code: func newMockGetDataFunc(mockStore *mock.NodeStore) func(addr Address) (data []byte, err error) {
  	return func(addr Address) (data []byte, err error) {
  		data, err = mockStore.Get(addr)
  		if err == mock.ErrNotFound {
  			// preserve ErrChunkNotFound error
  			err = ErrChunkNotFound
  		}
  		return data, err
  	}
  }
docs: // newMockGetFunc returns a function that reads chunk data from
  // the mock database, which is used as the value for DbStore.getFunc
  // to bypass the default functionality of DbStore with a mock store.