}

func (c *cancelCtx) Err() error {
	c.mu.Lock()
	err := c.err
	c.mu.Unlock()
	return err
}

// propagateCancel arranges for child to be canceled when parent is.
// It sets the parent context of cancelCtx.
func (c *cancelCtx) propagateCancel(parent Context, child canceler) {
	c.Context = parent

	done := parent.Done()
	if done == nil {
		return // parent is never canceled
	}