通过 select 进行超时限制

func TestSelectTimeLimiter(t *testing.T) {
    var res string

    done := make(chan struct{}, 1)

    go func() {
        time.Sleep(500 * time.Millisecond)
        res = "ok"
        done <- struct{}{}
    }()

    select {
    case <-done:
    case <-time.After(300 * time.Millisecond):
        res = "time out"
    //default: default 的出现会导致永远无法进入第二个 case
        //    println("default")
    }

    println(res)
}

// After waits for the duration to elapse and then sends the current time
// on the returned channel.
// It is equivalent to NewTimer(d).C.
// The underlying Timer is not recovered by the garbage collector
// until the timer fires. If efficiency is a concern, use NewTimer
// instead and call Timer.Stop if the timer is no longer needed.
func After(d Duration) <-chan Time {
    return NewTimer(d).C
}