Is it possible to call WaitGroup.Add inside a goroutine, and Done at the end? Will this work correctly?

Calling WaitGroup.Add inside a goroutine is possible but requires caution. The main rule is that the number of calls to Add should be known and completed before calling Wait.

If you call Add inside a goroutine that has not yet started or begun work, and Wait has already been called in the main thread, there is a risk that Wait will start waiting before the counter has been increased, and will finish prematurely — leading to synchronization errors.

Correct approach:

• Call Add before starting goroutines to ensure the counter is set.
• Inside goroutines, only call Done.

Example of correct usage:

var wg sync.WaitGroup
wg.Add(3) // pre-increment the counter

for i := 0; i < 3; i++ {
    go func() {
        defer wg.Done()
        // goroutine work
    }()
}

wg.Wait()

If Add is called inside a goroutine, Wait may be called before the counter is increased, which can lead to panic or incorrect waiting.

In summary: it is not recommended to call Add inside a goroutine if Wait might already be called.