🐛 Fix race conditions

errgroup.go

@@ -28,15 +28,15 @@ type Group struct {
 
 	wg sync.WaitGroup
 
-	sem chan token
+	sem atomic.Value
 
 	errOnce sync.Once
 	err     atomic.Value
 }
 
 func (g *Group) done() {
-	if g.sem != nil {
-		<-g.sem
+	if sem := g.sema(); sem != nil {
+		<-sem
 	}
 	g.wg.Done()
 }
@@ -51,6 +51,20 @@ func WithContext(ctx context.Context) (*Group, context.Context) {
 	return &Group{cancel: cancel}, ctx
 }
 
+type semContainer struct{ sem chan token }
+
+func (g *Group) sema() chan token {
+	v := g.sem.Load()
+	if v == nil {
+		return nil
+	}
+	return v.(semContainer).sem
+}
+
+func (g *Group) setSema(ch chan token) {
+	g.sem.Store(semContainer{ch})
+}
+
 func (g *Group) error() error {
 	v := g.err.Load()
 	if v == nil {
@@ -94,8 +108,8 @@ func (g *Group) Go(f func() error) {
 	if g.error() != nil {
 		return
 	}
-	if g.sem != nil {
-		g.sem <- token{}
+	if sem := g.sema(); sem != nil {
+		sem <- token{}
 	}
 
 	g.wg.Add(1)
@@ -126,9 +140,9 @@ func (g *Group) TryGo(f func() error) bool {
 	if g.error() != nil {
 		return false
 	}
-	if g.sem != nil {
+	if sem := g.sema(); sem != nil {
 		select {
-		case g.sem <- token{}:
+		case sem <- token{}:
 			// Note: this allows barging iff channels in general allow barging.
 		default:
 			return false
@@ -159,12 +173,13 @@ func (egerr *ErrgroupLimitError) Error() string {
 // The limit must not be modified while any goroutines in the group are active.
 func (g *Group) SetLimit(n int) {
 	if n < 0 {
-		g.sem = nil
+		g.setSema(nil)
 		return
 	}
-	if len(g.sem) != 0 {
-		var err error = &ErrgroupLimitError{len(g.sem)}
+
+	if sem := g.sema(); sem != nil && len(sem) != 0 {
+		var err error = &ErrgroupLimitError{len(sem)}
 		panic(err)
 	}
-	g.sem = make(chan token, n)
+	g.setSema(make(chan token, n))
 }