func (p *Pinger) Err() error {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.ctx.err
}
func (p *Pinger) Err() error {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.ctx.err
}
func ( p * Pinger ) Err ( ) error { p . mu . Lock ( ) defer p . mu . Unlock ( ) return p . ctx . err }
// Err returns an error that is set by RunLoop(). It must be called after
// RunLoop(). If not, it causes panic.
Err returns an error that is set by RunLoop () . It must be called after RunLoop () . If not it causes panic .