Data race with concurrent reads across Reader.File

[wizardishungry]

func TestConcurrentAccess(t *testing.T) {
	file, _ := os.Open("../../testdata/cool_images.doc") // a file with small number of embedded images
	defer file.Close()
	doc, err := mscfb.New(file)
	if err != nil {
		log.Fatal(err)
	}
	var wg sync.WaitGroup
	wg.Add(len(doc.File))
	for _, f := range doc.File {
		go func() {
			defer wg.Done()
			_, err := io.Copy(io.Discard, f)
			if err != nil {
				log.Println(err)
			}
		}()
	}
	wg.Wait()
}

Race is on r.buf in mscfb.(*Reader).readAt()

	if _, err := r.ra.ReadAt(r.buf[:length], offset); err != nil {

[richardlehane]

thanks for this report @wizardishungry