options.Logger is incompatible to slog logger

[TLINDEN]

I am using the slog logger and wanted to enable the new Options.Logger feature, which fails:

lg := slog.Default()
opts.Logger = *lg
db, err := bolt.Open(db.Dbfile, 0600, opts)

Compilation fails:

cannot use *lg (variable of type slog.Logger) as bbolt.Logger value in assignment: slog.Logger does not implement bbolt.Logger (method Debug has pointer receiver)

So, in order to be able to use my existing logger setup, I'd have to implement a bolt.Logger wrapper around the slog logger.

I'd appreciate it, if slog would be supported directly.

best,
Tom

[ivanvc]

We covered this in the fortnightly etcd triage meeting. In #509, the decision was to implement a logger interface compatible with go.uber.org/zap.

I'm torn because implementing a bridge/wrapper struct for every existing logger doesn't make sense and may quickly become an issue. However, log/slog is part of Golang's stdlib.

The bridge/wrapper is pretty simple:

import (
        "fmt"
        "log/slog"

        bolt "go.etcd.io/bbolt"
)

type logger struct {
        *slog.Logger
}

func (l logger) Debug(v ...interface{})                   {}
func (l logger) Debugf(format string, v ...interface{})   { l.Logger.Debug(fmt.Sprintf(format, v...)) }
func (l logger) Error(v ...interface{})                   {}
func (l logger) Errorf(format string, v ...interface{})   { l.Logger.Error(fmt.Sprintf(format, v...)) }
func (l logger) Info(v ...interface{})                    {}
func (l logger) Infof(format string, v ...interface{})    { l.Logger.Info(fmt.Sprintf(format, v...)) }
func (l logger) Warning(v ...interface{})                 {}
func (l logger) Warningf(format string, v ...interface{}) { l.Logger.Warn(fmt.Sprintf(format, v...)) }
func (l logger) Fatal(v ...interface{})                   {}
func (l logger) Fatalf(format string, v ...interface{})   { l.Logger.Error(fmt.Sprintf(format, v...)) }
func (l logger) Panic(v ...interface{})                   {}
func (l logger) Panicf(format string, v ...interface{})   { l.Logger.Error(fmt.Sprintf(format, v...)) }

func main() {
        lg := logger{slog.Default()}
        db, err := bolt.Open("test.db", 0600, &bolt.Options{Logger: lg})
        ...
}

Note that we only use the *f (formatted) functions. So, the implementation for the functions that don't receive a format can be empty.

I believe it would make more sense to document rather than implement it. What do you think, @ahrtr?

[ahrtr]

We don't want to manage any specific logger, nor their configuration. It's the reason why we only expose an interface.

Also exp/slog was experimental when we started to add logger into bbolt. But it's part of the golang std lib (starting from go1.21) now.

It's definitely better to enhance the document at least at https://pkg.go.dev/go.etcd.io/bbolt@v1.4.0#Logger.

If the community pushes hard, then I I do not object to adding thin wrappers for log/slog and go.uber.org/zap into bbolt repo, and expose interfaces something like WithSLog(...), WithUberLog(...).

[TLINDEN]

Here's one vote for it ;)

[ahrtr]

Here's one vote for it ;)

Are you voting for below comment? :)

If the community pushes hard, then I I do not object to adding thin wrappers for log/slog and go.uber.org/zap into bbolt repo, and expose interfaces something like WithSLog(...), WithUberLog(...).

[TLINDEN]

I did but to be honest, I followed the suggestion here and just implemented the logger interface. It's ok.