sync/waitgroup.go

@@ -7,8 +7,8 @@ import (
 )
 
 type WaitGroup struct {
+	mu sync.Mutex
 	wg *sync.WaitGroup
-	drain *atomic.Bool
 	c  *atomic.Int64
 }
 
@@ -16,7 +16,6 @@ func WrapWaitGroup(wg *sync.WaitGroup) *WaitGroup {
 	g := &WaitGroup{
 		wg: wg,
 		c:  &(atomic.Int64{}),
-		drain: &(atomic.Bool{}),
 	}
 	return g
 }
@@ -26,22 +25,22 @@ func NewWaitGroup() *WaitGroup {
 	g := &WaitGroup{
 		wg: &wg,
 		c:  &(atomic.Int64{}),
-		drain: &(atomic.Bool{}),
 	}
 	return g
 }
 
 func (g *WaitGroup) Add(n int) {
+	g.mu.Lock()
 	g.c.Add(int64(n))
 	g.wg.Add(n)
+	g.mu.Unlock()
 }
 
 func (g *WaitGroup) Done() {
-	if g.drain.Load() {
+	g.mu.Lock()
-		return
-	}
 	g.c.Add(int64(-1))
-	g.wg.Done()
+	g.wg.Add(-1)
+	g.mu.Unlock()
 }
 
 func (g *WaitGroup) Wait() {
@@ -57,11 +56,17 @@ func (g *WaitGroup) WaitContext(ctx context.Context) {
 
 	select {
 	case <-ctx.Done():
-		g.drain.Store(true)
+		g.mu.Lock()
 		g.wg.Add(-int(g.c.Load()))
-		g.drain.Store(false)
+		<-done
+		g.wg.Add(int(g.c.Load()))
+		g.mu.Unlock()
 		return
 	case <-done:
 		return
 	}
 }
+
+func (g *WaitGroup) Waiters() int {
+	return int(g.c.Load())
+}

sync/waitgroup_test.go

@@ -6,7 +6,7 @@ import (
 	"time"
 )
 
-func TestWaitGroup(t *testing.T) {
+func TestWaitGroupContext(t *testing.T) {
 	wg := NewWaitGroup()
 	_ = t
 	wg.Add(1)
@@ -14,3 +14,24 @@ func TestWaitGroup(t *testing.T) {
 	defer cancel()
 	wg.WaitContext(ctx)
 }
+
+func TestWaitGroupReuse(t *testing.T) {
+	wg := NewWaitGroup()
+	defer func() {
+		if wg.Waiters() != 0 {
+			t.Fatal("lost goroutines")
+		}
+	}()
+
+	wg.Add(1)
+	defer wg.Done()
+	ctx, cancel := context.WithTimeout(context.TODO(), 1*time.Second)
+	defer cancel()
+	wg.WaitContext(ctx)
+
+	wg.Add(1)
+	defer wg.Done()
+	ctx, cancel = context.WithTimeout(context.TODO(), 1*time.Second)
+	defer cancel()
+	wg.WaitContext(ctx)
+}