Quantile[0].Value; !math.IsNaN(got) {
+		t.Errorf("got %f, want NaN after expiration", got)
+	}
 }
 
 func getBounds(vars []float64, q, ε float64) (min, max float64) {