Read a block of data from the file.

def Read(self, length):
    """Read a block of data from the file."""
    result = b""

    # The total available size in the file
    length = int(length)
    length = min(length, self.size - self.offset)

    while length > 0:
      data = self._ReadPartial(length)
      if not data:
        break

      length -= len(data)
      result += data
    return result