Merge branch '330622-show-default-default-branch-for-empty-projects' into 'master'

Show the default "default branch" in the API for empty projects

See merge request gitlab-org/gitlab!64944

lib/api/entities/basic_project_details.rb

@@ -6,7 +6,7 @@ module API
       include ::API::ProjectsRelationBuilder
       include Gitlab::Utils::StrongMemoize
 
-      expose :default_branch, if: -> (project, options) { Ability.allowed?(options[:current_user], :download_code, project) }
+      expose :default_branch_or_main, as: :default_branch, if: -> (project, options) { Ability.allowed?(options[:current_user], :download_code, project) }
       # Avoids an N+1 query: https://github.com/mbleigh/acts-as-taggable-on/issues/91#issuecomment-168273770
 
       expose :topic_names, as: :tag_list

spec/lib/api/entities/basic_project_details_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe API::Entities::BasicProjectDetails do
+  let_it_be(:project) { create(:project) }
+
+  let(:current_user) { project.owner }
+
+  subject(:output) { described_class.new(project, current_user: current_user).as_json }
+
+  describe '#default_branch' do
+    it 'delegates to Project#default_branch_or_main' do
+      expect(project).to receive(:default_branch_or_main).twice.and_call_original
+
+      expect(output).to include(default_branch: project.default_branch_or_main)
+    end
+
+    context 'anonymous user' do
+      let(:current_user) { nil }
+
+      it 'is not included' do
+        expect(output.keys).not_to include(:default_branch)
+      end
+    end
+  end
+end