go - Why concurrent read-write has different results depending on default value?

Question

Why panic stop occurring if we replace global variable with true?

package main

import (
    "time"
)

// If here written false, the panic is expected some times.
// But if we write true here, then panic will never happen.
// Why? 
var value = false

func main() {
    go func() { for { value = true } }()

    time.Sleep(time.Second)

    for {
        if !value {
            panic("FALSE!")
        }
    }
}

question from:https://stackoverflow.com/questions/65908699/why-concurrent-read-write-has-different-results-depending-on-default-value

Answer 1

It is a race condition. In such situation you need to use WaitGroup. Using time sleep is dangerous as we cannot predict how go routines are executed by the scheduler. See the following code:

func main() {
    var wait sync.WaitGroup
    wait.Add(1);

    go func() {
        for {
            if !value {
                value = true
                wait.Done()
            }
        }
    }()

    wait.Wait()
    //time.Sleep(time.Second)

    for {
        if !value {
            panic("FALSE!")
        }
    }
}