Demo errgroup.WithContext - one failed goroutine cancels the context that other goroutines are select'ing from

errgroup_demo.go

package main

import (
	"context"
	"fmt"
	"os"
	"time"

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

func main() {
	const numSuccess = 10

	g, ctx := errgroup.WithContext(context.Background())

	// Spin up some number of goroutines which will succeed.
	// Then spin up one that will fail and *should* cancel the others via the context.
	for i := 0; i < numSuccess; i++ {
		j := i
		g.Go(func() error {
			return succeed(ctx, j, 10*time.Second)
		})
	}
	g.Go(func() error {
		return fail(ctx, 1*time.Second)
	})
	if err := g.Wait(); err == nil {
		fmt.Fprintln(os.Stderr, "expected an error, got nil")
		os.Exit(1)
	}
}

func fail(ctx context.Context, delay time.Duration) error {
	timeout := time.After(delay)

	select {
	case <-ctx.Done():
		fmt.Println("context in goroutine canceled")
	case <-timeout:
		fmt.Println("failed goroutine timed out")
	}
	return fmt.Errorf("failed after %s", delay)
}

func succeed(ctx context.Context, i int, delay time.Duration) error {
	timeout := time.After(delay)

	select {
	case <-ctx.Done():
		fmt.Printf("context in goroutine %d canceled\n", i)
		return nil
	case <-timeout:
		fmt.Printf("goroutine %d timed out\n", i)
		return nil
	}
	return nil
}