code: func (s *blockStore) openWriteFile(fileNum uint32) (filer, error) {
  	// The current block file needs to be read-write so it is possible to
  	// append to it.  Also, it shouldn't be part of the least recently used
  	// file.
  	filePath := blockFilePath(s.basePath, fileNum)
  	file, err := os.OpenFile(filePath, os.O_RDWR|os.O_CREATE, 0666)
  	if err != nil {
  		str := fmt.Sprintf("failed to open file %q: %v", filePath, err)
  		return nil, makeDbErr(database.ErrDriverSpecific, str, err)
  	}
  
  	return file, nil
  }
docs: // openWriteFile returns a file handle for the passed flat file number in
  // read/write mode.  The file will be created if needed.  It is typically used
  // for the current file that will have all new data appended.  Unlike openFile,
  // this function does not keep track of the open file and it is not subject to
  // the maxOpenFiles limit.