c.mu.Lock()
	defer c.mu.Unlock()
	if c.err != nil {
		return
	}
	c.err = err
	for _, f := range c.afterFuncs {
		go f()
	}
	c.afterFuncs = nil
}

func TestCustomContextAfterFuncCancel(t *testing.T) {
	ctx0 := &afterFuncContext{}
	ctx1, cancel := context.WithCancel(ctx0)
	defer cancel()
	ctx0.cancel(context.Canceled)
	<-ctx1.Done()
}