.log(
                SEVERE,
                "Uh oh! An open ClosingFuture has leaked and will close: {0}",
                closingFutureToString());
        FluentFuture<V> unused = finishToFuture();
      }
    }

    String closingFutureToString() {
      // TODO(dpb): Better toString, in the style of Futures.transform etc.
      return toStringHelper("ClosingFuture")