Simple Race Condition in Go HTTP Handler - Is this really a race condition?

Question

Given the code below, I'm trying to understand WHY the Go race detector ( go run -race example.go ) does not complain about a race condition.

var count int

func main() {
    http.HandleFunc("/a/", func(w http.ResponseWriter, r *http.Request) {
        count++ 
        fmt.Println(count)
    })

    http.HandleFunc("/b/", func(w http.ResponseWriter, r *http.Request) {
        count++
        fmt.Println(count)
    })

    log.Fatal(http.ListenAndServe(":8080", nil))
}

It's my understanding that the Go HTTP Server responds to all requests in a separate goroutine. With that in mind, would not the increments made to the global count variable by the handler functions happen in a goroutine that is separate from the main goroutine and thus, constitute a data race?

If this is not a data race, I'd very much like to know why.

Answer 1

This is a data race, the race detector however does not report races that don't happen. You need to make sure that there are concurrent calls in your test, and ensuring GOMAXPROCS>1 can help flush them out as well.

Answer 2

That's a race condition. False negatives can happen with the race checker.

The race checker is dynamic: rather than checking the source for problems, it only can only see if read and a write actually occur with no sync operation in between. There's no synchronizing operation in your code, but if one occurs in net/http between the increments, it'll be fooled. Its author suggests , essentially, running concurrent stress tests to shake out problems:

• write good concurrent tests
• have continuous build with race detector
• run integration tests
• run race-enabled canaries in production

In Go 1.4 and below, you should also make sure your program runs on more than one core with, eg, runtime.GOMAXPROCS(runtime.NumCPU()) . In Go 1.5, to be released around the end of 2015, GOMAXPROCS will run your code on all available cores by default.

Answer 3

count++ is a data race. It does not happen atomically. It is the same as:

count = count + 1

If the race detector doesn't see it, you probably are not hitting the server hard enough.