CancellableCopy wants to stop an in-progress io.Copy, but suffers from a race

main.go

package main

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

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

	completeChan := make(chan error)
	go func() {
		completeChan <- thirdPartyConsumer(r)
	}()

	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())

	cancel := make(chan struct{})
	close(cancel)
	err = CancellableCopy(file, 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 thirdPartyConsumer(r io.Reader) error {
	writer := new(bytes.Buffer)
	_, err := io.Copy(writer, r)
	return err
}

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

here's the race:

13:50 $ go run -race main.go
==================
WARNING: DATA RACE
Read by goroutine 5:
  os.(*File).read()
      /usr/local/Cellar/go/1.3.3/libexec/src/pkg/os/file_unix.go:190 +0x97
  os.(*File).Read()
      /usr/local/Cellar/go/1.3.3/libexec/src/pkg/os/file.go:95 +0xcb
  bytes.(*Buffer).ReadFrom()
      /usr/local/Cellar/go/1.3.3/libexec/src/pkg/bytes/buffer.go:169 +0x41b
  io.Copy()
      /usr/local/Cellar/go/1.3.3/libexec/src/pkg/io/io.go:349 +0x162
  main.thirdPartyConsumer()
      /Users/pivotal/go/src/race_example/main.go:54 +0x93
  main.func·001()
      /Users/pivotal/go/src/race_example/main.go:16 +0x77

Previous write by main goroutine:
  os.(*file).close()
      /usr/local/Cellar/go/1.3.3/libexec/src/pkg/os/file_unix.go:108 +0x19b
  os.(*File).Close()
      /usr/local/Cellar/go/1.3.3/libexec/src/pkg/os/file_unix.go:97 +0x94
  main.CancellableCopy()
      /Users/pivotal/go/src/race_example/main.go:23 +0x2f2
  main.main()
      /Users/pivotal/go/src/race_example/main.go:36 +0x12b

Goroutine 5 (running) created at:
  main.CancellableCopy()
      /Users/pivotal/go/src/race_example/main.go:17 +0x16b
  main.main()
      /Users/pivotal/go/src/race_example/main.go:36 +0x12b
==================
Found 1 data race(s)
exit status 66