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

spec/services/activitypub/fetch_remote_featured_collection_service_spec.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::FetchRemoteFeaturedCollectionService do
+  subject { described_class.new }
+
+  let(:account) { Fabricate(:remote_account) }
+  let(:uri) { 'https://example.com/featured_collections/1' }
+  let(:status) { 200 }
+  let(:response) do
+    {
+      '@context' => 'https://www.w3.org/ns/activitystreams',
+      'id' => uri,
+      'type' => 'FeaturedCollection',
+      'name' => 'Incredible people',
+      'summary' => 'These are really amazing',
+      'attributedTo' => account.uri,
+      'sensitive' => false,
+      'discoverable' => true,
+      'totalItems' => 0,
+    }
+  end
+
+  before do
+    stub_request(:get, uri)
+      .to_return_json(
+        status: status,
+        body: response,
+        headers: { 'Content-Type' => 'application/activity+json' }
+      )
+  end
+
+  context 'when collection does not exist' do
+    it 'creates a new collection' do
+      collection = nil
+      expect { collection = subject.call(uri) }.to change(Collection, :count).by(1)
+
+      expect(collection.uri).to eq uri
+      expect(collection.name).to eq 'Incredible people'
+    end
+  end
+
+  context 'when collection already exists' do
+    let!(:collection) do
+      Fabricate(:remote_collection, account:, uri:, name: 'temp')
+    end
+
+    it 'returns the existing collection' do
+      expect do
+        expect(subject.call(uri)).to eq collection
+      end.to_not change(Collection, :count)
+    end
+  end
+
+  context 'when the URI can not be fetched' do
+    let(:response) { nil }
+    let(:status) { 404 }
+
+    it 'returns `nil`' do
+      expect(subject.call(uri)).to be_nil
+    end
+  end
+end