singleflight.Do 中 shared 参数始终为 true 的原因是什么？

package main

import (
    "fmt"
    "sync"
    "time"

    "golang.org/x/sync/singleflight"
)

func main() {
    g := singleflight.group{}

    wg := sync.waitgroup{}

    for i := 0; i < 8; i++ {
        wg.add(1)
        go func(j int) {
            defer wg.done()
            val, err, shared := g.do("userid", querymysql)
            if err != nil {
                fmt.println(err)
                return
            }
            fmt.printf("index: %d, val: %d, shared: %t\n", j, val, shared)
        }(i)
    }

    wg.wait()

}

// 模拟数据库查询方法
func querymysql() (interface{}, error) {
    time.sleep(time.millisecond * 1)
    fmt.println("mysql query")
    return 1, nil
}

index: 0, val: 1, shared: true
index: 1, val: 1, shared: true
index: 2, val: 1, shared: true
index: 3, val: 1, shared: true
index: 4, val: 1, shared: true
index: 5, val: 1, shared: true
index: 6, val: 1, shared: true
index: 7, val: 1, shared: true