lib/input-generators/sitemap-index.js
function generateInput (options) {
  return nodeCall(
    common.isUrl(options.source) ? smLib.getUrl : smLib.getFile,
    options,
    parse
  )
    .catch(function (err) {
      options._abort(err);
    })
    .then(function () {
      base.EOI(sitemapIndex);
    });
}
function generateInput (options) {
  return nodeCall(
    common.isUrl(options.source) ? smLib.getUrl : smLib.getFile,
    options,
    parse
  )
    .catch(function (err) {
      options._abort(err);
    })
    .then(function () {
      base.EOI(sitemapIndex);
    });
}
generateInput

Called back by base.run to generate the input for this input type.
This can return true on error for true async. An async error is supplied to listener
in this case via _abort.

@param {Object} options - Sitemap options.
@param {String} options.source - Sitemap URL or file path.