@Override
      public void onCancellation(DataSource<T> dataSource) {}
{@link DataSource} supplier that provides a data source which will forward results of the first
available data source.

<p>Data sources are obtained in order. Only if the current data source fails, or if it finishes
without result, the next one will be tried.
fbcore/src/main/java/com/facebook/datasource/FirstAvailableDataSourceSupplier.java