Fix worker random race condition

statsd/worker.go

@@ -7,21 +7,23 @@ import (
 )
 
 type worker struct {
-	pool   *bufferPool
-	buffer *statsdBuffer
-	sender *sender
-	random *rand.Rand
+	pool       *bufferPool
+	buffer     *statsdBuffer
+	sender     *sender
+	random     *rand.Rand
+	randomLock sync.Mutex
 	sync.Mutex
 
 	inputMetrics chan metric
 	stop         chan struct{}
 }
 
 func newWorker(pool *bufferPool, sender *sender) *worker {
-	// Each worker uses its own random source to prevent workers in separate
-	// goroutines from contending for the lock on the "math/rand" package-global
-	// random source (e.g. calls like "rand.Float64()" must acquire a shared
-	// lock to get the next pseudorandom number).
+	// Each worker uses its own random source and random lock to prevent
+	// workers in separate goroutines from contending for the lock on the
+	// "math/rand" package-global random source (e.g. calls like
+	// "rand.Float64()" must acquire a shared lock to get the next
+	// pseudorandom number).
 	// Note that calling "time.Now().UnixNano()" repeatedly quickly may return
 	// very similar values. That's fine for seeding the worker-specific random
 	// source because we just need an evenly distributed stream of float values.
@@ -71,9 +73,15 @@ func (w *worker) processMetric(m metric) error {
 }
 
 func (w *worker) shouldSample(rate float64) bool {
+	// rand.NewSource is not thread safe.
+	// TODO: use defer once the lowest Go version we support is 1.14 (defer
+	// has an overhead before that).
+	w.randomLock.Lock()
 	if rate < 1 && w.random.Float64() > rate {
+		w.randomLock.Unlock()
 		return false
 	}
+	w.randomLock.Unlock()
 	return true
 }