Only expand ancestors when searching

Not all_groups, since that would expose groups the user does not have
access to

app/controllers/concerns/group_tree.rb

 module GroupTree
   def render_group_tree(groups)
-    if params[:filter].present?
-      @groups = Gitlab::GroupHierarchy.new(groups).all_groups
-      @groups = Gitlab::GroupHierarchy.new(@groups.search(params[:filter])).base_and_ancestors
+    @groups = if params[:filter].present?
+                Gitlab::GroupHierarchy.new(groups.search(params[:filter]))
+                  .base_and_ancestors
               else
                 # Only show root groups if no parent-id is given
-      @groups = groups.where(parent_id: params[:parent_id])
+                groups.where(parent_id: params[:parent_id])
               end
     @groups = @groups.with_selects_for_list
                 .sort(@sort = params[:sort])

spec/controllers/concerns/group_tree_spec.rb

@@ -9,7 +9,7 @@ describe GroupTree do
     include GroupTree # rubocop:disable RSpec/DescribedClass
 
     def index
-      render_group_tree Group.all
+      render_group_tree GroupsFinder.new(current_user).execute
     end
   end
 
@@ -52,6 +52,17 @@ describe GroupTree do
 
         expect(assigns(:groups)).to contain_exactly(group, subgroup)
       end
+
+      it 'does not include groups the user does not have access to' do
+        parent = create(:group, :private)
+        subgroup = create(:group, :private, parent: parent, name: 'filter')
+        subgroup.add_developer(user)
+        _other_subgroup = create(:group, :private, parent: parent, name: 'filte')
+
+        get :index, filter: 'filt', format: :json
+
+        expect(assigns(:groups)).to contain_exactly(parent, subgroup)
+      end
     end
 
     context 'json content' do