Fix invalid visibility string comparison in project import

This resolves an "ArgumentError: comparison of String with 0 failed"
issue where the visibility_level is stored as a string in the project
import data because the value comes directly from the Web form. This
problem happened upon creating a project from a template or restoring a
project.

We now cast the value to an integer to guard against these kinds of
failures.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/61692

changelogs/unreleased/sh-project-import-visibility-error.yml

+---
+title: Fix invalid visibility string comparison in project import
+merge_request: 28612
+author:
+type: fixed

lib/gitlab/import_export/project_tree_restorer.rb

@@ -129,7 +129,7 @@ module Gitlab
 
       def visibility_level
         level = override_params['visibility_level'] || json_params['visibility_level'] || @project.visibility_level
-        level = @project.group.visibility_level if @project.group && level > @project.group.visibility_level
+        level = @project.group.visibility_level if @project.group && level.to_i > @project.group.visibility_level
 
         { 'visibility_level' => level }
       end

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -328,6 +328,19 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
     end
 
     context 'when the project has overridden params in import data' do
+      it 'handles string versions of visibility_level' do
+        # Project needs to be in a group for visibility level comparison
+        # to happen
+        group = create(:group)
+        project.group = group
+
+        project.create_import_data(data: { override_params: { visibility_level: Gitlab::VisibilityLevel::INTERNAL.to_s } })
+
+        restored_project_json
+
+        expect(project.visibility_level).to eq(Gitlab::VisibilityLevel::INTERNAL)
+      end
+
       it 'overwrites the params stored in the JSON' do
         project.create_import_data(data: { override_params: { description: "Overridden" } })