change arbiter to start loop when meter receives data, stop loop when no meters or metricsEnabled is false

metrics/init_test.go

@@ -1,5 +1,8 @@
 package metrics
 
+import "time"
+
 func init() {
 	metricsEnabled = true
+	MeterTickerInterval = 1 * time.Second
 }

metrics/meter.go

@@ -11,6 +11,8 @@ import (
 // new Meter.
 // Be sure to unregister the meter from the registry once it is of no use to
 // allow for garbage collection.
+var MeterTickerInterval = time.Second * 5
+
 func GetOrRegisterMeter(name string, r Registry) *Meter {
 	if r == nil {
 		r = DefaultRegistry
@@ -18,7 +20,7 @@ func GetOrRegisterMeter(name string, r Registry) *Meter {
 	return r.GetOrRegister(name, NewMeter).(*Meter)
 }
 
-// NewMeter constructs a new Meter and launches a goroutine.
+// NewMeter constructs a new Meter
 // Be sure to call Stop() once the meter is of no use to allow for garbage collection.
 func NewMeter() *Meter {
 	m := newMeter()
@@ -94,7 +96,15 @@ func (m *Meter) Stop() {
 }
 
 // Mark records the occurrence of n events.
+// It launches an arbiter goroutine to update meters if not started.
 func (m *Meter) Mark(n int64) {
+	if !metricsEnabled {
+		return
+	}
+	if !arbiter.started {
+		arbiter.started = true
+		go arbiter.loop()
+	}
 	m.uncounted.Add(n)
 }
 
@@ -130,7 +140,6 @@ var arbiter = meterTicker{meters: make(map[*Meter]struct{})}
 // meters are references in a set for future stopping.
 type meterTicker struct {
 	mu      sync.RWMutex
-
 	started bool
 	meters  map[*Meter]struct{}
 }
@@ -140,10 +149,6 @@ func (ma *meterTicker) add(m *Meter) {
 	ma.mu.Lock()
 	defer ma.mu.Unlock()
 	ma.meters[m] = struct{}{}
-	if !ma.started {
-		ma.started = true
-		go ma.loop()
-	}
 }
 
 // remove removes a meter from the set of ticked meters.
@@ -153,12 +158,13 @@ func (ma *meterTicker) remove(m *Meter) {
 	ma.mu.Unlock()
 }
 
-// loop ticks meters on a 5 second interval.
+// loop ticks meters on a configured interval.
 func (ma *meterTicker) loop() {
-	ticker := time.NewTicker(5 * time.Second)
+	ticker := time.NewTicker(MeterTickerInterval)
 	for range ticker.C {
-		if !metricsEnabled {
-			continue
+		if len(ma.meters) == 0 || !metricsEnabled {
+			ma.started = false
+			return
 		}
 		ma.mu.RLock()
 		for meter := range ma.meters {

metrics/meter_test.go

@@ -1,8 +1,11 @@
 package metrics
 
 import (
+	"runtime"
 	"testing"
 	"time"
+
+	"go.uber.org/goleak"
 )
 
 func BenchmarkMeter(b *testing.B) {
@@ -81,3 +84,27 @@ func TestMeterRepeat(t *testing.T) {
 		t.Errorf("m.Count(): 10100 != %v\n", count)
 	}
 }
+
+func TestMeterLazyInitialization(t *testing.T) {
+	defer goleak.VerifyNone(t)
+	// Removing all meters so that goroutine stops
+	for meter, _ := range arbiter.meters {
+		arbiter.remove(meter)
+	}
+	time.Sleep(MeterTickerInterval)
+	initialGoroutines := runtime.NumGoroutine()
+	m1 := NewMeter()
+	afterCreateGoroutines := runtime.NumGoroutine()
+	if afterCreateGoroutines != initialGoroutines {
+		t.Errorf("Expected no new goroutines after meter creation, got: before=%d, after=%d",
+			initialGoroutines, afterCreateGoroutines)
+	}
+	m1.Mark(1)
+	afterMarkGoroutines := runtime.NumGoroutine()
+	if afterMarkGoroutines != initialGoroutines+1 {
+		t.Errorf("Expected exactly one new goroutine after Mark(), got: before=%d, after=%d",
+			initialGoroutines, afterMarkGoroutines)
+	}
+	m1.Stop()
+	time.Sleep(MeterTickerInterval)
+}