Race on asynch close

main.go

package main

import (
	"bytes"
	"errors"
	"io"
	"io/ioutil"
	"os"
)

func CancellableCopy(r io.ReadCloser, w io.Writer, cancelChan chan struct{}) error {
	defer r.Close()

	completeChan := make(chan error)
	go func() {
		_, err := io.Copy(w, r)
		completeChan <- err
	}()

	select {
	case err := <-completeChan:
		return err
	case <-cancelChan:
		return errors.New("canceled")
	}
}

func main() {
	filename := createTmpFile()

	file, err := os.Open(filename)
	panicOnError(err)
	defer os.Remove(file.Name())

	writer := new(bytes.Buffer)
	cancel := make(chan struct{})
	close(cancel)
	err = CancellableCopy(file, writer, cancel)
}

func createTmpFile() string {
	tmpFile, err := ioutil.TempFile("", "some-tmp-file")
	panicOnError(err)

	_, err = tmpFile.WriteString("this is a line of text\n")
	panicOnError(err)

	err = tmpFile.Close()
	panicOnError(err)

	return tmpFile.Name()
}

func panicOnError(err error) {
	if err != nil {
		panic(err.Error())
	}
}