一文解析 Golang sync.Once 用法及原理

// 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.
//
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
// 	config.once.Do(func() { config.init(filename) })
//
// Because no call to Do returns until the one call to f returns, if f causes
// Do to be called, it will deadlock.
//
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
//
func (o *Once) Do(f func()) 

func main() {
 var once sync.Once
 once.Do(func() {
    once.Do(func() {
       fmt.Println("hello kenmawr.")
    })
 })
}

var populateMapsOnce sync.Once
var entity           map[string]rune

func populateMaps() {
    entity = map[string]rune{
        "AElig;":                           '\U000000C6',
        "AMP;":                             '\U00000026',
        "Aacute;":                          '\U000000C1',
        "Abreve;":                          '\U00000102',
        "Acirc;":                           '\U000000C2',
        // 省略后续键值对
    }
}

func UnescapeString(s string) string {
    populateMapsOnce.Do(populateMaps)
    i := strings.IndexByte(s, '&')

    if i < 0 {
            return s
    }
    // 省略后续的实现
    ...
}

package main
import (
   "fmt"
   "sync"
)
type Singleton struct{}
var singleton *Singleton
var once sync.Once

func GetSingletonObj() *Singleton {
   once.Do(func() {
      fmt.Println("Create Obj")
      singleton = new(Singleton)
   })
   return singleton
}
func main() {
   var wg sync.WaitGroup
   for i := 0; i < 5; i++ {
      wg.Add(1)
      go func() {
         defer wg.Done()
         obj := GetSingletonObj()
         fmt.Printf("%p\n", obj)
      }()
   }
   wg.Wait()
}
/*--------- 输出 -----------
Create Obj
0x119f428
0x119f428
0x119f428
0x119f428
0x119f428
**/

type T int
type MyChannel struct {
   c    chan T
   once sync.Once
}
func (m *MyChannel) SafeClose() {
   // 保证只关闭一次channel
   m.once.Do(func() {
      close(m.c)
   })
}

package sync
import (
   "sync/atomic"
)
// 一个 Once 实例在使用之后不能被拷贝继续使用
type Once struct {
   done uint32 // done 表明了动作是否已经执行
   m    Mutex
}
func (o *Once) Do(f func()) {
    if atomic.LoadUint32(&o.done) == 0 {
      o.doSlow(f)
   }
}

func (o *Once) doSlow(f func()) {
   o.m.Lock()
   defer o.m.Unlock()
   if o.done == 0 {
      defer atomic.StoreUint32(&o.done, 1)
      f()
   }
}

type Once struct {
	// done indicates whether the action has been performed.
	// It is first in the struct because it is used in the hot path.
	// The hot path is inlined at every call site.
	// Placing done first allows more compact instructions on some architectures (amd64/386),
	// and fewer instructions (to calculate offset) on other architectures.
	done uint32
	m    Mutex
}

func (o *OnceA) Do(f func()) {
  if !atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    return
  }
  f()
}

func (o *Once) Do(f func()) {
    if atomic.LoadUint32(&o.done) == 0 {
      o.doSlow(f)
   }
}
func (o *Once) doSlow(f func()) {
   o.m.Lock()
   defer o.m.Unlock()
   if o.done == 0 {
      defer atomic.StoreUint32(&o.done, 1)
      f()
   }
}