Make logger configurable in the embedded ferretdb package

ferretdb/ferretdb.go

@@ -23,6 +23,7 @@
 	"errors"
 	"fmt"
 	"net/url"
+	"sync"
 
 	"go.uber.org/zap"
 
@@ -38,6 +39,9 @@
 type Config struct {
 	Listener ListenerConfig
 
+	// Logger to use; if nil, it uses the default global logger.
+	Logger *zap.Logger
+
 	// Handler to use; one of `postgresql` or `sqlite`.
 	Handler string
 
@@ -113,8 +117,13 @@
 
 	metrics := connmetrics.NewListenerMetrics()
 
+	log := config.Logger
+	if log == nil {
+		log = getGlobalLogger()
+	}
+
 	h, closeBackend, err := registry.NewHandler(config.Handler, &registry.NewHandlerOpts{
-		Logger:        logger,
+		Logger:        log,
 		ConnMetrics:   metrics.ConnMetrics,
 		StateProvider: sp,
 		TCPHost:       config.Listener.TCP,
@@ -143,7 +152,7 @@
 		Mode:    clientconn.NormalMode,
 		Metrics: metrics,
 		Handler: h,
-		Logger:  logger,
+		Logger:  log,
 	})
 
 	return &FerretDB{
@@ -214,21 +223,23 @@
 	return u.String()
 }
 
-// logger is a global logger used by FerretDB.
-//
-// TODO https://github.com/FerretDB/FerretDB/issues/4014
-var logger *zap.Logger
+var (
+	loggerOnce sync.Once
+	logger     *zap.Logger
+)
 
-// Initialize the global logger there to avoid creating too many issues for zap users that initialize it in their
-// `main()` functions. It is still not a full solution; eventually, we should remove the usage of the global logger.
-//
-// TODO https://github.com/FerretDB/FerretDB/issues/4014
-func init() {
-	l := zap.ErrorLevel
-	if version.Get().DebugBuild {
-		l = zap.DebugLevel
-	}
+// getGlobalLogger retrieves or creates a global logger using
+// a loggerOnce to ensure it is created only once.
+func getGlobalLogger() *zap.Logger {
+	loggerOnce.Do(func() {
+		level := zap.ErrorLevel
+		if version.Get().DebugBuild {
+			level = zap.DebugLevel
+		}
+
+		logging.Setup(level, "console", "")
+		logger = zap.L()
+	})
 
-	logging.Setup(l, "console", "")
-	logger = zap.L()
+	return logger
 }

internal/bson2/slog.go

@@ -41,6 +41,7 @@ func slogValue(v any) slog.Value {
 		if v == nil {
 			return slog.StringValue("RawDocument(nil)")
 		}
+
 		return slog.StringValue("RawDocument(" + strconv.Itoa(len(v)) + " bytes)")
 
 	case *Array:
@@ -56,6 +57,7 @@ func slogValue(v any) slog.Value {
 		if v == nil {
 			return slog.StringValue("RawArray(nil)")
 		}
+
 		return slog.StringValue("RawArray(" + strconv.Itoa(len(v)) + " bytes)")
 
 	default: