Fix Webpack manifest not working with single-threaded Web servers

The removal of the webpack-rails gem in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42485 changed the
way the manifest was loaded from the Webpack dev server. Instead of
contacting directly to the HTTP port 3809, the change caused an external
HTTP(s) request to be made to the GDK URL
(e.g. https://gdk.test:3001/assets/manifest.json).  This would cause a
single-threaded like Thin to lock up.

This change restores the separate `manifest_host` and `manifest_port`
that was previously used to contact the Webpack server directly.

config/initializers/static_files.rb

@@ -21,12 +21,12 @@ if app.config.public_file_server.enabled
     settings = {
       enabled: true,
       host: dev_server.host,
+      manifest_host: dev_server.host,
+      manifest_port: dev_server.port,
       port: dev_server.port
     }
 
     if Rails.env.development?
-      # /assets are proxied through a Rails middlware to the Webpack
-      # server, so we have to use the local Rails settings.
       settings.merge!(
         host: Gitlab.config.gitlab.host,
         port: Gitlab.config.gitlab.port,

lib/gitlab/webpack/manifest.rb

@@ -88,10 +88,9 @@ module Gitlab
         end
 
         def load_dev_server_manifest
-          host = ::Rails.configuration.webpack.dev_server.host
-          port = ::Rails.configuration.webpack.dev_server.port
-          scheme = ::Rails.configuration.webpack.dev_server.https ? 'https' : 'http'
-          uri = Addressable::URI.new(scheme: scheme, host: host, port: port, path: dev_server_path)
+          host = ::Rails.configuration.webpack.dev_server.manifest_host
+          port = ::Rails.configuration.webpack.dev_server.manifest_port
+          uri = Addressable::URI.new(scheme: 'http', host: host, port: port, path: dev_server_path)
 
           # localhost could be blocked via Gitlab::HTTP
           response = HTTParty.get(uri.to_s, verify: false) # rubocop:disable Gitlab/HTTParty

spec/lib/gitlab/webpack/manifest_spec.rb

@@ -41,7 +41,9 @@ RSpec.describe Gitlab::Webpack::Manifest do
   before do
     # Test that config variables work while we're here
     ::Rails.configuration.webpack.dev_server.host = 'hostname'
-    ::Rails.configuration.webpack.dev_server.port = 2000
+    ::Rails.configuration.webpack.dev_server.port = 1999
+    ::Rails.configuration.webpack.dev_server.manifest_host = 'hostname'
+    ::Rails.configuration.webpack.dev_server.manifest_port = 2000
     ::Rails.configuration.webpack.manifest_filename = "my_manifest.json"
     ::Rails.configuration.webpack.public_path = "public_path"
     ::Rails.configuration.webpack.output_dir = "manifest_output"