New service to fetch remote collections (#38298)

app/services/activitypub/fetch_remote_featured_collection_service.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchRemoteFeaturedCollectionService < BaseService
+  include JsonLdHelper
+
+  def call(uri, on_behalf_of = nil)
+    json = fetch_resource(uri, true, on_behalf_of)
+
+    return unless supported_context?(json)
+    return unless json['type'] == 'FeaturedCollection'
+
+    # Fetching an unknown account should eventually also fetch its
+    # collections, so it should be OK to only handle known accounts here
+    account = Account.find_by(uri: json['attributedTo'])
+    return unless account
+
+    existing_collection = account.collections.find_by(uri:)
+    return existing_collection if existing_collection.present?
+
+    ActivityPub::ProcessFeaturedCollectionService.new.call(account, json)
+  end
+end

app/services/activitypub/process_featured_collection_service.rb

@@ -27,7 +27,7 @@ class ActivityPub::ProcessFeaturedCollectionService
         tag_name: @json.dig('topic', 'name')
       )
 
-      process_items!
+      process_items! if @json['totalItems'].positive?
 
       @collection
     end