Updates underlying hashers with a given buffer.

    Args:
      buf: A byte buffer (string object) that is going to be fed to the hashers.

def HashBuffer(self, buf):
    """Updates underlying hashers with a given buffer.

    Args:
      buf: A byte buffer (string object) that is going to be fed to the hashers.
    """
    for hasher in itervalues(self._hashers):
      hasher.update(buf)
      if self._progress:
        self._progress()

    self._bytes_read += len(buf)