Fix GraphiQL when GitLab is installed under a relative URL

When installing GitLab under a relative URL [1], GraphiQL should use a
relative URL too for GraphQL.

[1]: https://docs.gitlab.com/ee/install/relative_url.html

changelogs/unreleased/fix_graphiql_relative_url.yml

+---
+title: Fix GraphiQL when GitLab is installed under a relative URL
+merge_request: 23143
+author: Mathieu Parent
+type: fixed

config/routes/api.rb

 post '/api/graphql', to: 'graphql#execute'
-mount GraphiQL::Rails::Engine, at: '/-/graphql-explorer', graphql_path: '/api/graphql'
+mount GraphiQL::Rails::Engine, at: '/-/graphql-explorer', graphql_path: Gitlab::Utils.append_path(Gitlab.config.gitlab.relative_url_root, '/api/graphql')
 
 ::API::API.logger Rails.logger # rubocop:disable Gitlab/RailsLogger
 mount ::API::API => '/'

spec/features/graphiql_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'GraphiQL' do
+  context 'without relative_url_root' do
+    before do
+      visit '/-/graphql-explorer'
+    end
+
+    it 'has the correct graphQLEndpoint' do
+      expect(page.body).to include('var graphQLEndpoint = "/api/graphql";')
+    end
+  end
+
+  context 'with relative_url_root' do
+    before do
+      stub_config_setting(relative_url_root: '/gitlab/root')
+      Rails.application.reload_routes!
+
+      visit '/-/graphql-explorer'
+    end
+
+    after do
+      Rails.application.reload_routes!
+    end
+
+    it 'has the correct graphQLEndpoint' do
+      expect(page.body).to include('var graphQLEndpoint = "/gitlab/root/api/graphql";')
+    end
+  end
+end