Go singleton as locks or sync.Once

singleton.go

package main

import (
	"log"
	"sync"
)

type MyStruct struct {
	field int
}

var onceCached *MyStruct

var lockCached = struct {
	sync.RWMutex
	val *MyStruct
}{}

func initMyStruct() MyStruct {
	return MyStruct{field: 1}
}

var once sync.Once

func getIntOnce(ident int) *MyStruct {
	log.Println("No lock", ident)

	once.Do(func() {
		log.Println("Initializing GetInt")
		x := initMyStruct()
		onceCached = &x
	})

	log.Println("No lock return", ident)

	return onceCached
}

func getIntLock(ident int) *MyStruct {
	lockCached.Lock()

	log.Println("lock", ident)
	if lockCached.val == nil {
		log.Println("Initializing GetInt")
		x := initMyStruct()
		lockCached.val = &x
	}

	lockCached.Unlock()
	log.Println("lock freed", ident)

	return lockCached.val
}

func goGetInt(getter func(int) *MyStruct) {
	var wg sync.WaitGroup
	wg.Add(2)

	log.Println(getter(0))

	go func() {
		defer wg.Done()
		log.Println(getter(1))
	}()

	go func() {
		defer wg.Done()
		log.Println(getter(2))
	}()

	wg.Wait()
}

func main() {
	goGetInt(getIntOnce)
	log.Println("******************")
	goGetInt(getIntLock)
}