pendingDescription = pendingToString();
      } catch (RuntimeException e) {
        // Don't call getMessage or toString() on the exception, in case the exception thrown by the
        // subclass is implemented with bugs similar to the subclass.
        pendingDescription = "Exception thrown from implementation: " + e.getClass();
      }
      // The future may complete during or before the call to getPendingToString, so we use null

  //   timeout is small we shouldn't park(). This needs to be traded off with the cpu overhead of
  //   spinning, so we use SPIN_THRESHOLD_NANOS which is what AbstractQueuedSynchronizer uses for
  //   similar purposes.
  // * We want to behave reasonably for timeouts of 0
  // * We are more responsive to completion than timeouts. This is because parkNanos depends on