How do you make sure goroutines finish in a for-loop using WaitGroup?

Issue

I’m running a function in a goroutine each time a for-loop iterates, and I’m using sync.WaitGroup to make sure the goroutines all finish. However, I’m getting weird behavior testing the concurrency with counters. In the example below, I attempt to keep track of the thread count using 4 different techniques (w, x, y, z), and get 4 different results. The only result I understand is x, since it is incremented in the for-loop itself. What am I missing here?

package main

import "fmt"
import "sync"

var w = 0

func main() {
  x := 0
  y := 0
  z := 0
  var wg sync.WaitGroup
  for i := 0; i < 10000; i++ {
    wg.Add(1)
    x++
    go func() {
      z++
      test(&y)
      wg.Done()
    }()
  }
  wg.Wait()
  fmt.Println(w, x, y, z) // 8947 10000 8831 8816
}

func test(y *int) {
  w++
  *y++
}

Solution

The sync.Waitgroup is working as expected. w, y and z will not reach 10000 because multiple goroutines are incrementing them concurrently, and Go’s increment is not concurrent-safe: it is implemented as a normal fetch-increment-reassign operation.

You have two options.

option 1: mutex

type incrementer struct {
    sync.Mutex
    i int
}

func (i *incrementer) Add(n int) {
    i.Lock()
    defer i.Unlock()
    i.i += n
}

and use this type for w, y and z.

Full example: https://play.golang.org/p/6wWUK2xnOCW

option 2: sync.atomic

var w int32 = 0

go func(){
    // in the loop
    atomic.AddInt32(&w, 1)

}()

Full example: https://play.golang.org/p/oUCGgKYC1-Y

Answered By – Pierre Prinetti

Answer Checked By – Jay B. (GoLangFix Admin)

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