code: public static String getUpstreamIdentifier(AbstractBuild<?, ?> rootBuild) {
          if (rootBuild != null) {
              AbstractProject<?, ?> rootProject = rootBuild.getProject();
              return ExtractorUtils.sanitizeBuildName(rootProject.getFullName()) + "-" + rootBuild.getNumber();
          }
  
          return null;
      }
docs: Get the identifier from the <b>root</b> build. which is composed of {@link hudson.model.AbstractProject#getFullName()}
  -{@link hudson.model.Run#getNumber()}
  
  @param rootBuild The root build
  @return The upstream identifier.