mastodon/app/services/fetch_link_card_service.rb

# frozen_string_literal: true

class FetchLinkCardService < BaseService
  URL_PATTERN = %r{
    (#{Twitter::TwitterText::Regex[:valid_url_preceding_chars]})                                                                #   $1 preceding chars
    (                                                                                                                           #   $2 URL
      (https?:\/\/)                                                                                                             #   $3 Protocol (required)
      (#{Twitter::TwitterText::Regex[:valid_domain]})                                                                           #   $4 Domain(s)
      (?::(#{Twitter::TwitterText::Regex[:valid_port_number]}))?                                                                #   $5 Port number (optional)
      (/#{Twitter::TwitterText::Regex[:valid_url_path]}*)?                                                                      #   $6 URL Path and anchor
      (\?#{Twitter::TwitterText::Regex[:valid_url_query_chars]}*#{Twitter::TwitterText::Regex[:valid_url_query_ending_chars]})? #   $7 Query String
    )
  }iox

  def call(status)
    @status       = status
    @original_url = parse_urls

    return if @original_url.nil? || @status.preview_cards.any?

    @url = @original_url.to_s

    RedisLock.acquire(lock_options) do |lock|
      if lock.acquired?
        @card = PreviewCard.find_by(url: @url)
        process_url if @card.nil? || @card.updated_at <= 2.weeks.ago || @card.missing_image?
      else
        raise Mastodon::RaceConditionError
      end
    end

    attach_card if @card&.persisted?
  rescue HTTP::Error, OpenSSL::SSL::SSLError, Addressable::URI::InvalidURIError, Mastodon::HostValidationError, Mastodon::LengthValidationError => e
    Rails.logger.debug "Error fetching link #{@original_url}: #{e}"
    nil
  end

  private

  def process_url
    @card ||= PreviewCard.new(url: @url)

    attempt_oembed || attempt_opengraph
  end

  def html
    return @html if defined?(@html)

    Request.new(:get, @url).add_headers('Accept' => 'text/html', 'User-Agent' => Mastodon::Version.user_agent + ' Bot').perform do |res|
      # We follow redirects, and ideally we want to save the preview card for
      # the destination URL and not any link shortener in-between, so here
      # we set the URL to the one of the last response in the redirect chain
      @url  = res.request.uri.to_s
      @card = PreviewCard.find_or_initialize_by(url: @url) if @card.url != @url

      if res.code == 200 && res.mime_type == 'text/html'
        @html_charset = res.charset
        @html = res.body_with_limit
      else
        @html_charset = nil
        @html = nil
      end
    end
  end

  def attach_card
    @status.preview_cards << @card
    Rails.cache.delete(@status)
    Trends.links.register(@status)
  end

  def parse_urls
    urls = begin
      if @status.local?
        @status.text.scan(URL_PATTERN).map { |array| Addressable::URI.parse(array[1]).normalize }
      else
        document = Nokogiri::HTML(@status.text)
        links    = document.css('a')

        links.filter_map { |a| Addressable::URI.parse(a['href']) unless skip_link?(a) }.filter_map(&:normalize)
      end
    end

    urls.reject { |uri| bad_url?(uri) }.first
  end

  def bad_url?(uri)
    # Avoid local instance URLs and invalid URLs
    uri.host.blank? || TagManager.instance.local_url?(uri.to_s) || !%w(http https).include?(uri.scheme)
  end

  def mention_link?(anchor)
    @status.mentions.any? do |mention|
      anchor['href'] == ActivityPub::TagManager.instance.url_for(mention.account)
    end
  end

  def skip_link?(anchor)
    # Avoid links for hashtags and mentions (microformats)
    anchor['rel']&.include?('tag') || anchor['class']&.match?(/u-url|h-card/) || mention_link?(anchor)
  end

  def attempt_oembed
    service         = FetchOEmbedService.new
    url_domain      = Addressable::URI.parse(@url).normalized_host
    cached_endpoint = Rails.cache.read("oembed_endpoint:#{url_domain}")

    embed   = service.call(@url, cached_endpoint: cached_endpoint) unless cached_endpoint.nil?
    embed ||= service.call(@url, html: html) unless html.nil?

    return false if embed.nil?

    url = Addressable::URI.parse(service.endpoint_url)

    @card.type          = embed[:type]
    @card.title         = embed[:title]         || ''
    @card.author_name   = embed[:author_name]   || ''
    @card.author_url    = embed[:author_url].present? ? (url + embed[:author_url]).to_s : ''
    @card.provider_name = embed[:provider_name] || ''
    @card.provider_url  = embed[:provider_url].present? ? (url + embed[:provider_url]).to_s : ''
    @card.width         = 0
    @card.height        = 0

    case @card.type
    when 'link'
      @card.image_remote_url = (url + embed[:thumbnail_url]).to_s if embed[:thumbnail_url].present?
    when 'photo'
      return false if embed[:url].blank?

      @card.embed_url        = (url + embed[:url]).to_s
      @card.image_remote_url = (url + embed[:url]).to_s
      @card.width            = embed[:width].presence  || 0
      @card.height           = embed[:height].presence || 0
    when 'video'
      @card.width            = embed[:width].presence  || 0
      @card.height           = embed[:height].presence || 0
      @card.html             = Formatter.instance.sanitize(embed[:html], Sanitize::Config::MASTODON_OEMBED)
      @card.image_remote_url = (url + embed[:thumbnail_url]).to_s if embed[:thumbnail_url].present?
    when 'rich'
      # Most providers rely on <script> tags, which is a no-no
      return false
    end

    @card.save_with_optional_image!
  end

  def attempt_opengraph
    return if html.nil?

    link_details_extractor = LinkDetailsExtractor.new(@url, @html, @html_charset)

    @card = PreviewCard.find_or_initialize_by(url: link_details_extractor.canonical_url) if link_details_extractor.canonical_url != @card.url
    @card.assign_attributes(link_details_extractor.to_preview_card_attributes)
    @card.save_with_optional_image! unless @card.title.blank? && @card.html.blank?
  end

  def lock_options
    { redis: Redis.current, key: "fetch:#{@original_url}", autorelease: 15.minutes.seconds }
  end
end