sync.Once的用处
sync.Once 主要用途执行只需要执行一次的初始化函数，比如验证器的初始，http.Client初始化，都可以派上大用场。
来个直观的例子
下面的例子比较贴近实战代码，你提供了一个包给别人使用，这个包里面有初始化函数。他对你提供的初始化函数和方法二次封装。只暴露一个函数出来，这时候就可以使用sync.Once包下初始化函数。
package main
import (
"fmt"
"sync"
)
type Client struct {
url string
}
func New() *Client {
return &Client{}
}
func (c *Client) Look(id int) {
fmt.Printf("url is %d\n", id)
}
var (
defaultClient *Client
once sync.Once
)
func look(id int) {
once.Do(func() {
defaultClient = New()
fmt.Printf("init ok\n")
})
defaultClient.Look(id)
}
func main() {
for i := 0; i < 5; i++ {
look(i)
}
}
sync.Once数据结构
done uint32标记位。如果用户的函数初始化过，标记位置为1。
m Mutext 锁的作用，是锁住回调函数的执行过程，第一个函数在资源没准备好时，第二个函数过来就要乖乖地等待。
// Once is an object that will perform exactly one action.
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/x86),
// and fewer instructions (to calculate offset) on other architectures.
done uint32
m Mutex
}
实现
-
if atomic.LoadUint32(&o.done) == 0，是if o.Done == 0的线程安全写法，如果 o.done等于0才会进下来的逻辑，初始化过自然就不会进的。atomic.LoadUint32是原子变量取值函数，其实atomic下面包是汇编指令的抽象实现，不同平台会生成该平台对应机器指令，。 -
defer atomic.StoreUint32(&o.done, 1)是o.done = 1线程安全写法。当然取值的时候也是要用atomic.LoadUint32取值才有效果的，如果写的时候用atomic.StoreUint32，取值直接用if o.done == 1等于没用。
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()
}
}
注意(错误的实现)
这段是注释里面特别说明的，很多cas狂魔闭眼就能写出如下代码。这段代码其实有问题，
回调函数还没有初始化成功，这时第二个调用过来立马返回继续执行，资源没准备好就往下跑，panic少不了。
if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
f()
}
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