1 files changed, 68 insertions, 0 deletions
diff --git a/app/services/fetch_resource_service.rb b/app/services/fetch_resource_service.rb
new file mode 100644
index 00000000000..c0473f3ad29
--- /dev/null
+++ b/app/services/fetch_resource_service.rb
@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+class FetchResourceService < BaseService
+  include JsonLdHelper
+
+  ACCEPT_HEADER = 'application/activity+json, application/ld+json; profile="https://www.w3.org/ns/activitystreams", text/html'
+
+  def call(url)
+    return if url.blank?
+
+    process(url)
+  rescue HTTP::Error, OpenSSL::SSL::SSLError, Addressable::URI::InvalidURIError, Mastodon::HostValidationError, Mastodon::LengthValidationError => e
+    Rails.logger.debug "Error fetching resource #{@url}: #{e}"
+    nil
+  end
+
+  private
+
+  def process(url, terminal = false)
+    @url = url
+
+    perform_request { |response| process_response(response, terminal) }
+  end
+
+  def perform_request(&block)
+    Request.new(:get, @url).add_headers('Accept' => ACCEPT_HEADER).perform(&block)
+  end
+
+  def process_response(response, terminal = false)
+    return nil if response.code != 200
+
+    if ['application/activity+json', 'application/ld+json'].include?(response.mime_type)
+      body = response.body_with_limit
+      json = body_to_json(body)
+
+      [json['id'], { prefetched_body: body, id: true }, :activitypub] if supported_context?(json) && (equals_or_includes_any?(json['type'], ActivityPub::FetchRemoteAccountService::SUPPORTED_TYPES) || expected_type?(json))
+    elsif !terminal
+      link_header = response['Link'] && parse_link_header(response)
+
+      if link_header&.find_link(%w(rel alternate))
+        process_link_headers(link_header)
+      elsif response.mime_type == 'text/html'
+        process_html(response)
+      end
+    end
+  end
+
+  def expected_type?(json)
+    equals_or_includes_any?(json['type'], ActivityPub::Activity::Create::SUPPORTED_TYPES + ActivityPub::Activity::Create::CONVERTED_TYPES)
+  end
+
+  def process_html(response)
+    page      = Nokogiri::HTML(response.body_with_limit)
+    json_link = page.xpath('//link[@rel="alternate"]').find { |link| ['application/activity+json', 'application/ld+json; profile="https://www.w3.org/ns/activitystreams"'].include?(link['type']) }
+
+    process(json_link['href'], terminal: true) unless json_link.nil?
+  end
+
+  def process_link_headers(link_header)
+    json_link = link_header.find_link(%w(rel alternate), %w(type application/activity+json)) || link_header.find_link(%w(rel alternate), ['type', 'application/ld+json; profile="https://www.w3.org/ns/activitystreams"'])
+
+    process(json_link.href, terminal: true) unless json_link.nil?
+  end
+
+  def parse_link_header(response)
+    LinkHeader.parse(response['Link'].is_a?(Array) ? response['Link'].first : response['Link'])
+  end
+end