Drill 6: Trade-off — Goroutines and channels

func FetchAll(ctx context.Context, urls []string) ([]string, error) {
    results := make(chan string, len(urls))
    var group sync.WaitGroup

    for _, url := range urls {
        group.Add(1)
        go func(url string) {
            defer group.Done()
            body, err := fetch(ctx, url)
            if err == nil {
                results <- body
            }
        }(url)
    }

    group.Wait()
    close(results)

    var bodies []string
    for body := range results {
        bodies = append(bodies, body)
    }
    return bodies, nil
}