serrors - Structured Errors

serrors allows you to add tags/fields/kv-pairs to your errors.

Usage

package main

import (
	"os"
	"fmt"
	"log/slog"
	
	"github.com/Eun/serrors"
)

func validateUserName(name string) error {
	const maxLength = 10
	if len(name) > maxLength {
		return serrors.New("username is too long").
			With("username", name).
			With("max_length", maxLength)
	}
	return nil
}

func main() {
	user := os.Getenv("USER")
	err := validateUserName(user)
	if err != nil {
		slog.Error("name validation failed",
			"error", err.Error(),
			slog.Group("details", serrors.GetFieldsAsCombinedSlice(err)...),
			"stack", serrors.GetStack(err),
		)
		return
	}
	fmt.Println("Welcome ", user)
}

Problem

We use structured loggers like slog to create nice formatted log messages
and add important context to error messages.
Take this code as an example:

func validateUserNameLength(name string) error {
	const maxLength = 10
	if len(name) > maxLength {
		slog.Error("username is too long", "username", name, "max_length", maxLength)
		return errors.New("username is too long")
	}
	return nil
}

Not only do we return an error, but we also log the error using slog.
Lets look at the calling function:

func addUserToRole(userName, roleName string) error {
	if err := validateUserNameLength(userName); err != nil {
		slog.Error("validation of username failed", "username", name)
		return fmt.Errorf("validation of username failed: %w", err)
	}
	// ...
}

Again, we return the error (with the underlying error), and we also log it - because we need the context in our messages.

In this case we end up with at least two error messages:

1. slog.Error("username is too long", ...)
2. slog.Error("validation of username failed", ...)
3. when we handle addUserToRole: validation of username failed: username is too long

The last error that will be logged or printed won't contain any useful information on why this problem actually occurred.

One possible solution would be to use something like fmt.Errorf("username is too long [username=%s]", name). However, this could lead to some funny unreadable errors like:

validation of username failed [username=MisterDolittle]: username is too long [username=MisterDolittle] [max_length=10]

This package attempts to solve this problem by providing methods to add tags/fields/kv-pairs to errors that can later be retrieved.

Builder Usage

You could save some code duplication by using the builder functionality:

func validateUserName(name string) error {
	serr := serrors.NewBuilder().
		With("username", name)

	if name == "" {
		return serr.New("username cannot be empty")
	}

	if err := validateUserNameLength(name); err != nil {
		return serr.Wrap(err, "username has invalid length")
	}

	reservedNames := []string{"root", "admin"}
	for _, s := range reservedNames {
		if name == s {
			return serr.Errorf("username cannot be %q, it is reserved", name).
				With("reserved", reservedNames)
		}
	}
	return nil
}

Building without Stack

By default serrors collects stack information, this behaviour can be disabled by setting the build tag serrors_without_stack:

go build -tags serrors_without_stack ...