class Rake::Promise

def value

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

Return the value of this promise. 
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