code: @CheckForNull
      public UpdateSite getCoreSource() {
          for (UpdateSite s : sites) {
              Data data = s.getData();
              if (data!=null && data.core!=null)
                  return s;
          }
          return null;
      }
docs: Gets the {@link UpdateSite} from which we receive updates for {@code jenkins.war}.
  
  @return
  {@code null} if no such update center is provided.