Fix permission checking when for epic todos

Since we support confidential epics, it's not sufficient to check user
can read the epic's group, we should check that user can read the epic
itself.

ee/app/controllers/groups/todos_controller.rb

@@ -13,7 +13,7 @@ class Groups::TodosController < Groups::ApplicationController
     strong_memoize(:epic) do
       next if params[:issuable_type] != 'epic'
 
-      @group.epics.find_by(id: params[:issuable_id])
+      EpicsFinder.new(current_user, group_id: @group.id).find(params[:issuable_id])
     end
   end
   # rubocop: enable CodeReuse/ActiveRecord

ee/spec/controllers/groups/todos_controller_spec.rb

@@ -3,10 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Groups::TodosController do
-  let(:user)   { create(:user) }
-  let(:group)  { create(:group, :private) }
-  let(:epic)   { create(:epic, group: group) }
-  let(:parent) { group }
+  let_it_be(:user) { create(:user) }
 
   describe 'POST create' do
     def post_create
@@ -19,6 +16,50 @@ RSpec.describe Groups::TodosController do
         format: :json
     end
 
-    it_behaves_like 'todos actions'
+    shared_examples_for 'todo for inaccessible resource' do
+      it 'does not create todo because resource can not be found' do
+        sign_in(user)
+
+        expect do
+          post_create
+        end.to change { user.todos.count }.by(0)
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    context 'when epic is not confidential' do
+      let_it_be(:group) { create(:group, :private) }
+      let_it_be(:epic) { create(:epic, group: group) }
+      let(:parent) { group }
+
+      context 'when epics are available' do
+        before do
+          stub_licensed_features(epics: true)
+        end
+
+        it_behaves_like 'todos actions'
+      end
+
+      context 'when epics are not available' do
+        before do
+          stub_licensed_features(epics: false)
+          group.add_developer(user)
+        end
+
+        it_behaves_like 'todo for inaccessible resource'
+      end
+    end
+
+    context 'when the user can not access confidential epic in public group' do
+      let_it_be(:group) { create(:group) }
+      let_it_be(:epic) { create(:epic, :confidential, group: group) }
+
+      before do
+        stub_licensed_features(epics: true)
+      end
+
+      it_behaves_like 'todo for inaccessible resource'
+    end
   end
 end