Add preview_path to autoload_paths in after_initialize

Only config.autoload_paths is frozen, so add the preview_path
to ActiveSupport::Dependencies.autoload_paths directly in an
after_initialize block. Also protect against a blank preview_path
being added to autoload_paths which can cause a serious slowdown
as Dir[] tries to load all *_preview.rb files under /

Fixes #13372

2 files changed, 14 insertions, 6 deletions

diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index 43d9ec4bb5..ecceaf8c70 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -56,12 +56,18 @@ module ActionMailer
 
       protected
         def load_previews #:nodoc:
-          Dir["#{preview_path}/**/*_preview.rb"].each{ |file| require_dependency file }
+          if preview_path?
+            Dir["#{preview_path}/**/*_preview.rb"].each{ |file| require_dependency file }
+          end
         end
 
         def preview_path #:nodoc:
           Base.preview_path
         end
+
+        def preview_path? #:nodoc:
+          Base.preview_path?
+        end
     end
   end
 end
diff --git a/actionmailer/lib/action_mailer/railtie.rb b/actionmailer/lib/action_mailer/railtie.rb
index af8009ba97..c893ddfef5 100644
--- a/actionmailer/lib/action_mailer/railtie.rb
+++ b/actionmailer/lib/action_mailer/railtie.rb
@@ -19,6 +19,10 @@ module ActionMailer
       options.javascripts_dir ||= paths["public/javascripts"].first
       options.stylesheets_dir ||= paths["public/stylesheets"].first
 
+      if Rails.env.development?
+        options.preview_path  ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
+      end
+
       # make sure readers methods get compiled
       options.asset_host          ||= app.config.asset_host
       options.relative_url_root   ||= app.config.relative_url_root
@@ -41,11 +45,9 @@ module ActionMailer
       end
     end
 
-    initializer "action_mailer.configure_mailer_previews", before: :set_autoload_paths do |app|
-      if Rails.env.development?
-        options = app.config.action_mailer
-        options.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
-        app.config.autoload_paths << options.preview_path
+    config.after_initialize do
+      if ActionMailer::Base.preview_path?
+        ActiveSupport::Dependencies.autoload_paths << ActionMailer::Base.preview_path
       end
     end
   end