class PostThumbnailUploadValidator
  MAX_THUMBNAIL_BYTES = 20 * 1024 * 1024

  class InvalidUpload < StandardError; end

  def self.validate! thumbnail
    return if thumbnail.blank?
    raise InvalidUpload, 'thumbnail upload が不正です．' unless thumbnail.is_a?(ActionDispatch::Http::UploadedFile)
    raise InvalidUpload, 'thumbnail file size が大きすぎます．' if thumbnail.size > MAX_THUMBNAIL_BYTES

    attachment = Post.resized_thumbnail_attachment(thumbnail)
    attachment[:io].close if attachment[:io].respond_to?(:close)
  rescue MiniMagick::Error
    raise InvalidUpload, 'サムネイル画像の変換に失敗しました．'
  ensure
    thumbnail&.rewind if thumbnail.respond_to?(:rewind)
  end
end