handler.go

package slogkafka

import (
	"context"
	"encoding/json"
	"time"

	"log/slog"

	slogcommon "github.com/samber/slog-common"
	"github.com/segmentio/kafka-go"
)

type Option struct {
	// log level (default: debug)
	Level slog.Leveler

	// Kafka Writer
	KafkaWriter *kafka.Writer
	Timeout     time.Duration // default: 60s

	// optional: customize Kafka event builder
	Converter Converter
	// optional: custom marshaler
	Marshaler func(v any) ([]byte, error)
	// optional: fetch attributes from context
	AttrFromContext []func(ctx context.Context) []slog.Attr

	// optional: see slog.HandlerOptions
	AddSource   bool
	ReplaceAttr func(groups []string, a slog.Attr) slog.Attr
}

func (o Option) NewKafkaHandler() slog.Handler {
	if o.Level == nil {
		o.Level = slog.LevelDebug
	}

	if o.KafkaWriter == nil {
		panic("missing Kafka writer")
	}

	if o.Timeout == 0 {
		o.Timeout = 60 * time.Second
	}

	if o.Converter == nil {
		o.Converter = DefaultConverter
	}

	if o.Marshaler == nil {
		o.Marshaler = json.Marshal
	}

	if o.AttrFromContext == nil {
		o.AttrFromContext = []func(ctx context.Context) []slog.Attr{}
	}

	return &KafkaHandler{
		option: o,
		attrs:  []slog.Attr{},
		groups: []string{},
	}
}

var _ slog.Handler = (*KafkaHandler)(nil)

type KafkaHandler struct {
	option Option
	attrs  []slog.Attr
	groups []string
}

func (h *KafkaHandler) Enabled(_ context.Context, level slog.Level) bool {
	return level >= h.option.Level.Level()
}

func (h *KafkaHandler) Handle(ctx context.Context, record slog.Record) error {
	fromContext := slogcommon.ContextExtractor(ctx, h.option.AttrFromContext)
	payload := h.option.Converter(h.option.AddSource, h.option.ReplaceAttr, append(h.attrs, fromContext...), h.groups, &record)

	return h.publish(record.Time, payload)
}

func (h *KafkaHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	return &KafkaHandler{
		option: h.option,
		attrs:  slogcommon.AppendAttrsToGroup(h.groups, h.attrs, attrs...),
		groups: h.groups,
	}
}

func (h *KafkaHandler) WithGroup(name string) slog.Handler {
	// https://cs.opensource.google/go/x/exp/+/46b07846:slog/handler.go;l=247
	if name == "" {
		return h
	}

	return &KafkaHandler{
		option: h.option,
		attrs:  h.attrs,
		groups: append(h.groups, name),
	}
}

func (h *KafkaHandler) publish(timestamp time.Time, payload map[string]interface{}) error {
	key, err := timestamp.MarshalBinary()
	if err != nil {
		return err
	}

	// bearer:disable go_lang_deserialization_of_user_input
	values, err := h.option.Marshaler(payload)
	if err != nil {
		return err
	}

	// we ignore cancel, since the call to WriteMessages might be non-blocking
	ctx, cancel := context.WithTimeout(context.Background(), h.option.Timeout) //nolint:lostcancel
	defer cancel()

	return h.option.KafkaWriter.WriteMessages(
		ctx,
		kafka.Message{
			Key:   key,
			Value: values,
		},
	)
}