code: public CmsUser update(CmsObject cms, String siteRoot) {
  
          CmsUser originalUser = m_map.getIfPresent(siteRoot);
          if ((originalUser == null) || originalUser.equals(cms.getRequestContext().getCurrentUser())) {
              m_map.put(siteRoot, cms.getRequestContext().getCurrentUser());
              return null;
          }
          return originalUser;
      }
docs: Tries to update or create an entry for the given user/site root combination.<p>
  
  If this method succeeds, it will return null, but if another user has created an entry for the site root,
  it will return that user.<p>
  
  @param cms the current CMS context
  @param siteRoot the site root
  
  @return null of the user who has already created an entry