侦听频道并进行循环处理时重写频道

Question

While trying few experiments in channels, I came up with below code:

var strChannel = make(chan string, 30)
var mutex = &sync.Mutex{}

func main() {

    go sampleRoutine()

    for i := 0; i < 10; i++ {
        mutex.Lock()
        strChannel <- strconv.FormatInt(int64(i), 10)
        mutex.Unlock()
        time.Sleep(1 * time.Second)
    }

    time.Sleep(10 * time.Second)
}

func sampleRoutine() {
/*  A: for msg := range strChannel{*/
/*  B: for {
        msg := <-strChannel*/
        log.Println("got message ", msg, strChannel)
        if msg == "3" {
            mutex.Lock()
            strChannel = make(chan string, 20)
            mutex.Unlock()
        }
    }
}

Basically here while listening to a given channel, I am assigning the channel variable to a new channel in a specific condition (here when msg == 3).

When I use the code in comment block B it works as expected, i.e. the loop moves on to the newly created channel and prints 4-10.

However comment block A which I believe is just a different way to write the loop doesn't work i.e. after printing "3" it stops.

Could someone please let me know the reason for this behavior?

And also is code like this where a routine listening on a channel, creating a new one safe?

Answer 1

In Go, for statement evaluate the value on the right side of range before the loop begins.

That means changing the value of the variable on the right side of range will take no effects. So in your code, in Variant A, msg is ever-iterating over the orignal channel and never changed. In Varaint B, it works as intended as the channel is being evaluated per iteration.

The concept is a little bit tricky. It does not mean you cannot modify items of the slice or map on the right side of range. If you look deeper into it, you will find that in Go, map and slice stores a pointer, and modify its item does not change that pointer, so it has effects.

It is even more trickier in case of array. Modifying item of an array on the right side of range has no effects. This is due to Go's mechanism about storing array as a value.

Playground examples: https://play.golang.org/p/wzPfGHFYrnv