golang中sync.Once只执行一次的原理解析

func TestOnce(t *testing.T) {
   once := sync.Once{}
   f1 := func() {
      fmt.Println("f1 func")
   }
   wg := sync.WaitGroup{}
   for i := 0; i < 10; i++ {
      wg.Add(1)
      go func() {
         defer wg.Done()
         once.Do(f1)
      }()
   }
   wg.Wait()
}

// Once is an object that will perform exactly one action.
//
// A Once must not be copied after first use.
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
}
// Once只对外提供一个Do方法
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.
//
// 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()) {
   // Note: Here is an incorrect implementation of Do:
   //
   // if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
   //    f()
   // }
   //
   // Do guarantees that when it returns, f has finished.
   // This implementation would not implement that guarantee:
   // given two simultaneous calls, the winner of the cas would
   // call f, and the second would return immediately, without
   // waiting for the first's call to f to complete.
   // This is why the slow path falls back to a mutex, and why
   // the atomic.StoreUint32 must be delayed until after f returns.
   if atomic.LoadUint32(&o.done) == 0 {
      // Outlined slow-path to allow inlining of the fast-path.
      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()
   }
}

func TestOnce(t *testing.T) {
   once := sync.Once{}
   f1 := func() {
      fmt.Println("f1 func")
   }
   f2 := func() {
      fmt.Println("f2 func")
   }
   // f1执行成功
   once.Do(f1)
   // f2不会执行
   once.Do(f2)
}

if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
   f()
}

package main
import (
   "fmt"
   "sync"
)
func main() {
   once := Once{}
   var wg sync.WaitGroup
   wg.Add(2)
   go func() {
      once.Do(print)
      wg.Done()
   }()
   go func() {
      once.Do(print)
      wg.Done()
   }()
   wg.Wait()
   fmt.Println("end")
}
func print() {
   fmt.Println("qqq")
}
type Once struct {
   done uint32
   m    sync.Mutex
}
func (o *Once) Do(f func()) {
   // 原来：atomic.LoadUint32(&o.done) == 0
   if o.done == 0 {
      o.doSlow(f)
   }
}
func (o *Once) doSlow(f func()) {
   o.m.Lock()
   defer o.m.Unlock()
   if o.done == 0 {
      // 原来：atomic.StoreUint32(&o.done, 1)
      defer func() {
         o.done = 1
      }()
      f()
   }
}

 go run -race main.go

qqq
==================
WARNING: DATA RACE
Write at 0x00c0000bc014 by goroutine 7:
  main.(*Once).doSlow.func1()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:44 +0x32
  runtime.deferreturn()
      /usr/local/go/src/runtime/panic.go:436 +0x32
  main.(*Once).Do()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:35 +0x52
  main.main.func1()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:13 +0x37
Previous read at 0x00c0000bc014 by goroutine 8:
  main.(*Once).Do()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:34 +0x3c
  main.main.func2()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:17 +0x37
Goroutine 7 (running) created at:
  main.main()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:12 +0x136
Goroutine 8 (running) created at:
  main.main()
      /Users/cr/Documents/golang/src/ahut.com/go/demo/main.go:16 +0x1da
==================
end
Found 1 data race(s)
exit status 66