errgroup.Groupの亜種(最初のエラーで他のgoroutineをキャンセルする)

main.go

package main

import (
	"context"
	"errors"
	"fmt"
	"os"
	"sync"
	"time"
)

// This is a variant of https://github.com/golang/sync/blob/master/errgroup/errgroup.go which
// cancels all the other goroutines on first error.

func main() {
	fmt.Println("Hello, playground")

	var errOnce sync.Once

	var cause error

	var wg sync.WaitGroup

	ch1 := make(chan string)
	ch2 := make(chan string)
	
	defer close(ch1)
	defer close(ch2)

	go func() {
		ch1 <- "to1: a"
		ch2 <- "to2: a"
		ch1 <- "to1: b"
		ch1 <- "err"
	}()

	ctx := context.Background()
	ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
	defer cancel()

	wg.Add(1)
	go func() {
		defer wg.Done()

		if err := procCh(ctx, "proc1", ch1); err != nil {
			errOnce.Do(func() {
				cause = err
				cancel()
			})
		}
	}()

	wg.Add(1)
	go func() {
		defer wg.Done()

		if err := procCh(ctx, "proc2", ch2); err != nil {
			errOnce.Do(func() {
				cause = err
				cancel()
			})
		}
	}()

	wg.Wait()
	if cause != nil {
		println("main", cause.Error())
		os.Exit(1)
	}
}

func procCh(ctx context.Context, text string, ch chan string) error {
	defer println(text, "stopping")

	for {
		select {
		case <-ctx.Done():
			return nil
		case t := <-ch:
			if t == "err" {
				println(text, "failed with", t)
				return errors.New(t)
			}
			println(text, "received", t)
		}
	}
}