Add goroutine limit to the Group

group.go

@@ -24,6 +24,8 @@ type Group struct {
 	ctx    context.Context
 	cancel func()
 
+	withoutGoroutines bool
+
 	// Task runner and a heap of tasks to be run.
 	wg    sync.WaitGroup
 	mu    sync.Mutex
@@ -34,9 +36,19 @@ type Group struct {
 	lenC chan int
 }
 
+type GroupOption func(*Group)
+
+// WithoutGoroutines is the option for handling tasks
+// which makes Group not spawn a new goroutine for each task.
+func WithoutGoroutines() GroupOption {
+	return func(g *Group) {
+		g.withoutGoroutines = true
+	}
+}
+
 // New creates a new Group which will use ctx for cancelation. If cancelation
 // is not a concern, use context.Background().
-func New(ctx context.Context) *Group {
+func New(ctx context.Context, opts ...GroupOption) *Group {
 	// Monitor goroutine context and cancelation.
 	mctx, cancel := context.WithCancel(ctx)
 
@@ -50,6 +62,10 @@ func New(ctx context.Context) *Group {
 		lenC: make(chan int),
 	}
 
+	for _, opt := range opts {
+		opt(g)
+	}
+
 	g.wg.Add(1)
 	go func() {
 		defer g.wg.Done()
@@ -210,11 +226,18 @@ func (g *Group) trigger(now time.Time) time.Time {
 		next := &g.tasks[0]
 		if next.Deadline.After(now) {
 			// Earliest scheduled task is not ready.
+
 			return next.Deadline
 		}
 
 		// This task is ready, pop it from the heap and run it.
 		t := heap.Pop(&g.tasks).(task)
+
+		if g.withoutGoroutines {
+			t.Call()
+			continue
+		}
+
 		g.wg.Add(1)
 		go func() {
 			defer g.wg.Done()