errgroup — Fan-Out with Cancellation

// go get golang.org/x/sync/errgroup
package main

import (
    "context"
    "fmt"
    "net/http"

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

func fetchAll(ctx context.Context, urls []string) error {
    g, ctx := errgroup.WithContext(ctx)        // ctx cancels when first error happens
    g.SetLimit(8)                              // optional: cap concurrency

    for _, url := range urls {
        url := url                             // capture per iteration
        g.Go(func() error {
            req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
            resp, err := http.DefaultClient.Do(req)
            if err != nil { return fmt.Errorf("get %s: %w", url, err) }
            resp.Body.Close()
            if resp.StatusCode >= 400 {
                return fmt.Errorf("get %s: status %d", url, resp.StatusCode)
            }
            fmt.Println("OK", url)
            return nil
        })
    }
    return g.Wait()                            // returns first error or nil
}

func main() {
    err := fetchAll(context.Background(), []string{
        "https://example.com",
        "https://golang.org",
    })
    if err != nil { fmt.Println("failed:", err) }
}