Merge branch '49107-prefetching-of-assets-and-cdn-domain' into 'master'

DNS Prefetching + Preconnect of assets_host (CDN Domain)

Closes #49107

See merge request gitlab-org/gitlab-ce!20781

app/views/layouts/_head.html.haml

@@ -3,6 +3,11 @@
 - site_name = "GitLab"
 %head{ prefix: "og: http://ogp.me/ns#" }
   %meta{ charset: "utf-8" }
+
+  - if Feature.enabled?('asset_host_prefetch') && ActionController::Base.asset_host
+    %link{ rel: 'dns-prefetch', href: ActionController::Base.asset_host }
+    %link{ rel: 'preconnnect', href: ActionController::Base.asset_host, crossorigin: '' }
+
   %meta{ 'http-equiv' => 'X-UA-Compatible', content: 'IE=edge' }
 
   -# Open Graph - http://ogp.me/

changelogs/unreleased/49107-prefetching-of-assets-and-cdn-domain.yml

+---
+title: DNS prefetching if asset_host for CDN hosting is set
+merge_request: 20781
+author:
+type: performance

spec/views/layouts/_head.html.haml_spec.rb

@@ -29,6 +29,39 @@ describe 'layouts/_head' do
     expect(rendered).to match(%{content="foo" http-equiv="refresh"})
   end
 
+  context 'when an asset_host is set and feature is activated in the config it will' do
+    let(:asset_host) { 'http://assets' }
+
+    before do
+      stub_feature_flags(asset_host_prefetch: true)
+      allow(ActionController::Base).to receive(:asset_host).and_return(asset_host)
+    end
+
+    it 'add a link dns-prefetch tag' do
+      render
+      expect(rendered).to match('<link href="http://assets" rel="dns-prefetch">')
+    end
+
+    it 'add a link preconnect tag' do
+      render
+      expect(rendered).to match('<link crossorigin="" href="http://assets" rel="preconnnect">')
+    end
+  end
+
+  context 'when an asset_host is set and feature is not activated in the config it will' do
+    let(:asset_host) { 'http://assets' }
+
+    before do
+      stub_feature_flags(asset_host_prefetch: false)
+      allow(ActionController::Base).to receive(:asset_host).and_return(asset_host)
+    end
+
+    it 'not add a link dns-prefetch tag' do
+      render
+      expect(rendered).not_to match('<link href="http://assets" rel="dns-prefetch">')
+    end
+  end
+
   def stub_helper_with_safe_string(method)
     allow_any_instance_of(PageLayoutHelper).to receive(method)
       .and_return(%q{foo" http-equiv="refresh}.html_safe)