Fix panic when logging is disabled

fixtures/config.toml

@@ -28,8 +28,7 @@ invalid-usernames = ["用户名"]
 invalid-username-message = "Invalid username %s. Please check https://vlab.ustc.edu.cn/docs/login/ssh/#username for more information."
 
 [logger]
-enabled = true
-endpoint = "udp://127.0.0.1:5556"
+enabled = false
 
 [proxy-protocol]
 enabled = true

sshmux.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/base64"
 	"fmt"
+	"io"
 	"log"
 	"log/slog"
 	"net"
@@ -28,7 +29,7 @@ type Server struct {
 	Banner         string
 	SSHConfig      *ssh.ServerConfig
 	Authenticator  Authenticator
-	LogWriter      *net.Conn
+	LogWriter      io.Writer
 	ProxyPolicy    ProxyPolicyConfig
 	UsernamePolicy UsernamePolicyConfig
 	PasswordPolicy PasswordPolicyConfig
@@ -78,7 +79,7 @@ func makeServer(config Config) (*Server, error) {
 	if err != nil {
 		return nil, err
 	}
-	var loggerEndpoint *net.Conn = nil
+	var logWriter io.Writer
 	if config.Logger.Enabled {
 		loggerURL, err := url.Parse(config.Logger.Endpoint)
 		if err != nil {
@@ -89,17 +90,19 @@ func makeServer(config Config) (*Server, error) {
 			if err != nil {
 				log.Fatalf("Logger Dial failed: %s\n", err)
 			}
-			loggerEndpoint = &conn
+			logWriter = conn
 		} else {
 			log.Fatalf("unsupported logger endpoint: %s\n", config.Logger.Endpoint)
 		}
+	} else {
+		logWriter = io.Discard
 	}
 	sshmux := &Server{
 		Address:       config.Address,
 		Banner:        config.SSH.Banner,
 		SSHConfig:     sshConfig,
 		Authenticator: makeAuthenticator(config.Auth, config.Recovery),
-		LogWriter:     loggerEndpoint,
+		LogWriter:     logWriter,
 		ProxyPolicy:   proxyPolicyConfig,
 		UsernamePolicy: UsernamePolicyConfig{
 			InvalidUsernames:       config.Auth.InvalidUsernames,
@@ -145,10 +148,7 @@ func (s *Server) handler(conn net.Conn) {
 	}
 	defer session.Close()
 
-	var logger *slog.Logger = nil
-	if s.LogWriter != nil {
-		logger = slog.New(slog.NewJSONHandler(*s.LogWriter, nil))
-	}
+	logger := slog.New(slog.NewJSONHandler(s.LogWriter, nil))
 	logger = logger.With(
 		slog.Int64("connect_time", time.Now().Unix()),
 		slog.String("remote_ip", conn.RemoteAddr().String()),