SlogLogger loses error context
Author: oPOCCOMAXAoCreated Mar 16, 2025Updated Mar 16, 2025
Describe the bug In function slogErr errors are converted into string and lose metadata, like stacktrace or extra fields of error structs.
//fxevent/slog.go:270
func slogErr(err error) slog.Attr {
return slog.String("error", err.Error())
}To Reproduce Example:
package example
import (
"context"
"fmt"
"log/slog"
"testing"
"github.com/pkg/errors"
"github.com/stretchr/testify/assert"
"github.com/stretchr/testify/require"
"go.uber.org/fx"
"go.uber.org/fx/fxevent"
)
type TestService struct{}
func NewTestService(
lc fx.Lifecycle,
) *TestService {
lc.Append(fx.Hook{
OnStart: func(context.Context) error {
return errors.WithStack(assert.AnError)
},
})
return &TestService{}
}
var _ slog.Handler = (*CustomLogger)(nil)
type CustomLogger struct {
SeenError bool
Text string
}
func (c *CustomLogger) Enabled(context.Context, slog.Level) bool { return true }
func (c *CustomLogger) Handle(_ context.Context, rec slog.Record) error {
rec.Attrs(func(attr slog.Attr) bool {
if err, ok := attr.Value.Any().(error); ok && err != nil {
c.SeenError = true
}
if attr.Key == "error" {
c.Text = attr.Value.String()
}
return true
})
return nil
}
func (c *CustomLogger) WithAttrs([]slog.Attr) slog.Handler { return c }
func (c *CustomLogger) WithGroup(string) slog.Handler { return c }
func TestExample(t *testing.T) {
slogHandler := CustomLogger{}
app := fx.New(
fx.WithLogger(func() fxevent.Logger {
return &fxevent.SlogLogger{
Logger: slog.New(&slogHandler),
}
}),
fx.Provide(NewTestService),
fx.Invoke(func(_ *TestService, sd fx.Shutdowner) {
sd.Shutdown()
}),
)
err := app.Start(context.Background())
require.Error(t, err, "test error")
<-app.Done()
assert.False(t, slogHandler.SeenError)
assert.Equal(t, "assert.AnError general error for testing", slogHandler.Text)
// here partial checks as path is not stable from machine to machine
fullErrorText := fmt.Sprintf("%+v", err)
t.Log(fullErrorText)
assert.Contains(t, fullErrorText, "assert.AnError")
assert.Contains(t, fullErrorText, "example_test.go:23")
assert.Contains(t, fullErrorText, "go.uber.org/[email protected]/internal/lifecycle/lifecycle.go:265")
}Here slog handler doesn't have access to original error, but receives untraceable string.
Original error has extra data - stacktrace, which could be retrieved in slog handler like this:
// here partial checks as path is not stable from machine to machine
fullErrorText := fmt.Sprintf("%+v", err)
t.Log(fullErrorText)
assert.Contains(t, fullErrorText, "assert.AnError")
assert.Contains(t, fullErrorText, "example_test.go:23")
assert.Contains(t, fullErrorText, "go.uber.org/[email protected]/internal/lifecycle/lifecycle.go:265")Also repo with code: https://github.com/oPOCCOMAXAo/fxexample/blob/master/example_test.go
Expected behavior
Custom slog Handler must access to original error value and process it as required by business logic. Like in this example:
func (c *CustomLogger) Handle(_ context.Context, rec slog.Record) error {
rec.Attrs(func(attr slog.Attr) bool {
if err, ok := attr.Value.Any().(error); ok && err != nil {
c.SeenError = true
}
if attr.Key == "error" {
c.Text = attr.Value.String()
}
return true
})
return nil
}Additional context
This will fix the issue:
//fxevent/slog.go:270
func slogErr(err error) slog.Attr {
return slog.Any("error", err)
}Source: uber-go/fx