go - channel 缓冲区比 Go 中的预期多取一个值

Question

我认为 Go 中的 channel 默认只保存 1 个值，除非指定缓冲区大小。我读到 here .但是当我运行这个时:

func main (){
    for i := range numGen(6) {
        log.Println("taking from channel", i)
    }
}

func numGen(num int) chan int {
    c := make(chan string)
    go func() {
        for i := 0; i < num; i++ {
            log.Println("passing to channel", i)
            c <- i
        }
        close(c)
    }
    return c
}

我的输出是:

2017/06/13 18:09:08 passing to channel 0
2017/06/13 18:09:08 passing to channel 1
2017/06/13 18:09:08 taking from channel 0
2017/06/13 18:09:08 taking from channel 1
2017/06/13 18:09:08 passing to channel 2
2017/06/13 18:09:08 passing to channel 3
2017/06/13 18:09:08 taking from channel 2
2017/06/13 18:09:08 taking from channel 3
2017/06/13 18:09:08 passing to channel 4
2017/06/13 18:09:08 passing to channel 5
2017/06/13 18:09:08 taking from channel 4
2017/06/13 18:09:08 taking from channel 5

这表明 channel 一次保存 2 个值。像这样指定缓冲区大小

c := make(chan int, 0)

什么都不做。有什么方法可以让它只包含 1，值，而不是 2？

Answer 1

which shows that the channel is holding 2 values at a time.

事实并非如此。代码是这样执行的:

1. ma​​in goroutine block 在 channel 上读取
2. 第二个 goroutine 写入 channel 并继续执行。
3. 第二个 goroutine 在第二次写入尝试时阻塞，因为没有人在读取
4. ma​​in goroutine 继续执行，打印读取的数字
5. ma​​in goroutine 读取另一个数字，因为有人正在写入它
6. ma​​in goroutine 打印读取次数并在下一次读取时阻塞
7. 第二个 goroutine 在 2 步继续执行。

没有缓冲区，只有并发。