Why Doesn\'t My Go sync.WaitGroup Ever Complete?

Golang sync.WaitGroup Never Completes

The provided code demonstrates concurrency by fetching a list of URLs and conditionally downloading and saving files to the file system. The main goroutine waits for all files to be fetched using a sync.WaitGroup. However, the program fails to exit even after completing all requests.

To understand the issue, consider the WaitGroup's functionality. It maintains a count of the executing goroutines. If the count ever reaches zero, the waitgroup returns immediately, indicating all goroutines have completed execution.

In the given code, the WaitGroup's Add method is used to increment the count, while the Done method is used to decrement it. If the Add method is called more times than the Done method, or vice versa, the WaitGroup will never reach zero, causing the main goroutine to wait indefinitely.

There are two errors in the provided code:

1. The downloadFromURL function receives a copy of the WaitGroup, not a pointer to it. This results in a copy of the WaitGroup's internal data being passed around, so any modifications made to it within the function (such as calling Done) are not visible to the main goroutine. To fix this issue, pass a pointer to the WaitGroup instead:

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

2. The defer wg.Done() statement is not placed as the first statement in the downloadFromURL function. If an error occurs or any other logic is executed before calling Done, the decrement operation will not be registered, and the WaitGroup's count will never reach zero. To fix this, ensure defer wg.Done() is the first statement in the function:

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

With these corrections in place, the WaitGroup will function correctly, decrementing its count as goroutines complete their tasks, ultimately leading to the main goroutine's successful exit.

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