Support additional LabelsFinder parameters for group labels

In some situations (listing labels for epics) we want to
list only group ancestor labels, this is already supported
in LabelsFinder we just need to enable additional parameters.

Also `set_issuables_index` method now loads project labels only if
@project is set (which is not used for epic group labels).

app/controllers/concerns/issuable_collections.rb

@@ -17,7 +17,7 @@ module IssuableCollections
     set_pagination
     return if redirect_out_of_range(@total_pages)
 
-    if params[:label_name].present?
+    if params[:label_name].present? && @project
       labels_params = { project_id: @project.id, title: params[:label_name] }
       @labels = LabelsFinder.new(current_user, labels_params).execute
     end

app/controllers/groups/labels_controller.rb

@@ -14,7 +14,13 @@ class Groups::LabelsController < Groups::ApplicationController
       end
 
       format.json do
-        available_labels = LabelsFinder.new(current_user, group_id: @group.id).execute
+        available_labels = LabelsFinder.new(
+          current_user,
+          group_id: @group.id,
+          only_group_labels: params[:only_group_labels],
+          include_ancestor_groups: params[:include_ancestor_groups]
+        ).execute
+
         render json: LabelSerializer.new.represent_appearance(available_labels)
       end
     end

spec/controllers/groups/labels_controller_spec.rb

@@ -3,6 +3,7 @@ require 'spec_helper'
 describe Groups::LabelsController do
   let(:group) { create(:group) }
   let(:user)  { create(:user) }
+  let(:project) { create(:project, namespace: group) }
 
   before do
     group.add_owner(user)
@@ -10,6 +11,34 @@ describe Groups::LabelsController do
     sign_in(user)
   end
 
+  describe 'GET #index' do
+    let!(:label_1) { create(:label, project: project, title: 'label_1') }
+    let!(:group_label_1) { create(:group_label, group: group, title: 'group_label_1') }
+
+    it 'returns group and project labels by default' do
+      get :index, group_id: group, format: :json
+
+      label_ids = json_response.map {|label| label['title']}
+      expect(label_ids).to match_array([label_1.title, group_label_1.title])
+    end
+
+    context 'with ancestor group', :nested_groups do
+      let(:subgroup) { create(:group, parent: group) }
+      let!(:subgroup_label_1) { create(:group_label, group: subgroup, title: 'subgroup_label_1') }
+
+      before do
+        subgroup.add_owner(user)
+      end
+
+      it 'returns ancestor group labels', :nested_groups do
+        get :index, group_id: subgroup, include_ancestor_groups: true, only_group_labels: true, format: :json
+
+        label_ids = json_response.map {|label| label['title']}
+        expect(label_ids).to match_array([group_label_1.title, subgroup_label_1.title])
+      end
+    end
+  end
+
   describe 'POST #toggle_subscription' do
     it 'allows user to toggle subscription on group labels' do
       label = create(:group_label, group: group)