Fix 404s when C++ .gitignore template selected

Due to a overly-stringent regex, the project template API was 404'ing
when C++ was requested as the template. Loosen the regex to allow `+`
and `%` for URL-encoded characters.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57857

changelogs/unreleased/sh-fix-cpp-templates-404.yml

+---
+title: Fix 404s when C++ .gitignore template selected
+merge_request: 25416
+author:
+type: fixed

lib/api/project_templates.rb

@@ -36,7 +36,10 @@ module API
         optional :project, type: String, desc: 'The project name to use when expanding placeholders in the template. Only affects licenses'
         optional :fullname, type: String, desc: 'The full name of the copyright holder to use when expanding placeholders in the template. Only affects licenses'
       end
-      get ':id/templates/:type/:name', requirements: { name: /[\w\.-]+/ } do
+      # The regex is needed to ensure a period (e.g. agpl-3.0)
+      # isn't confused with a format type. We also need to allow encoded
+      # values (e.g. C%2B%2B for C++), so allow % and + as well.
+      get ':id/templates/:type/:name', requirements: { name: /[\w%.+-]+/ } do
         template = TemplateFinder
           .build(params[:type], user_project, name: params[:name])
           .execute

spec/requests/api/project_templates_spec.rb

@@ -92,6 +92,22 @@ describe API::ProjectTemplates do
       expect(json_response['name']).to eq('Actionscript')
     end
 
+    it 'returns C++ gitignore' do
+      get api("/projects/#{public_project.id}/templates/gitignores/C++")
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(response).to match_response_schema('public_api/v4/template')
+      expect(json_response['name']).to eq('C++')
+    end
+
+    it 'returns C++ gitignore for URL-encoded names' do
+      get api("/projects/#{public_project.id}/templates/gitignores/C%2B%2B")
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(response).to match_response_schema('public_api/v4/template')
+      expect(json_response['name']).to eq('C++')
+    end
+
     it 'returns a specific gitlab_ci_yml' do
       get api("/projects/#{public_project.id}/templates/gitlab_ci_ymls/Android")
 
@@ -125,6 +141,18 @@ describe API::ProjectTemplates do
       expect(response).to have_gitlab_http_status(200)
       expect(response).to match_response_schema('public_api/v4/license')
     end
+
+    shared_examples 'path traversal attempt' do |template_type|
+      it 'rejects invalid filenames' do
+        get api("/projects/#{public_project.id}/templates/#{template_type}/%2e%2e%2fPython%2ea")
+
+        expect(response).to have_gitlab_http_status(500)
+      end
+    end
+
+    TemplateFinder::VENDORED_TEMPLATES.each do |template_type, _|
+      it_behaves_like 'path traversal attempt', template_type
+    end
   end
 
   describe 'GET /projects/:id/templates/licenses/:key' do