func flag(w http.ResponseWriter, req *http.Request, img string, ctxt *fs.Context) { if !isImgName(img) && !isTagName(img) { fmt.Fprintf(w, "Invalid image.\n") return } data, _, _ := ctxt.Read("qr/flag/" + img) data = append(data, '!') ctxt.Write("qr/flag/"+img, data) fmt.Fprintf(w, "Thank you. The image has been reported.\n") }