etcdserver/api/rafthttp/urlpick.go
func (p *urlPicker) unreachable(u url.URL) {
	p.mu.Lock()
	defer p.mu.Unlock()
	if u == p.urls[p.picked] {
		p.picked = (p.picked + 1) % len(p.urls)
	}
}
func (p *urlPicker) unreachable(u url.URL) {
	p.mu.Lock()
	defer p.mu.Unlock()
	if u == p.urls[p.picked] {
		p.picked = (p.picked + 1) % len(p.urls)
	}
}
// unreachable notices the picker that the given url is unreachable,
// and it should use other possible urls.