golang如何操作二进制流呢？

panic

func main() {
    http.HandleFunc("/pdf", function(w ResponseWriter, r *http.Request) {
        w.Header().Add("Content-Type", "application/pdf");
        fp, err:= os.Open("./go.pdf")
        if err!=nil {
            http.Error(w, fmt.Errorf("internal server error"), http.StatusInternalServerError)
            return;
        }
        defer fp.Close()
        
        if _, err:=io.Copy(w, fp);err!=nil {
            http.Error(w, fmt.Errorf("internal server error"), http.StatusInternalServerError)
           return;
        }
    });
}