lib: Fix Close method for empty Metrics

Fixes #461

lib/metrics.go

@@ -88,7 +88,12 @@ func (m *Metrics) Add(r *Result) {
 // Close implements the Close method of the Report interface by computing
 // derived summary metrics which don't need to be run on every Add call.
 func (m *Metrics) Close() {
+	if m.Requests == 0 {
+		return
+	}
+
 	m.init()
+
 	m.Rate = float64(m.Requests)
 	m.Throughput = float64(m.success)
 	m.Duration = m.Latest.Sub(m.Earliest)

lib/metrics_test.go

@@ -1,6 +1,7 @@
 package vegeta
 
 import (
+	"io/ioutil"
 	"math/rand"
 	"reflect"
 	"testing"
@@ -98,6 +99,18 @@ func TestMetrics_NonNilErrorsOnClose(t *testing.T) {
 	}
 }
 
+// https://github.com/tsenart/vegeta/issues/461
+func TestMetrics_EmptyMetricsCanBeReported(t *testing.T) {
+	t.Parallel()
+
+	var m Metrics
+	m.Close()
+
+	reporter := NewJSONReporter(&m)
+	if err := reporter(ioutil.Discard); err != nil {
+		t.Error(err)
+	}
+}
 func BenchmarkMetrics(b *testing.B) {
 	b.StopTimer()
 	b.ResetTimer()