task package: replace waitgroup with channel, fix stuck

internal/task/debug.go

@@ -0,0 +1,26 @@
+package task
+
+import "strings"
+
+// debug only.
+func (t *Task) listChildren() []string {
+	var children []string
+	allTasks.Range(func(child *Task) bool {
+		if child.parent == t {
+			children = append(children, strings.TrimPrefix(child.name, t.name+"."))
+		}
+		return true
+	})
+	return children
+}
+
+// debug only.
+func (t *Task) listCallbacks() []string {
+	var callbacks []string
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	for c := range t.callbacks {
+		callbacks = append(callbacks, c.about)
+	}
+	return callbacks
+}