goerr

Package goerr provides more contextual error handling in Go.

Features

goerr provides the following features:

• Stack traces
  • Compatible with github.com/pkg/errors.
  • Structured stack traces with goerr.Stack is available.
• Contextual variables to errors using WithValue(key, value).
• errors.Is to identify errors and errors.As to unwrap errors.
• slog.LogValuer interface to output structured logs with slog.

Usage

Stack trace

goerr records stack trace when creating an error. The format is compatible with github.com/pkg/errors and it can be used for sentry.io, etc.

func someAction(fname string) error {
	if _, err := os.Open(fname); err != nil {
		return goerr.Wrap(err, "failed to open file")
	}
	return nil
}

func main() {
	if err := someAction("no_such_file.txt"); err != nil {
		log.Fatalf("%+v", err)
	}
}

Output:

2024/04/06 10:30:27 failed to open file: open no_such_file.txt: no such file or directory
main.someAction
        /Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/stacktrace_print/main.go:12
main.main
        /Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/stacktrace_print/main.go:18
runtime.main
        /usr/local/go/src/runtime/proc.go:271
runtime.goexit
        /usr/local/go/src/runtime/asm_arm64.s:1222
exit status 1

You can not only print the stack trace, but also extract the stack trace by goerr.Unwrap(err).Stacks().

if err := someAction("no_such_file.txt"); err != nil {
  // NOTE: `errors.Unwrap` also works
  if goErr := goerr.Unwrap(err); goErr != nil {
    for i, st := range goErr.Stacks() {
      log.Printf("%d: %v\n", i, st)
    }
  }
  log.Fatal(err)
}

Stacks() returns a slice of goerr.Stack struct, which contains Func, File, and Line.

2024/04/06 10:35:30 0: &{main.someAction /Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/stacktrace_extract/main.go 12}
2024/04/06 10:35:30 1: &{main.main /Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/stacktrace_extract/main.go 18}
2024/04/06 10:35:30 2: &{runtime.main /usr/local/go/src/runtime/proc.go 271}
2024/04/06 10:35:30 3: &{runtime.goexit /usr/local/go/src/runtime/asm_arm64.s 1222}
2024/04/06 10:35:30 failed to open file: open no_such_file.txt: no such file or directory
exit status 1

NOTE: If the error is wrapped by goerr multiply, %+v will print the stack trace of the deepest error.

Tips: If you want not to print the stack trace for current stack frame, you can use Unstack method. Also, UnstackN method removes the top multiple stack frames.

if err := someAction("no_such_file.txt"); err != nil {
	// Unstack() removes the current stack frame from the error message.
	return goerr.Wrap(err, "failed to someAction").Unstack()
}

Add/Extract contextual variables

goerr provides the WithValue(key, value) method to add contextual variables to errors. The standard way to handle errors in Go is by injecting values into error messages. However, this approach makes it difficult to aggregate various errors. On the other hand, goerr's With method allows for adding contextual information to errors without changing error message, making it easier to aggregate error logs. Additionally, error handling services like Sentry.io can handle errors more accurately with this feature.

var errFormatMismatch = errors.New("format mismatch")

func someAction(tasks []task) error {
	for _, t := range tasks {
		if err := validateData(t.Data); err != nil {
			return goerr.Wrap(err, "failed to validate data").WithValue("name", t.Name)
		}
	}
	// ....
	return nil
}

func validateData(data string) error {
	if !strings.HasPrefix(data, "data:") {
		return goerr.Wrap(errFormatMismatch).WithValue("data", data)
	}
	return nil
}

type task struct {
	Name string
	Data string
}

func main() {
	tasks := []task{
		{Name: "task1", Data: "data:1"},
		{Name: "task2", Data: "invalid"},
		{Name: "task3", Data: "data:3"},
	}
	if err := someAction(tasks); err != nil {
		if goErr := goerr.Unwrap(err); goErr != nil {
			for k, v := range goErr.Values() {
				log.Printf("var: %s => %v\n", k, v)
			}
		}
		log.Fatalf("msg: %s", err)
	}
}

Output:

2024/04/06 14:40:59 var: data => invalid
2024/04/06 14:40:59 var: name => task2
2024/04/06 14:40:59 msg: failed to validate data: : format mismatch
exit status 1

If you want to send the error to sentry.io with SDK, you can extract the contextual variables by goErr.Values() and set them to the scope.

// Sending error to Sentry
hub := sentry.CurrentHub().Clone()
hub.ConfigureScope(func(scope *sentry.Scope) {
  if goErr := goerr.Unwrap(err); goErr != nil {
    for k, v := range goErr.Values() {
      scope.SetExtra(k, v)
    }
  }
})
evID := hub.CaptureException(err)

Structured logging

goerr provides slog.LogValuer interface to output structured logs with slog. It can be used to output not only the error message but also the stack trace and contextual variables. Additionally, unwrapped errors can be output recursively.

var errRuntime = errors.New("runtime error")

func someAction(input string) error {
	if err := validate(input); err != nil {
		return goerr.Wrap(err, "failed validation")
	}
	return nil
}

func validate(input string) error {
	if input != "OK" {
		return goerr.Wrap(errRuntime, "invalid input").WithValue("input", input)
	}
	return nil
}

func main() {
	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
	if err := someAction("ng"); err != nil {
		logger.Error("aborted myapp", slog.Any("error", err))
	}
}

Output:

{
  "time": "2024-04-06T11:32:40.350873+09:00",
  "level": "ERROR",
  "msg": "aborted myapp",
  "error": {
    "message": "failed validation",
    "stacktrace": [
      "/Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/logging/main.go:16 main.someAction",
      "/Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/logging/main.go:30 main.main",
      "/usr/local/go/src/runtime/proc.go:271 runtime.main",
      "/usr/local/go/src/runtime/asm_arm64.s:1222 runtime.goexit"
    ],
    "cause": {
      "message": "invalid input",
      "values": {
        "input": "ng"
      },
      "stacktrace": [
        "/Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/logging/main.go:23 main.validate",
        "/Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/logging/main.go:15 main.someAction",
        "/Users/mizutani/.ghq/github.com/m-mizutani/goerr/examples/logging/main.go:30 main.main",
        "/usr/local/go/src/runtime/proc.go:271 runtime.main",
        "/usr/local/go/src/runtime/asm_arm64.s:1222 runtime.goexit"
      ],
      "cause": "runtime error"
    }
  }
}

Builder

goerr provides goerr.NewBuilder() to create an error with pre-defined contextual variables. It is useful when you want to create an error with the same contextual variables in multiple places.

type object struct {
	id    string
	color string
}

func (o *object) Validate() error {
	eb := goerr.NewBuilder().WithValue("id", o.id)

	if o.color == "" {
		return eb.New("color is empty")
	}

	return nil
}

func main() {
	obj := &object{id: "object-1"}

	if err := obj.Validate(); err != nil {
		slog.Default().Error("Validation error", "err", err)
	}
}

Output:

2024/10/19 14:19:54 ERROR Validation error err.message="color is empty" err.values.id=object-1 (snip)

License

The 2-Clause BSD License. See LICENSE for more detail.