WaitGroup.Add Called Inside Goroutine

Why This Happens

sync.WaitGroup.Add must be called before the goroutine starts, not inside it. If Add runs concurrently with Wait, there is a race: Wait might return before Add increments the counter, causing the program to exit before all goroutines finish.

The Problem

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 5; i++ {
        go func() {
            wg.Add(1) // race with wg.Wait()
            defer wg.Done()
            doWork()
        }()
    }
    wg.Wait()
}

The Fix

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 5; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            doWork()
        }()
    }
    wg.Wait()
}

Step-by-Step Fix

1. 1

   Identify the misplaced Add

   Find wg.Add calls that are inside the goroutine function instead of before the go statement.

2. 2

   Move Add before go

   Place wg.Add(1) immediately before the go statement that launches the goroutine.

3. 3

   Verify Done matches Add

   Ensure every goroutine calls wg.Done() exactly once, usually with defer.