Fix #6311: Replace relative URLs in CSS only for Premailer (#6335)

config/webpack/configuration.js

@@ -27,7 +27,7 @@ function formatPublicPath(host = '', path = '') {
 
 const output = {
   path: resolve('public', settings.public_output_path),
-  publicPath: formatPublicPath(env.ASSET_HOST || env.WEB_DOMAIN || env.LOCAL_DOMAIN, settings.public_output_path),
+  publicPath: formatPublicPath(env.ASSET_HOST, settings.public_output_path),
 };
 
 module.exports = {

lib/mastodon/premailer_webpack_strategy.rb

@@ -2,16 +2,21 @@
 
 module PremailerWebpackStrategy
   def load(url)
-    public_path_host = ENV['ASSET_HOST'] || ENV['LOCAL_DOMAIN']
-    url = url.gsub(/\A\/\/#{public_path_host}/, '')
+    asset_host = ENV['ASSET_HOST'] || ENV['WEB_DOMAIN'] || ENV['LOCAL_DOMAIN']
 
     if Webpacker.dev_server.running?
-      url = File.join("#{Webpacker.dev_server.protocol}://#{Webpacker.dev_server.host_with_port}", url)
+      asset_host = "#{Webpacker.dev_server.protocol}://#{Webpacker.dev_server.host_with_port}"
+      url        = File.join(asset_host, url)
+    end
+
+    css = if url.start_with?('http')
             HTTP.get(url).to_s
           else
             url = url[1..-1] if url.start_with?('/')
             File.read(Rails.root.join('public', url))
           end
+
+    css.gsub(/url\(\//, "url(#{asset_host}/")
   end
 
   module_function :load