errgroup for parallel tasks with error propagation and cancellation

sync.WaitGroup does not handle errors or cancellation elegantly. When one parallel task fails, you often want to cancel all others and return the first error.

Use golang.org/x/sync/errgroup:

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

func fetchAll(ctx context.Context, urls []string) ([]Result, error) {
    g, ctx := errgroup.WithContext(ctx)
    results := make([]Result, len(urls))

    for i, url := range urls {
        i, url := i, url // capture
        g.Go(func() error {
            r, err := fetch(ctx, url) // ctx cancelled when any task fails
            if err != nil {
                return err
            }
            results[i] = r
            return nil
        })
    }

    if err := g.Wait(); err != nil {
        return nil, err
    }
    return results, nil
}

Limit concurrency:

g.SetLimit(10) // at most 10 goroutines at a time

errgroup.WithContext creates a derived context that is cancelled when any goroutine returns a non-nil error. g.Wait returns the first non-nil error after all goroutines complete.