code: func (t *Torrent) readAt(b []byte, off int64) (n int, err error) {
  	p := &t.pieces[off/t.info.PieceLength]
  	p.waitNoPendingWrites()
  	return p.Storage().ReadAt(b, off-p.Info().Offset())
  }
docs: // Non-blocking read. Client lock is not required.