Tuesday, August 11, 2020

Go: Convert errors.Wrap calls to fmt.Errorf

 I was a longtime fan of https://github.com/pkg/errors. It was a great way to add context to why an error was being returned which made tracing them easier. The need for pkg/errors has gone away with the new fmt.Errorf %w directive, errors.Is(), and errors.As().

I used errors.Wrap() a lot so naturally my code has lots of function calls I need to migrate. One repo had close to 300 calls to errors.Wrap() which is more than I'm willing to do by hand. I wrote a simple tool to take care of the most common case I have: errors.Wrap(err, "<message>").

  • Any line it doesn't know how to handle it leaves unchanged.
  • It looks specifically for errors.Wrap(err, "
  • On that same line it expects to find a double quote followed by a closing paren
  • The existing context string has : %w appended to it
  • It does not edit your imports; you should run goimports or a similar tool
  • By default the tool just outputs to stdout; use -o to overwrite the file in-place
  • Fix everything by doing for i in $(grep -R errors.Wrap `ls`); do errors_wrap_convert -in $i -o; end
  • Definitely make sure you have a snapshot of your code to revert back to in case this tool does bad things
  • This could have been done better using gofix but I was in too much of a hurry to learn how to extend gofix.
# errors_wrap_convert.go
package main

import (

var (
        fIn        = flag.String("in", "", "input file")
        fOverwrite = flag.Bool("o", false, "overwrite the existing file")

func fatalIfError(err error, msg string) {
        if err != nil {
                log.Fatal("error ", msg, ": ", err)

func main() {
        b, err := ioutil.ReadFile(*fIn)
        fatalIfError(err, "reading input file")

        var out io.WriteCloser = os.Stdout
        if *fOverwrite {
                out, err = os.Create(*fIn)
                fatalIfError(err, "opening output file")
        defer out.Close()

        scanner := bufio.NewScanner(bytes.NewBuffer(b))
        for scanner.Scan() {
                fmt.Fprintln(out, Rewrite(scanner.Text()))
        fatalIfError(scanner.Err(), "scanner error")

func Rewrite(in string) string {
        idx := strings.Index(in, `errors.Wrap(err, "`)
        if idx == -1 {
                return in

        eIdx := strings.Index(in[idx:], ")")
        if eIdx == -1 {
                return in
        eIdx += idx

        q1Idx := strings.Index(in[idx:], `"`)
        if q1Idx == -1 {
                return in
        q1Idx += idx

        q2Idx := eIdx - 1
        if in[q2Idx] != '"' {
                return in

        out := in[:idx] +
                `fmt.Errorf(` +
                in[q1Idx:q2Idx] +
                `: %w", err)` +
        return out

And a couple of basic tests:

# errors_convert_test.go
package main

import "testing"

func TestRewrite(t *testing.T) {
        for in, want := range map[string]string{
                "": "",
                `               return nil, errors.Wrap(err, "bad thing") // foo bar`: `                return nil, fmt.Errorf("bad thing: %w", err) // foo bar`,
                `return nil, errors.Wrap(err, "foo " + blarg + " bar")`: `return nil, fmt.Errorf("foo " + blarg + " bar: %w", err)`,
        } {
                got := Rewrite(in)
                if got != want {
                        t.Fatalf("got %q, want %q, for %q", got, want, in)

I had searched for a tool to do this but it either doesn't exist or my searching ability failed me. If you would like to pick this up and generalize I'd happily refer to your version as canonical.

No comments:

Post a Comment