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

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

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

railties/test/application/mailer_previews_test.rb

@@ -225,6 +225,46 @@ module ApplicationTests
       assert_no_match '<li><a href="/rails/mailers/notifier/bar">bar</a></li>', last_response.body
     end
 
+    test "mailer previews are reloaded from a custom preview_path" do
+      add_to_config "config.action_mailer.preview_path = '#{app_path}/lib/mailer_previews'"
+
+      app('development')
+
+      get "/rails/mailers"
+      assert_no_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      app_file 'lib/mailer_previews/notifier_preview.rb', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+
+      remove_file 'lib/mailer_previews/notifier_preview.rb'
+      sleep(1)
+
+      get "/rails/mailers"
+      assert_no_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+    end
+
     test "mailer preview not found" do
       app('development')
       get "/rails/mailers/notifier"
@@ -366,7 +406,7 @@ module ApplicationTests
     private
       def build_app
         super
-        app_file 'config/routes.rb', "Rails.application.routes.draw do; end"
+        app_file "config/routes.rb", "Rails.application.routes.draw do; end"
       end
 
       def mailer(name, contents)