When accessing a public var that's been modified inside a for loop, i cannot see a change

Issue

I have a public variable which we call Tpus, I modify this value within an infinite for loop, however i get 2 different results when i try to print Tpus in the for loop, and in a goroutine outside the for loop, What im looking to do is get the same result in the goroutine as i get in the for loop.

    var Tpus []string
    
    func TPU_Calc(destination string) {
        clusternodes := GetClusterNodes(destination)
        go func() {
            wg.Add(1)
            time.Sleep(2 * time.Second)
            for {
                time.Sleep(150 * time.Millisecond)
                //fmt.Printf("\n + %e", Tpus)
            }
        }()
        for {
            slot := GetSlot(destination)
            slotleaders := GetSlotLeaders(destination, strconv.FormatUint(slot+10, 10))
            for x := 0; x < 80; x++ {
                for z := 0; z < len(clusternodes.Result); z++ {
                    if slotleaders[x] == clusternodes.Result[z].Pubkey {
                        if len(Tpus) >= 2 {
                            X := RemoveIndex(Tpus, 0)
                            Tpus = append(X, clusternodes.Result[x].Tpu)
                            fmt.Printf("\n + %e", Tpus)
                        } else {
                            Tpus = append(Tpus, clusternodes.Result[x].Tpu)
                        }
                    }
                }
            }
        }
    }

Result From Code Above:

  • [%!e(string=136.144.49.213:8004) %!e(string=198.55.56.164:8004)]
  • [%!e(string=198.55.56.164:8004) %!e(string=13.124.174.97:8003)]
  • [%!e(string=13.124.174.97:8003) %!e(string=185.16.38.73:8003)]
  • [%!e(string=185.16.38.73:8003) %!e(string=103.28.52.250:8003)]
  • [%!e(string=103.28.52.250:8003) %!e(string=18.214.103.198:8004)]
  • [%!e(string=18.214.103.198:8004) %!e(string=185.188.42.43:9003)]
  • [%!e(string=185.188.42.43:9003) %!e(string=135.181.115.253:8003)]
    var Tpus []string
    
    func TPU_Calc(destination string) {
        clusternodes := GetClusterNodes(destination)
        go func() {
            wg.Add(1)
            time.Sleep(2 * time.Second)
            for {
                time.Sleep(150 * time.Millisecond)
                fmt.Printf("\n + %e", Tpus)
            }
        }()
        for {
            slot := GetSlot(destination)
            slotleaders := GetSlotLeaders(destination, strconv.FormatUint(slot+10, 10))
            for x := 0; x < 80; x++ {
                for z := 0; z < len(clusternodes.Result); z++ {
                    if slotleaders[x] == clusternodes.Result[z].Pubkey {
                        if len(Tpus) >= 2 {
                            X := RemoveIndex(Tpus, 0)
                            Tpus = append(X, clusternodes.Result[x].Tpu)
                            //fmt.Printf("\n + %e", Tpus)
                        } else {
                            Tpus = append(Tpus, clusternodes.Result[x].Tpu)
                        }
                    }
                }
            }
        }
    }

Result From Code Above:

  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]
  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]
  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]
  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]
  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]
  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]
  • [%!e(string=142.132.248.119:8003) %!e(string=3.231.94.117:8004)]

Solution

var globalvar []string

func main() {
    var mu sync.Mutex

    go func() {
        for {
            // Block access to a global variable, so that
            // no one can change it outside the goroutine.
            // If it's already locked outside the goroutine,
            // then wait for unlocking.
            mu.Lock()

            // Some actions with a global variable...
            fmt.Printf("%v\n", globalvar)

            // Unlocking access to a global variable
            mu.Unlock()

            // Some code...
        }
    }()

    for i := 0; i < 255; i++ {
        // Block access to a global variable.
        // If it's already locked inside the goroutine,
        // then wait for unlocking.
        mu.Lock()

        // Some actions with a global variable
        globalvar = append(globalvar, "Str #"+strconv.Itoa(i))

        // Unlock access
        mu.Unlock()

        // Some code...
    }
}

Also you can define a special structure with mutex, value and methods for change it value. Something like this:


type TpusContainer struct {
    mu    sync.Mutex
    value []string
}

func (t *TpusContainer) RemoveIndex(i int) {
    t.mu.Lock()
    defer t.mu.Unlock()
    t.value = RemoveIndex(t.value, i)
}

func (t *TpusContainer) Append(elem string) {
    t.mu.Lock()
    defer t.mu.Unlock()
    t.value = append(t.value, elem)
}

func (t *TpusContainer) String() string {
    t.mu.Lock()
    defer t.mu.Unlock()
    return fmt.Sprintf("%v", t.value)
}

var Tpus TpusContainer

func main() {
    go func() {
        for {
            fmt.Printf("%v\n", Tpus)
        }
    }()

    for i := 0; i < 255; i++ {
        Tpus.RemoveIndex(0)
        Tpus.Append("Some string #"+strconv.Itoa(i))
    }
}

Personally, I prefer the second approach.

Answered By – Ivan Pirog

Answer Checked By – Jay B. (GoLangFix Admin)

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