How to Write Buffer Plugin

def evacuate_chunk(chunk)
  unless chunk.is_a?(Fluent::Plugin::Buffer::FileChunk)
    raise ArgumentError, "The chunk must be FileChunk, but it was #{chunk.class}."
  end

  backup_dir = File.join(backup_base_dir, 'buffer', safe_owner_id)
  FileUtils.mkdir_p(backup_dir, mode: system_config.dir_permission || Fluent::DEFAULT_DIR_PERMISSION) unless Dir.exist?(backup_dir)

  FileUtils.copy([chunk.path, chunk.meta_path], backup_dir)
  log.warn "chunk files are evacuated to #{backup_dir}.", chunk_id: dump_unique_id_hex(chunk.unique_id)
rescue => e
  log.error "unexpected error while evacuating chunk files.", error: e
end