docs: def checksum(self, chunk_size=None, progress_callback=None, **kwargs):
          """Compute checksum of file."""
          fp = self.open(mode='rb')
          try:
              value = self._compute_checksum(
                  fp, size=self._size, chunk_size=None,
                  progress_callback=progress_callback)
          except StorageError:
              raise
          finally:
              fp.close()
          return value