The best practice of Singleton in Golang

Let's consider how to create a singleton? We need a reference can't be change by anyone at all the time except initialize. Is that possible in Go? Yes, package level reference.

var s *singleton

What's next? It have to be initialize before using. Basic implementation you can easier thought out is

func GetInstance() *singleton {
    if s == nil {
	s = &singleton{} // for clean, nothing at here
    }
    return s
}

Seems perfect, but buggy actually. Why? Because the check can facing the race condition when using a lot goroutine access our singleton. Now we know the problem, how to solve it? We have two choices. One is init God function, another is sync.Once let only do once all the time. Let's start from first solution.

func init() {
    s = &singleton{}
}

func GetInstance() *singleton {
    return s
}

Why this will work? Because init must be executed after anything trying to using anything in a package. So here s will be initialized before this package be using, and the race condition won't exist by Go's guarantee.

Second way is sync.Once.

var once sync.Once

func GetInstance() *singleton {
    once.Do(func() {
	s = &singleton{}
    })
    return s
}

Because sync.Once will do one time at all process. So it will be a safety way to initialize the singleton