如何干净地处理结构状态错误？

Question

So I'm just getting my feet wet with Go, and I'm trying to cleanly handle errors in the Go-way. One result is that having a type Movie struct with methods to update the record, and sync to a data store. So an example method:

func (movie Movie) SetTitle(title string) : error {
    prevTitle := movie.Title
    movie.Title = title
    json, err := json.Marshal(movie)
    if (err != nil) {
        movie.Title = prevTitle
        return err
    }

    err = db.SetValue(movie.id, json)
    if (err != nil) {
        movie.Title = prevTitle
        return err
    }

    return nil
}

The issue above is the 'cleanup' for a failed operation is to save the previous state, and restore as appropriate. But this feels very unmaintainable - it's so easy for a new error check to be made in the future, only to return without the proper cleanup. This would get even worse with multiple such 'cleanups'.

So to contrast, in other languages, I would wrap the whole bit in a try/catch, and handle cleanup just inside a single catch block. Here, I can:

1. Create a copy of movie, do all the stuff on the copy, and then only copy back to the original object once everything is successful
2. Change the logic to:

if json, err := json.Marshal(movie); err == nil {
  if err = db.SetValue(...); err == nil {
    return nil
  }
}

movie.Title = prevTitle;
return err

Which works, but I don't love having a level of nesting per check.

1. Change the error return to indicate it was updated locally, but not saved
2. Do it as described above
3. Break out the save logic into an func Update() : err function to minimize the number of checks needed (just thought of this - think I like this one)

I feel like none of these are perfect; am I missing something obvious?

Answer 1

Update

The way you are persisting can cause numerous problems:

• you could incandescently mutate original object
• you mix different layers in one method, it makes code very fragile
• method, does too much

I suggest, separate update and persistence. Playground

type Movie struct {
    id    int
    Title string
}

func (m *Movie) Persist() error {
    json, err := json.Marshal(m)
    if err != nil {
        return err
    }

    log.Printf("Storing in db: %s", json)

    return nil
}

func main() {
    m := &Movie{1, "Matrix"}
    m.Title = "Iron Man"

    if err := m.Persist(); err != nil {
        log.Fatalln(err)
    }

}

---

Old answer

In your example you use by-value receiver. In this case, you get a copy of the struct in the method, you free to modify that copy, but all changes will not be visible outside.

func (movie Movie) SetTitleValueSemantic(title string) error {
    movie.Title = title
    json, err := json.Marshal(movie)
    if err != nil {
        return err
    }

    log.Printf("Serialized: %s", json)

    return nil
}

playgorund: https://play.golang.org/p/mVnQ66TCaG9

---

I would strongly recommend avoiding such a coding style. In case you REALLY need some of this kind, here is an example for inspiration:

playground: https://play.golang.org/p/rHacnsRLkEE

func (movie *Movie) SetTitle(title string) (result *Movie, e error) {
    movieCopy := new(Movie)
    *movieCopy = *movie
    result = movie

    defer func() {
        if e != nil {
            result = movieCopy
        }
    }()

    movie.Title = title
    serialized, e := json.Marshal(movie)
    if e != nil {
        return
    }

    log.Printf("Serialized: %s", serialized)

    return
}