public void didRollback(){
synchronized (this) {
    if (pending != null) {
      pending.cancel(false);
      pending=null;
    }
    docsSinceCommit.set(0);
  }
}

public void didRollback ( ) { synchronized ( this ) { if ( pending != null ) { pending . cancel ( false ) ; pending = null ; } docsSinceCommit . set ( 0 ) ; } }
inform tracker that a rollback has occurred , cancel any pending commits
