discourse/app/jobs/regular/crawl_topic_link.rb

require 'open-uri'
require 'nokogiri'
require 'excon'
require_dependency 'retrieve_title'
require_dependency 'topic_link'

module Jobs
  class CrawlTopicLink < Jobs::Base

    def execute(args)
      raise Discourse::InvalidParameters.new(:topic_link_id) unless args[:topic_link_id].present?

      topic_link = TopicLink.find_by(id: args[:topic_link_id], internal: false, crawled_at: nil)
      return if topic_link.blank?

      # Look for a topic embed for the URL. If it exists, use its title and don't crawl
      topic_embed = TopicEmbed.where(embed_url: topic_link.url).includes(:topic).references(:topic).first
      # topic could be deleted, so skip
      if topic_embed && topic_embed.topic
        TopicLink.where(id: topic_link.id).update_all(['title = ?, crawled_at = CURRENT_TIMESTAMP', topic_embed.topic.title[0..255]])
        return
      end

      begin
        crawled = false

        # Special case: Images
        # If the link is to an image, put the filename as the title
        if FileHelper.is_supported_image?(topic_link.url)
          uri = URI(topic_link.url)
          filename = File.basename(uri.path)
          crawled = (TopicLink.where(id: topic_link.id).update_all(["title = ?, crawled_at = CURRENT_TIMESTAMP", filename]) == 1)
        end

        unless crawled
          # Fetch the beginning of the document to find the title
          title = RetrieveTitle.crawl(topic_link.url)
          if title.present?
            crawled = (TopicLink.where(id: topic_link.id).update_all(['title = ?, crawled_at = CURRENT_TIMESTAMP', title[0..254]]) == 1)
          end
        end
      rescue Exception
        # If there was a connection error, do nothing
      ensure
        TopicLink.where(id: topic_link.id).update_all('crawled_at = CURRENT_TIMESTAMP') if !crawled && topic_link.present?
      end
    end

  end
end