Add dns-prefetch if using different host for assets or uploads (#8942)

app/helpers/application_helper.rb

@@ -81,4 +81,20 @@ module ApplicationHelper
     output << 'rtl' if locale_direction == 'rtl'
     output.reject(&:blank?).join(' ')
   end
+
+  def cdn_host
+    ENV['CDN_HOST'].presence
+  end
+
+  def cdn_host?
+    cdn_host.present?
+  end
+
+  def storage_host
+    ENV['S3_ALIAS_HOST'].presence || ENV['S3_CLOUDFRONT_HOST'].presence
+  end
+
+  def storage_host?
+    storage_host.present?
+  end
 end

app/views/layouts/application.html.haml

@@ -3,6 +3,13 @@
   %head
     %meta{ charset: 'utf-8' }/
     %meta{ name: 'viewport', content: 'width=device-width, initial-scale=1' }/
+
+    - if cdn_host?
+      %link{ rel: 'dns-prefetch', href: cdn_host }/
+
+    - if storage_host?
+      %link{ rel: 'dns-prefetch', href: storage_host }/
+
     %link{ rel: 'icon', href: favicon_path, type: 'image/x-icon' }/
     %link{ rel: 'apple-touch-icon', sizes: '180x180', href: '/apple-touch-icon.png' }/
     %link{ rel: 'mask-icon', href: '/mask-icon.svg', color: '#2B90D9' }/