szt-sketch opened a new issue #961:
URL: https://github.com/apache/servicecomb-service-center/issues/961
**Describe the bug**
gopool 实现不能复用协程, gopool 包下的goroutines.go如下代码实现导致问题：
``` go
func (g *Pool) Do(f func(context.Context)) *Pool {
defer log.Recover()
select {
case g.pending <- f: // block if workers are busy
case g.workers <- struct{}{}:
g.wg.Add(1)
go g.loop(f)
}
return g
}
```
select 多个分支条件满足时，是公平调度的，所以pending 的case语句不一定先执行，当前实现会面临50%的新创建协程。
实测协程复用率偏低。
提供一个参考写法：
```
func (g *Pool) Do(f func(context.Context)) *Pool {
select {
case g.pending <- f: // block if workers are busy
fmt.Printf("eureka!")
default:
select {
case g.pending <- f: // block if workers are busy
case g.workers <- struct{}{}:
g.wg.Add(1)
go g.loop(f)
}
}
return g
}
```
**To Reproduce**
复现demo：
```
func main() {
var ids sync.Map
for i := 0; i < 100; i++ {
Go(func(context context.Context) {
id := Goid(); // 获取携程唯一标识id，
ids.Store(id, struct {}{}) //用于统计携程创建梳理
fmt.Println(id, " msg: ", i)
})
time.Sleep(10 * time.Millisecond)
}
defaultGo.wg.Wait()
size := 0
ids.Range(func(_, _ interface{}) bool {
size +=1
return true
})
fmt.Println("len: ", size)
}
```
**Expected behavior**
协程能复用，不创建新协程
**Platform And Runtime (please complete the following information):**
环境无差别，稳定复现
**Additional context**
无
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