Go http server and global variables

Question

I have a http server. It is written with Go. I have this code:

package main
import (
    "net/http"
    "runtime"
)
var cur = 0
func handler(w http.ResponseWriter, r *http.Request) {
    cur = cur + 1;
}
func main() {
    runtime.GOMAXPROCS(runtime.NumCPU())
    http.HandleFunc("/", handler)
    http.ListenAndServe(":9010", nil)
}

Is it safe? May be I need to use a mutex?

Answer 1

No it is not safe, yes you will need locking of some form. Each connection is handled in its own goroutine. See the Serve() implementation for details.

A general pattern is to use a goroutine which checks for a channel and accepts changes via a channel:

var counterInput = make(chan int)

func handler(w http.ResponseWriter, r *http.Request) {
    counterInput <- 1
}

func counter(c <- chan int) {
    cur := 0
    for v := range c {
        cur += v
    }
}

func main() {
    go counter(counterInput)
    // setup http
}

Related: Is "net/http"'s use of global variables considered a good practice in golang?.

Answer 2

Unless I'm overlooking something, in this case, instead of using a lock (or a channel), you could use the tools found in the sync/atomic package (though you'll need to make your type either int32 or int64)

The documentation itself recommends you otherwise, though.

These functions require great care to be used correctly. Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package. Share memory by communicating; don't communicate by sharing memory.