io.go

package main

import (
	"fmt"
	"io"
	"sync"
)

func bidirectCopy(left io.ReadWriteCloser, right io.ReadWriteCloser) (int, int, error) {
	var (
		n1   = 0
		n2   = 0
		err  error
		err1 error
		err2 error
		wg   sync.WaitGroup
	)

	wg.Add(2)

	go func() {
		if n, err := io.Copy(right, left); err != nil {
			err1 = err
		} else {
			n1 = int(n)
		}

		right.Close()
		wg.Done()
	}()

	go func() {
		if n, err := io.Copy(left, right); err != nil {
			err2 = err
		} else {
			n2 = int(n)
		}

		left.Close()
		wg.Done()
	}()

	wg.Wait()

	if err1 != nil && err2 != nil {
		err = fmt.Errorf("both copier failed; left: %s; right: %s", err1, err2)
	} else {
		if err1 != nil {
			err = err1
		} else if err2 != nil {
			err = err2
		}
	}

	return n1, n2, err
}