* ListenableFuture} listeners, should take care not to do so while holding a lock. Additionally,
   * as a further line of defense, prefer not to perform any locking inside a task that will be run
   * under {@code directExecutor}: Not only might the wait for a lock be long, but if the running
   * thread was holding a lock, the listener may deadlock or break lock isolation.
   *
   * <p>This instance is equivalent to: