Example of non atomic ++

simple_increment_failure.go

package main

import (
	"fmt"
	"time"
	"sync"
)

var glob int64 = 0
var mu sync.Mutex

func main() {
	var wg sync.WaitGroup

	for i := 0; i < 1000; i++ {
		wg.Add(1)

		go func (wg *sync.WaitGroup) {
			time.Sleep(200 * time.Millisecond)
			for j := 0; j < 1000; j++ {
				// mu.Lock()
				glob++
				// mu.Unlock()
			}

			wg.Done()
		} (&wg)
	}


	wg.Wait()
	fmt.Println("I = " , glob)

}

We expect that I = 1 000 000, but it's not determined cause ++ is not atomic (i'll add a link to the post which describes why).
How to improve this:

1. First use mu.Lock() and mu.Unlock
2. Second: optimize it by incrementing local variable first and at the end add this local value to the global one (and don't forget mutex!)