Program: 158767
Project: 158767
Buggy source:
public String registerUserHandling(String userName) throws MageException {
  this.isAdmin=false;
  if (userName.equals("String_Node_Str")) {
    return "String_Node_Str";
  }
  if (userName.length() > ConfigSettings.getInstance().getMaxUserNameLength()) {
    return new StringBuilder("String_Node_Str").append(ConfigSettings.getInstance().getMaxUserNameLength()).append("String_Node_Str").toString();
  }
  if (userName.length() < ConfigSettings.getInstance().getMinUserNameLength()) {
    return new StringBuilder("String_Node_Str").append(ConfigSettings.getInstance().getMinUserNameLength()).append("String_Node_Str").toString();
  }
  Pattern p=Pattern.compile(ConfigSettings.getInstance().getUserNamePattern(),Pattern.CASE_INSENSITIVE);
  Matcher m=p.matcher(userName);
  if (m.find()) {
    return new StringBuilder("String_Node_Str").append(userName).append("String_Node_Str").toString();
  }
  User user=UserManager.getInstance().createUser(userName,host);
  boolean reconnect=false;
  if (user == null) {
    user=UserManager.getInstance().findUser(userName);
    if (user.getHost().equals(host)) {
      user.updateLastActivity(null);
      this.userId=user.getId();
      if (user.getSessionId().isEmpty()) {
        logger.info("String_Node_Str" + userName);
        reconnect=true;
      }
 else {
        logger.info("String_Node_Str" + userName);
        SessionManager.getInstance().disconnect(user.getSessionId(),DisconnectReason.ConnectingOtherInstance);
      }
    }
 else {
      return new StringBuilder("String_Node_Str").append(userName).append("String_Node_Str").toString();
    }
  }
  if (!UserManager.getInstance().connectToSession(sessionId,user.getId())) {
    return new StringBuilder("String_Node_Str").append(userName).toString();
  }
  this.userId=user.getId();
  if (reconnect) {
    UUID chatId=GamesRoomManager.getInstance().getRoom(GamesRoomManager.getInstance().getMainRoomId()).getChatId();
    if (chatId != null) {
      ChatManager.getInstance().joinChat(chatId,userId);
    }
    ChatManager.getInstance().sendReconnectMessage(userId);
  }
  return null;
}

Fixed version (ground truth):
public String registerUserHandling(String userName) throws MageException {
  this.isAdmin=false;
  if (userName.equals("String_Node_Str")) {
    return "String_Node_Str";
  }
  if (userName.length() > ConfigSettings.getInstance().getMaxUserNameLength()) {
    return "String_Node_Str" + ConfigSettings.getInstance().getMaxUserNameLength() + "String_Node_Str";
  }
  if (userName.length() < ConfigSettings.getInstance().getMinUserNameLength()) {
    return "String_Node_Str" + ConfigSettings.getInstance().getMinUserNameLength() + "String_Node_Str";
  }
  Pattern p=Pattern.compile(ConfigSettings.getInstance().getUserNamePattern(),Pattern.CASE_INSENSITIVE);
  Matcher m=p.matcher(userName);
  if (m.find()) {
    return "String_Node_Str" + userName + "String_Node_Str";
  }
  User user=UserManager.getInstance().createUser(userName,host);
  boolean reconnect=false;
  if (user == null) {
    user=UserManager.getInstance().findUser(userName);
    if (user.getHost().equals(host)) {
      user.updateLastActivity(null);
      this.userId=user.getId();
      if (user.getSessionId().isEmpty()) {
        logger.info("String_Node_Str" + userName);
        reconnect=true;
      }
 else {
        logger.info("String_Node_Str" + userName);
        SessionManager.getInstance().disconnect(user.getSessionId(),DisconnectReason.ConnectingOtherInstance);
      }
    }
 else {
      return "String_Node_Str" + userName + "String_Node_Str";
    }
  }
  if (!UserManager.getInstance().connectToSession(sessionId,user.getId())) {
    return "String_Node_Str" + userName;
  }
  this.userId=user.getId();
  if (reconnect) {
    UUID chatId=GamesRoomManager.getInstance().getRoom(GamesRoomManager.getInstance().getMainRoomId()).getChatId();
    if (chatId != null) {
      ChatManager.getInstance().joinChat(chatId,userId);
    }
    ChatManager.getInstance().sendReconnectMessage(userId);
  }
  return null;
}