Add ActivityPub Link object type handling

Author: angusmcleod

lib/discourse_activity_pub/ap/link.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+module DiscourseActivityPub
+  module AP
+    class Link
+      attr_accessor :value
+
+      def initialize(value = nil)
+        @value = value
+      end
+
+      def type
+        'Link'
+      end
+
+      def href
+        return value if value.is_a?(String)
+        return value[:href] if value.is_a?(Hash)
+      end
+
+      def media_type
+        return value[:mediaType] if value.is_a?(Hash)
+      end
+
+      def name
+        return value[:name] if value.is_a?(Hash)
+      end
+    end
+  end
+end

lib/discourse_activity_pub/ap/object.rb

@@ -51,7 +51,11 @@ def actor?
       end
 
       def url
-        stored.respond_to?(:url) && stored&.url
+        if stored
+          stored.respond_to?(:url) && stored&.url
+        elsif json.present?
+          Link.new(json[:url])
+        end
       end
 
       def audience
@@ -95,7 +99,11 @@ def summary
       end
 
       def name
-        stored.respond_to?(:name) && stored&.name
+        if stored.present?
+          stored.respond_to?(:name) && stored&.name
+        elsif json.present?
+          json[:name]
+        end
       end
 
       def context
@@ -110,6 +118,19 @@ def delivered_to
         @delivered_to ||= []
       end
 
+      def media_type
+        json[:mediaType] if json.present?
+      end
+
+      def attachment
+        if json.present? && json[:attachment].present?
+          json[:attachment].each_with_object([]) do |attachment_json, result|
+            obj = AP::Object.factory(attachment_json)
+            result << obj if obj.present?
+          end
+        end
+      end
+
       def cache
         @cache ||= {}
       end

plugin.rb

@@ -47,6 +47,7 @@
   require_relative "lib/discourse_activity_pub/context_resolver"
   require_relative "lib/discourse_activity_pub/ap"
   require_relative "lib/discourse_activity_pub/ap/handlers"
+  require_relative "lib/discourse_activity_pub/ap/link"
   require_relative "lib/discourse_activity_pub/ap/object"
   require_relative "lib/discourse_activity_pub/ap/actor"
   require_relative "lib/discourse_activity_pub/ap/actor/group"
@@ -1164,16 +1165,22 @@
                   )
           end
 
-          if object.json[:attachment].present?
-            object.json[:attachment].each do |attachment|
+          if object.attachment.present?
+            object.attachment.each do |attachment|
+              # Some platforms (e.g. Mastodon) put attachment url media types on the attachment itself,
+              # instead of on a Link object in the url attribute. Technically this violates the specification,
+              # but we need to support it nevertheless. See further https://www.w3.org/TR/activitystreams-vocabulary/#dfn-mediatype
+              media_type = attachment.url.media_type || attachment.media_type
+              name = attachment.url.name || attachment.name
+
               begin
                 DiscourseActivityPubAttachment.create(
                   object_id: object.stored.id,
                   object_type: "DiscourseActivityPubObject",
-                  ap_type: attachment[:type],
-                  url: attachment[:url],
-                  name: attachment[:name],
-                  media_type: attachment[:mediaType],
+                  ap_type: attachment.type,
+                  url: attachment.url.href,
+                  name: name,
+                  media_type: media_type,
                 )
               rescue ActiveRecord::RecordInvalid => error
                 # fail silently if an attachment does not validate

spec/lib/discourse_activity_pub/ap/activity/create_spec.rb

@@ -477,6 +477,20 @@
             { type: "Image", mediaType: "invalid-type", url: attachment_url_3 },
           ]
         end
+        let!(:attachments_with_link_objects) do
+          [
+            {
+              type: "Document",
+              url: {
+                type: "Link",
+                href: attachment_url_1,
+                mediaType: "image/png",
+                name: attachment_name_1,
+              },
+            },
+            { type: "Image", mediaType: "image/jpeg", url: attachment_url_2 },
+          ]
+        end
 
         context "with supported media types" do
           let!(:new_post_json) do
@@ -588,6 +602,34 @@
             expect(post.present?).to be(true)
           end
         end
+
+        context "with Link objects" do
+          let!(:new_post_json) do
+            build_activity_json(
+              actor: actor,
+              object:
+                build_object_json(
+                  name: "My cool topic title",
+                  attributed_to: actor,
+                  attachments: attachments_with_link_objects,
+                ),
+              type: "Create",
+            )
+          end
+
+          it "creates records for all attachments" do
+            perform_process(new_post_json, delivered_to)
+            object =
+              DiscourseActivityPubObject.find_by(ap_type: "Note", attributed_to_id: actor.ap_id)
+            expect(object.attachments.size).to eq(2)
+            expect(object.attachments.first.ap_type).to eq("Document")
+            expect(object.attachments.first.url).to eq(attachment_url_1)
+            expect(object.attachments.first.media_type).to eq("image/png")
+            expect(object.attachments.second.ap_type).to eq("Image")
+            expect(object.attachments.second.url).to eq(attachment_url_2)
+            expect(object.attachments.second.media_type).to eq("image/jpeg")
+          end
+        end
       end
 
       context "with unsupported attachment types" do