通过sync.Once实例来保证一个Once实例的Do只会执行一次，无论Do里的func有多个或者一个，利用这个特性来实现设计模式里的单例模式，golang里没有类这个东西，所以拿结构体来测试:
type singleton struct{}
var ins *singleton
var once sync.Once
func GetIns() *singleton {
once.Do(func(){
ins = &singleton{}
//ins = new(singleton)
})
return ins
}
sync.Once doc:
type Once
Once is an object that will perform exactly one action.
type Once struct {
// contains filtered or unexported fields
}
func (*Once) Do
func (o *Once) Do(f func())
Do calls the function f if and only if Do is being called for the first time for this instance of Once. In other words, given
var once Once
if once.Do(f) is called multiple times, only the first call will invoke f, even if f has a different value in each invocation. A new instance of Once is required for each function to execute.
