Update how delayed extensions are loaded, fixes #3509

Changes.md

@@ -2,11 +2,13 @@
 
 [Sidekiq Changes](https://github.com/mperham/sidekiq/blob/master/Changes.md) | [Sidekiq Pro Changes](https://github.com/mperham/sidekiq/blob/master/Pro-Changes.md) | [Sidekiq Enterprise Changes](https://github.com/mperham/sidekiq/blob/master/Ent-Changes.md)
 
-HEAD
+5.0.3
 -----------
 
 - Fix overriding `class_attribute` core extension from ActiveSupport with Sidekiq one [PikachuEXE, #3499]
 - Allow job logger to be overridden [AlfonsoUceda, #3502]
+- Set a default Redis client identifier for debugging [#3516]
+- Fix "Uninitialized constant" errors on startup with the delayed extensions [#3509]
 
 5.0.2
 -----------

lib/sidekiq/delay.rb

@@ -3,12 +3,17 @@ module Sidekiq
 
     def self.enable_delay!
       if defined?(::ActiveSupport)
-        ActiveSupport.on_load(:active_record) do
         require 'sidekiq/extensions/active_record'
+        require 'sidekiq/extensions/action_mailer'
+
+        # Need to patch Psych so it can autoload classes whose names are serialized
+        # in the delayed YAML.
+        Psych::Visitors::ToRuby.prepend(Sidekiq::Extensions::PsychAutoload)
+
+        ActiveSupport.on_load(:active_record) do
           include Sidekiq::Extensions::ActiveRecord
         end
         ActiveSupport.on_load(:action_mailer) do
-          require 'sidekiq/extensions/action_mailer'
           extend Sidekiq::Extensions::ActionMailer
         end
       end
@@ -17,5 +22,19 @@ module Sidekiq
       Module.__send__(:include, Sidekiq::Extensions::Klass)
     end
 
+    module PsychAutoload
+      def resolve_class(klass_name)
+        # constantize
+        names = klass_name.split('::')
+        names.shift if names.empty? || names.first.empty?
+
+        names.inject(Object) do |constant, name|
+          constant.const_defined?(name) ? constant.const_get(name) : constant.const_missing(name)
+        end
+      rescue NameError
+        super
       end
     end
+  end
+end
+