fixed data race

metric.go

@@ -37,7 +37,7 @@ type Measurer interface {
 // and a counter.
 type Metric struct {
 	sync.Mutex
-	sample sample // The sample used to build a histogram
+	data   sample // The sample used to build a histogram
 	count  int32  // The number of samples observed
 	create int64  // The first updated time
 	name   string // The name of the metric
@@ -52,7 +52,7 @@ const (
 func NewMetric(name string) *Metric {
 	return &Metric{
 		name:   name,
-		sample: make([]int32, 0, reservoirSize),
+		data:   make([]int32, reservoirSize, reservoirSize),
 		create: time.Now().Unix(),
 	}
 }
@@ -62,8 +62,7 @@ func (m *Metric) Reset() {
 	m.Lock()
 	defer m.Unlock()
 
-	m.count = 0
-	m.sample = m.sample[:0]
+	atomic.StoreInt32(&m.count, 0)
 	m.create = time.Now().Unix()
 }
 
@@ -84,6 +83,15 @@ func (m *Metric) Window() (time.Time, time.Time) {
 	return time.Unix(m.create, 0), time.Now()
 }
 
+// sample returns the usable sample
+func (m *Metric) sample() sample {
+	count := m.count
+	if count > reservoirSize {
+		count = reservoirSize
+	}
+	return m.data[:count]
+}
+
 // Count returns the number of samples recorded, which may exceed the
 // reservoir size.
 func (m *Metric) Count() int {
@@ -97,38 +105,40 @@ func (m *Metric) Count() int {
 func (m *Metric) Max() int {
 	m.Lock()
 	defer m.Unlock()
-	return m.sample.Max()
+	return m.sample().Max()
 }
 
 // Mean returns the mean of the values in the sample.
 func (m *Metric) Mean() float64 {
 	m.Lock()
 	defer m.Unlock()
-	return m.sample.Mean()
+	return m.sample().Mean()
 }
 
 // Min returns the minimum value in the sample, which may not be the minimum
 // value ever to be part of the sample.
 func (m *Metric) Min() int {
 	m.Lock()
 	defer m.Unlock()
-	return m.sample.Min()
+	return m.sample().Min()
 }
 
 // Quantile returns a slice of arbitrary quantiles of the sample.
 func (m *Metric) Quantile(quantiles ...float64) []float64 {
 	m.Lock()
 	defer m.Unlock()
-	return m.sample.Quantile(quantiles...)
+	return m.sample().Quantile(quantiles...)
 }
 
 // Snapshot returns a read-only copy of the sample.
 func (m *Metric) Snapshot() *Snapshot {
 	m.Lock()
 	defer m.Unlock()
 
-	dest := make([]int32, len(m.sample))
-	copy(dest, m.sample)
+	// Snapshot the data
+	sample := m.sample()
+	dest := make([]int32, len(sample))
+	copy(dest, sample)
 	return &Snapshot{
 		Metric: m.name,
 		Label:  m.tag,
@@ -143,14 +153,14 @@ func (m *Metric) Snapshot() *Snapshot {
 func (m *Metric) StdDev() float64 {
 	m.Lock()
 	defer m.Unlock()
-	return m.sample.StdDev()
+	return m.sample().StdDev()
 }
 
 // Variance returns the variance of the values in the sample.
 func (m *Metric) Variance() float64 {
 	m.Lock()
 	defer m.Unlock()
-	return m.sample.Variance()
+	return m.sample().Variance()
 }
 
 // Rate returns a operation per second rate since the creation of the metric.
@@ -162,16 +172,16 @@ func (m *Metric) Rate() float64 {
 // Update samples a new value into the metric.
 func (m *Metric) Update(v int32) {
 	count := atomic.AddInt32(&m.count, 1)
-	if count < reservoirSize {
+	if count <= reservoirSize {
 		m.Lock()
-		m.sample = append(m.sample, v)
+		m.data[count-1] = v
 		m.Unlock()
 		return
 	}
 
-	if r := int(rand.Int31n(count)); r < (reservoirSize - 1) {
+	if r := int(rand.Int31n(count)); r < reservoirSize {
 		m.Lock()
-		m.sample[r] = v
+		m.data[r] = v
 		m.Unlock()
 		return
 	}

metric_test.go

@@ -17,6 +17,7 @@
 package stats
 
 import (
+	"sync"
 	"testing"
 	"time"
 
@@ -46,6 +47,29 @@ func BenchmarkMetricUpdate(b *testing.B) {
 	}
 }
 
+func TestMetricConcurrency(t *testing.T) {
+	h := NewMetric("x")
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+
+	go func() {
+		for i := int32(0); i < 50000; i++ {
+			h.Update(i)
+		}
+		wg.Done()
+	}()
+
+	go func() {
+		for i := int32(0); i < 50000; i++ {
+			h.Reset()
+		}
+		wg.Done()
+	}()
+
+	wg.Wait()
+}
+
 func TestMetric(t *testing.T) {
 	h := NewMetric("x")
 	for i := int32(0); i < 100; i++ {
@@ -74,3 +98,13 @@ func TestMetric(t *testing.T) {
 	assert.Equal(t, 0, h.Count())
 	assert.Equal(t, 0, h.Max())
 }
+
+func TestSampleClamp(t *testing.T) {
+	h := NewMetric("x")
+	for i := int32(0); i < 2000; i++ {
+		h.Update(i)
+	}
+
+	sample := h.sample()
+	assert.Equal(t, reservoirSize, len(sample))
+}