Why Is My Go sync.WaitGroup Never Completing?

Debugging "golang sync.WaitGroup never completes" Issue

In concurrent Go programs, sync.WaitGroup ensures that the main goroutine waits for other goroutines to finish executing. However, in some cases, the main goroutine may hang indefinitely, indicating that WaitGroup is not functioning as intended.

One reason for this issue is improper usage of WaitGroup. In the code below, the error is that the downloadFromURL function is passed a copy of the WaitGroup, not a pointer:

func main() {
    ...
    go downloadFromURL(url, wg)
    ...
}

This prevents the Done method from signaling the WaitGroup in the main goroutine. To fix this, pass a pointer:

func main() {
    ...
    go downloadFromURL(url, &wg)
    ...
}

Another error is that the Done method is not called early enough in the downloadFromURL function. If an error occurs and the function returns before Done is called, the WaitGroup will not register the completion. Place Done as one of the first statements:

func downloadFromURL(url string, wg *sync.WaitGroup) error {
    defer wg.Done()
    ...
}

By ensuring that WaitGroup is used correctly, you can prevent deadlocks and ensure that the program exits as expected.

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