Read as much as possible, but not more than length.

def _ReadPartial(self, length):
    """Read as much as possible, but not more than length."""
    chunk = self.offset // self.chunksize
    chunk_offset = self.offset % self.chunksize

    # If we're past the end of the file, we don't have a chunk to read from, so
    # we can't read anymore. We return the empty string here so we can read off
    # the end of a file without raising, and get as much data as is there.
    if chunk > self.last_chunk:
      return ""

    available_to_read = min(length, self.chunksize - chunk_offset)

    fd = self._GetChunkForReading(chunk)

    fd.seek(chunk_offset)

    result = fd.read(available_to_read)
    self.offset += len(result)

    return result