def value
      unless complete?
        stat :sleeping_on, :item_id => object_id
        @mutex.synchronize do
          stat :has_lock_on, :item_id => object_id
          chore
          stat :releasing_lock_on, :item_id => object_id
        end
      end
      error? ? raise(@error) : @result
    end
def value
      unless complete?
        stat :sleeping_on, :item_id => object_id
        @mutex.synchronize do
          stat :has_lock_on, :item_id => object_id
          chore
          stat :releasing_lock_on, :item_id => object_id
        end
      end
      error? ? raise(@error) : @result
    end
Create a promise to do the chore specified by the block.
 Return the value of this promise.

 If the promised chore is not yet complete, then do the work
 synchronously. We will wait.