common/logger/logger.go

// Package logger provides structured logging with OpenTelemetry trace integration.
//
// This package offers multiple logging configurations for different deployment scenarios:
//   - Text logging to stderr with optional timestamp removal for systemd
//   - OTLP (OpenTelemetry Protocol) logging for observability pipelines
//   - Multi-logger setup that outputs to both text and OTLP simultaneously
//   - Context-aware logging with trace ID correlation
//
// The package automatically detects systemd environments and adjusts timestamp handling
// accordingly. It supports debug level configuration via environment variables and
// provides compatibility bridges for legacy logging interfaces.
//
// Key features:
//   - Automatic OpenTelemetry trace and span ID inclusion in log entries
//   - Configurable log levels via DEBUG environment variable (with optional prefix)
//   - Systemd-compatible output (no timestamps when INVOCATION_ID is present)
//   - Thread-safe logger setup with sync.Once protection
//   - Context propagation for request-scoped logging
//
// Environment variables:
//   - DEBUG: Enable debug level logging (configurable prefix via ConfigPrefix)
//   - INVOCATION_ID: Systemd detection for timestamp handling
package logger

import (
	"context"
	"log"
	"log/slog"
	"os"
	"strconv"
	"sync"

	slogtraceid "github.com/remychantenay/slog-otel"
	slogmulti "github.com/samber/slog-multi"
	"go.opentelemetry.io/contrib/bridges/otelslog"
)

// ConfigPrefix allows customizing the environment variable prefix for configuration.
// When set, environment variables like DEBUG become {ConfigPrefix}_DEBUG.
// This enables multiple services to have independent logging configuration.
var ConfigPrefix = ""

var (
	textLogger  *slog.Logger
	otlpLogger  *slog.Logger
	multiLogger *slog.Logger
)

var (
	setupText  sync.Once // this sets the default
	setupOtlp  sync.Once // this never sets the default
	setupMulti sync.Once // this sets the default, and will always run after the others
	mu         sync.Mutex
)

func setupStdErrHandler() slog.Handler {
	programLevel := new(slog.LevelVar) // Info by default

	envVar := "DEBUG"
	if len(ConfigPrefix) > 0 {
		envVar = ConfigPrefix + "_" + envVar
	}

	if opt := os.Getenv(envVar); len(opt) > 0 {
		if debug, _ := strconv.ParseBool(opt); debug {
			programLevel.Set(slog.LevelDebug)
		}
	}

	logOptions := &slog.HandlerOptions{Level: programLevel}

	if len(os.Getenv("INVOCATION_ID")) > 0 {
		// don't add timestamps when running under systemd
		log.Default().SetFlags(0)

		logOptions.ReplaceAttr = logRemoveTime
	}

	logHandler := slogtraceid.OtelHandler{
		Next: slog.NewTextHandler(os.Stderr, logOptions),
	}

	return logHandler
}

func setupOtlpLogger() *slog.Logger {
	setupOtlp.Do(func() {
		otlpLogger = slog.New(
			newLogFmtHandler(otelslog.NewHandler("common")),
		)
	})
	return otlpLogger
}

// SetupMultiLogger creates a logger that outputs to both text (stderr) and OTLP simultaneously.
// This is useful for services that need both human-readable logs and structured observability data.
//
// The multi-logger combines:
//   - Text handler: Stderr output with OpenTelemetry trace integration
//   - OTLP handler: Structured logs sent via OpenTelemetry Protocol
//
// On first call, this logger becomes the default logger returned by Setup().
// The function is thread-safe and uses sync.Once to ensure single initialization.
func SetupMultiLogger() *slog.Logger {
	setupMulti.Do(func() {
		textHandler := Setup().Handler()
		otlpHandler := setupOtlpLogger().Handler()

		multiHandler := slogmulti.Fanout(
			textHandler,
			otlpHandler,
		)
		mu.Lock()
		defer mu.Unlock()
		multiLogger = slog.New(multiHandler)
		slog.SetDefault(multiLogger)
	})

	return multiLogger
}

// SetupOLTP creates a logger that sends structured logs via OpenTelemetry Protocol.
// This logger is designed for observability pipelines and log aggregation systems.
//
// The OTLP logger formats log messages similarly to the text logger for better
// compatibility with Loki + Grafana, while still providing structured attributes.
// Log attributes are available both in the message format and as OTLP attributes.
//
// This logger does not become the default logger and must be used explicitly.
// It requires OpenTelemetry tracing configuration to be set up via the tracing package.
//
// See: https://github.com/grafana/loki/issues/14788 for formatting rationale.
func SetupOLTP() *slog.Logger {
	return setupOtlpLogger()
}

// Setup creates and returns the standard text logger for the application.
// This is the primary logging function that most applications should use.
//
// Features:
//   - Text formatting to stderr with human-readable output
//   - Automatic OpenTelemetry trace_id and span_id inclusion when available
//   - Systemd compatibility: omits timestamps when INVOCATION_ID environment variable is present
//   - Debug level support via DEBUG environment variable (respects ConfigPrefix)
//   - Thread-safe initialization with sync.Once
//
// On first call, this logger becomes the slog default logger. If SetupMultiLogger()
// has been called previously, Setup() returns the multi-logger instead of the text logger.
//
// The logger automatically detects execution context:
//   - Systemd: Removes timestamps (systemd adds its own)
//   - Debug mode: Enables debug level logging based on environment variables
//   - OpenTelemetry: Includes trace correlation when tracing is active
func Setup() *slog.Logger {
	setupText.Do(func() {
		h := setupStdErrHandler()
		textLogger = slog.New(h)
		slog.SetDefault(textLogger)
	})

	mu.Lock()
	defer mu.Unlock()

	if multiLogger != nil {
		return multiLogger
	}
	return textLogger
}

type loggerKey struct{}

// NewContext stores a logger in the context for request-scoped logging.
// This enables passing request-specific loggers (e.g., with request IDs,
// user context, or other correlation data) through the call stack.
//
// Use this to create context-aware logging where different parts of the
// application can access the same enriched logger instance.
//
// Example:
//
//	logger := slog.With("request_id", requestID)
//	ctx := logger.NewContext(ctx, logger)
//	// Pass ctx to downstream functions
func NewContext(ctx context.Context, l *slog.Logger) context.Context {
	return context.WithValue(ctx, loggerKey{}, l)
}

// FromContext retrieves a logger from the context.
// If no logger is stored in the context, it returns the default logger from Setup().
//
// This function provides a safe way to access context-scoped loggers without
// needing to check for nil values. It ensures that logging is always available,
// falling back to the application's standard logger configuration.
//
// Example:
//
//	log := logger.FromContext(ctx)
//	log.Info("processing request") // Uses context logger or default
func FromContext(ctx context.Context) *slog.Logger {
	if l, ok := ctx.Value(loggerKey{}).(*slog.Logger); ok {
		return l
	}
	return Setup()
}

func logRemoveTime(groups []string, a slog.Attr) slog.Attr {
	// Remove time
	if a.Key == slog.TimeKey && len(groups) == 0 {
		return slog.Attr{}
	}
	return a
}