Why Doesn\'t My Golang WaitGroup Ever Complete?

WaitGroup Never Completes in Golang

A recent inquiry on a popular coding platform raised concerns over a Golang program that fails to exit after completing a series of file downloads. The main goroutine was waiting for all file downloads to finish using a sync.WaitGroup, but the program never terminated.

To investigate the issue, it's crucial to understand potential causes. One possibility is that the number of goroutines registered with the WaitGroup became inflated due to excessive additions (Add()) or inadequate decrements (Done()). Another factor to consider is identifying the current routine count within the WaitGroup for debugging purposes.

Resolution:

Upon examination, two key issues were identified in the provided code:

1. Pointer Reference to WaitGroup: The downloadFromURL() function must receive a pointer to the WaitGroup (&wg) instead of a copy. This ensures that changes made to the WaitGroup are visible to the main goroutine.
2. Deferred Done() Placement: The defer wg.Done() statement should appear as one of the first actions in downloadFromURL(). This guarantees that even if the function terminates before reaching that statement, the Done() will still be executed, decrementing the WaitGroup.

By implementing these corrections, the code can properly track the completion of file downloads using a WaitGroup, and the main goroutine will successfully exit when all downloads are complete.

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