How does golang.org/x/sync/errgroup simplify concurrent error handling?

import "golang.org/x/sync/errgroup"

// Basic errgroup — collect first error
func fetchAll(urls []string) error {
    g := new(errgroup.Group)
    for _, url := range urls {
        url := url // capture for goroutine (pre-Go 1.22)
        g.Go(func() error {
            resp, err := http.Get(url)
            if err != nil { return err }
            defer resp.Body.Close()
            return nil
        })
    }
    return g.Wait() // waits for all; returns first non-nil error
}

// errgroup with context — cancel all on first error
func fetchWithCancel(ctx context.Context, urls []string) error {
    g, gctx := errgroup.WithContext(ctx)
    // gctx is cancelled when any goroutine returns an error

    for _, url := range urls {
        url := url
        g.Go(func() error {
            req, _ := http.NewRequestWithContext(gctx, "GET", url, nil)
            resp, err := http.DefaultClient.Do(req)
            if err != nil { return err }
            defer resp.Body.Close()
            return nil
        })
    }
    return g.Wait()
}

// Limit concurrency (Go 1.20+)
g.SetLimit(10) // max 10 goroutines — g.Go blocks when at limit
for _, url := range urls {
    url := url
    g.Go(func() error { return fetch(url) })
}