io.MultiWriter example with cancelable context

MultiWriter.go

package main

import (
	"context"
	"fmt"
	"io"
	"math/rand"
	"os"
	"sync"
	"time"
)

var random *rand.Rand

func init() {
	random = rand.New(rand.NewSource(time.Now().UnixNano()))
}

func main() {
	ctx, cancel := context.WithCancel(context.Background())

	var wg sync.WaitGroup
	cerr := make(chan error)
	done := make(chan struct{})
	bye := make(chan struct{})
	go func() {
		defer close(bye)
		for {
			select {
			case err := <-cerr:
				if err != nil && err != context.Canceled {
					fmt.Println("err", err)
					cancel()
				}
			case <-done:
				return
			}
		}
	}()

	wg.Add(10)

	for i := 0; i < 5; i++ {
		i := i

		// async
		go func() {
			ra, wa := io.Pipe()
			rb, wb := io.Pipe()

			name := fmt.Sprintf("f%d", i)

			go func() { cerr <- send(ctx, name, ra); ra.Close(); wg.Done() }()
			go func() { cerr <- send(ctx, "bk/"+name, rb); rb.Close(); wg.Done() }()

			w := io.MultiWriter(wa, wb)
			_, _ = fmt.Fprintf(w, "%d", i)
			_, _ = fmt.Fprintf(w, "~%d", i)

			wa.Close()
			wb.Close()
		}()
	}

	wg.Wait()
	close(done)
	<-bye
}

// req, _ = http.NewRequest(url, contentType, r)
// req = req.WithContext(ctx)
// _, _ = http.DefaultClient.Do(req)
func send(ctx context.Context, name string, r io.Reader) error {
	fmt.Println("+", name)

	select {
	case <-ctx.Done():
		return ctx.Err()
	case <-time.After(time.Duration(rand.New(random).Intn(499))):
	}

	if name == "f2" {
		return fmt.Errorf("f2 failed")
	}

	f, err := os.Create(name)
	if err != nil {
		return err
	}
	defer f.Close()

	_, err = io.Copy(f, r)
	if err != nil {
		return err
	}

	return nil
}