* `futures` into a local variable (in preparation for propagating cancellation to them). In
     * either case, no one needs to read `futures` for cancellation purposes later. (And
     * cancellation purposes are the main reason to access `futures`, as discussed in its docs.)
     */
    this.futures = null;
  }

  enum ReleaseResourcesReason {
    OUTPUT_FUTURE_DONE,
    ALL_INPUT_FUTURES_PROCESSED,
  }

  /**