Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/websocket/tcpserver/server.go

@@ -33,15 +33,38 @@ type Server struct {
 	hub  *websocket.Hub
 }
 
+// Option configures a Server.
+type Option func(*Server)
+
+// WithLogger makes the server log to logger instead of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(s *Server) {
+		s.log = logger
+	}
+}
+
 // New builds a server for hub's plain TCP clients. Run starts it; it shuts
 // hub down when it stops, so hub should not be shared with another server.
-// Push to hub to reach the server's clients.
-func New(conf *config.JSCServer, hub *websocket.Hub, log *slog.Logger) *Server {
-	return &Server{
+// Push to hub to reach the server's clients. It logs to slog.Default()
+// unless given WithLogger.
+func New(conf *config.JSCServer, hub *websocket.Hub, opts ...Option) *Server {
+	server := &Server{
 		conf: conf,
-		log:  log,
 		hub:  hub,
 	}
+	for _, opt := range opts {
+		opt(server)
+	}
+	return server
+}
+
+// logger returns the logger the server logs to, tagged with its component.
+func (s *Server) logger() *slog.Logger {
+	log := s.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", "tcp")
 }
 
 // Run listens on the configured host and port and serves plain TCP clients
@@ -71,14 +94,14 @@ func (s *Server) serve(ctx context.Context, listen net.Listener) error {
 	})
 	defer stop()
 
-	s.log.Info("listening", "addr", listen.Addr().String())
+	s.logger().Info("listening", "addr", listen.Addr().String())
 	acceptErr := s.accept(ctx, connCtx, listen)
 	if ctx.Err() != nil {
 		acceptErr = nil // cancelling ctx is how the server is stopped
 	}
 	listen.Close() //nolint:errcheck,gosec // closed already when ctx was cancelled
 
-	s.log.Info("shutting down")
+	s.logger().Info("shutting down")
 	shutdownCtx := context.Background()
 	if s.conf.Timeouts.ShutdownTimeout > 0 {
 		var cancel context.CancelFunc
@@ -117,7 +140,7 @@ func (s *Server) accept(
 
 			// Back off and retry e.g. when out of file descriptors.
 			delay = min(max(2*delay, 5*time.Millisecond), time.Second)
-			s.log.Warn("tcp accept failed", "error", err, "retry_in", delay)
+			s.logger().Warn("tcp accept failed", "error", err, "retry_in", delay)
 
 			select {
 			case <-time.After(delay):
@@ -131,7 +154,7 @@ func (s *Server) accept(
 		if limiter != nil {
 			ip := remoteIP(newConn)
 			if allowed, _ := limiter.Allow(ip); !allowed {
-				s.log.Warn("tcp connection rate limited", "ip", ip)
+				s.logger().Warn("tcp connection rate limited", "ip", ip)
 				go reject(newConn, "rate limited")
 				continue
 			}