go语言中channel疑惑

Question

1. 《Go memory model》中的定义：A receive from an unbuffered channel happens before the send on that channel completes.
   这句话怎么理解？

2. 为何channel关闭之后case：<-ch:……分支一直执行

func main() {
    ch := make(chan string)
    go sendData(ch)
    go recvData(ch)
    time.Sleep(2*1e9)
}

func sendData(ch chan  string)  {
    time.Sleep(1e9)
    close(ch)
}

func recvData(ch chan string)  {
    for {
        select {
        case <-ch:
            fmt.Println("Closed")
        default:
            fmt.Println("ing")
            time.Sleep(1e8)
        }
    }
}

而使用

func recvData(ch chan string)  {
    for {
        select {
        case <-ch:
            fmt.Println("Closed")
            select{}
        default:
            fmt.Println("ing")
            time.Sleep(1e8)
        }
    }
}

则case：<-ch:……分支只会执行一次。

Answer 1

http://dave.cheney.net/tag/golang-3

A closed channel never blocks

http://dave.cheney.net/2014/03/19/channel-axioms

A receive from a closed channel returns the zero value immediately

Answer 2

case <-ch:
    fmt.Println("Closed")
    select{}

收到ch关闭事件，select{}执行这句的时候，会阻塞协成,知道主进程执行完退出。