What is the difference between using wg.Add inside a loop and before the loop in the context of synchronization in Go?

In Go, wg.Add increments the WaitGroup counter. The difference between calling wg.Add inside a loop and before the loop:

• Calling wg.Add before the loop: the counter is increased once for the total number of goroutines that will be launched. This is safe and recommended, as the counter is set beforehand.

wg.Add(3) // increase the counter beforehand
for i := 0; i < 3; i++ {
    go func() {
        defer wg.Done()
        // work
    }()
}
wg.Wait()

• Calling wg.Add inside the loop: the counter is increased in each iteration, which also works but can lead to race conditions if goroutines are launched asynchronously and call Done before Add has been called for all.

for i := 0; i < 3; i++ {
    wg.Add(1) // increase the counter in the loop
    go func() {
        defer wg.Done()
        // work
    }()
}
wg.Wait()

Recommendation: it is better to call wg.Add before launching goroutines to avoid potential synchronization issues.