Fix `expose_url` helper does not include custom base url if it is set

changelogs/unreleased/fj-46459-fix-expose-url-when-base-url-set.yml

+---
+title: Fixed bug where generated api urls didn't add the base url if set
+merge_request: 19003
+author:
+type: fixed

lib/api/helpers/related_resources_helpers.rb

@@ -13,9 +13,14 @@ module API
 
       def expose_url(path)
         url_options = Gitlab::Application.routes.default_url_options
-        protocol, host, port = url_options.slice(:protocol, :host, :port).values
+        protocol, host, port, script_name = url_options.values_at(:protocol, :host, :port, :script_name)
 
-        URI::Generic.build(scheme: protocol, host: host, port: port, path: path).to_s
+        # Using a blank component at the beginning of the join we ensure
+        # that the resulted path will start with '/'. If the resulted path
+        # does not start with '/', URI::Generic#build will fail
+        path_with_script_name = File.join('', [script_name, path].select(&:present?))
+
+        URI::Generic.build(scheme: protocol, host: host, port: port, path: path_with_script_name).to_s
       end
 
       private

spec/lib/api/helpers/related_resources_helpers_spec.rb

@@ -9,9 +9,9 @@ describe API::Helpers::RelatedResourcesHelpers do
     let(:path) { '/api/v4/awesome_endpoint' }
     subject(:url) { helpers.expose_url(path) }
 
-    def stub_default_url_options(protocol: 'http', host: 'example.com', port: nil)
+    def stub_default_url_options(protocol: 'http', host: 'example.com', port: nil, script_name: '')
       expect(Gitlab::Application.routes).to receive(:default_url_options)
-        .and_return(protocol: protocol, host: host, port: port)
+        .and_return(protocol: protocol, host: host, port: port, script_name: script_name)
     end
 
     it 'respects the protocol if it is HTTP' do
@@ -37,5 +37,17 @@ describe API::Helpers::RelatedResourcesHelpers do
 
       is_expected.to start_with('http://example.com:8080/')
     end
+
+    it 'includes the relative_url before the path if it is set' do
+      stub_default_url_options(script_name: '/gitlab')
+
+      is_expected.to start_with('http://example.com/gitlab/api/v4')
+    end
+
+    it 'includes the path after the host' do
+      stub_default_url_options
+
+      is_expected.to start_with('http://example.com/api/v4')
+    end
   end
 end