Go语言如何判断一个chan被关闭

package main

import "fmt"

func main() {
    c := make(chan int, 5)
    c <- 123
    close(c)
    
    fmt.Println(<-c)
    fmt.Println(<-c)
}

123
0

package main

import "fmt"

func main() {
    c := make(chan int, 10)
    c <- 123
    close(c)

    var res int
    var ok bool

    res, ok = <-c
    fmt.Println(res, ok)

    res, ok = <-c
    fmt.Println(res, ok) //此时ok为false
}

123 true
0 false