如何从Go中的匿名函数访问全局变量？

Question

I have a code:

var test string

func main() {
    http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
        test = "index"
    })

    fmt.Println(test)

    if error := http.ListenAndServe(":9001", nil); error != nil {
        log.Fatal("Error!", error)
    }
}

How to change the value of the test variable in the anonymous function? I will be thankful!

Answer 1

The HTTP handler does change your global variable; however you don't protect access to the global variable and thus have a race condition. That is to say fmt.Println(test) runs before your http.HandleFunc.

I'm assuming this is a toy example: If you want to change your code to wait for the value to be changed / wait for an HTTP hit, before terminating, then you could do this:

var test string
var doneCh = make(chan bool, 1)

func main() {
    http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
        test = "index"
        select {
        case doneCh <- true:
        default:
        }
    })

    go func() {
        <-doneCh
        fmt.Println(test)
    }()

    if error := http.ListenAndServe(":9001", nil); error != nil {
        log.Fatal("Error!", error)
    }
}

This uses a channel to guard the state of test. In real code it would be more likely to use sync.Once or sync.Mutex. Also, I should mention (and I hope you already realize) mutating global state is always something to be avoided when possible.