DEV: apply allow origin response header for CDN requests. (#11893)

Currently, it creates a CORS error while accessing those static files.

lib/discourse.rb

@@ -17,6 +17,7 @@ end
 
 module Discourse
   DB_POST_MIGRATE_PATH ||= "db/post_migrate"
+  REQUESTED_HOSTNAME ||= "REQUESTED_HOSTNAME"
 
   require 'sidekiq/exception_handler'
   class SidekiqExceptionHandler
@@ -917,6 +918,24 @@ module Discourse
   def self.is_parallel_test?
     ENV['RAILS_ENV'] == "test" && ENV['TEST_ENV_NUMBER']
   end
+
+  CDN_REQUEST_METHODS ||= ["GET", "HEAD", "OPTIONS"]
+
+  def self.is_cdn_request?(env, request_method)
+    return unless CDN_REQUEST_METHODS.include?(request_method)
+
+    cdn_hostnames = GlobalSetting.cdn_hostnames
+    return if cdn_hostnames.blank?
+
+    requested_hostname = env[REQUESTED_HOSTNAME] || env[Rack::HTTP_HOST]
+    cdn_hostnames.include?(requested_hostname)
+  end
+
+  def self.apply_cdn_headers(headers)
+    headers['Access-Control-Allow-Origin'] = '*'
+    headers['Access-Control-Allow-Methods'] = CDN_REQUEST_METHODS.join(", ")
+    headers
+  end
 end
 
 # rubocop:enable Style/GlobalVars