Teach ActiveJob to set configs on itself

Previously configs of the form `config.active_job.X` were only forwarded to
`ActiveJob::Base`. This teaches Active Job to set them on `ActiveJob` directly
instead, if the setter exists.

For consistency, this more or less mirrors the way that Active Record does it.

Co-authored-by: Adrianna Chang <adrianna.chang@shopify.com>
Co-authored-by: Sam Bostock <sam.bostock@shopify.com>

---

Fix use_big_decimal_serializer Rails 7.1 default

This config should be enabled for new Rails 7.1 apps, or apps that have updated
their config to `load_defaults 7.1`, not disabled.

This also clarifies the config accessor comment.

---

Add contributor documentation comment to load_defaults

The process for introducing a change in behavior in Rails can be confusing to
new contributors, so a comment is added roughly explaining how to do so, and
what belongs in `load_defaults` and `new_framework_defaults`.

This comment is aimed at contributors, not consumers, so it is added within the
method, rather than above it.

activejob/lib/active_job.rb

@@ -46,7 +46,7 @@ module ActiveJob
   # :singleton-method:
   # If false, Rails will preserve the legacy serialization of BigDecimal job arguments as Strings.
   # If true, Rails will use the new BigDecimalSerializer to (de)serialize BigDecimal losslessly.
-  # This behavior will be removed in Rails 7.2.
+  # Legacy serialization will be removed in Rails 7.2, along with this config.
   singleton_class.attr_accessor :use_big_decimal_serializer
   self.use_big_decimal_serializer = false
 end

activejob/lib/active_job/railtie.rb

@@ -25,16 +25,29 @@ class Railtie < Rails::Railtie # :nodoc:
       options = app.config.active_job
       options.queue_adapter ||= :async
 
+      config.after_initialize do
+        options.each do |k, v|
+          k = "#{k}="
+          if ActiveJob.respond_to?(k)
+            ActiveJob.send(k, v)
+          end
+        end
+      end
+
       ActiveSupport.on_load(:active_job) do
         # Configs used in other initializers
         options = options.except(
           :log_query_tags_around_perform,
           :custom_serializers
         )
 
-        options.each do  |k, v|
+        options.each do |k, v|
           k = "#{k}="
-          send(k, v) if respond_to? k
+          if ActiveJob.respond_to?(k)
+            ActiveJob.send(k, v)
+          elsif respond_to? k
+            send(k, v)
+          end
         end
       end
 

railties/lib/rails/application/configuration.rb

@@ -87,6 +87,20 @@ def initialize(*)
       # {configuration guide}[https://guides.rubyonrails.org/configuring.html#versioned-default-values]
       # for the default values associated with a particular version.
       def load_defaults(target_version)
+        # To introduce a change in behavior, follow these steps:
+        # 1. Add an accessor on the target object (e.g. the ActiveJob class for global Active Job config).
+        # 2. Set a default value there preserving existing behavior for existing applications.
+        # 3. Implement the behavior change based on the config value.
+        # 4. In the section below corresponding to the next release of Rails, set the new value.
+        # 5. Add a commented out section in the `new_framework_defaults` template, setting the new value.
+        # 6. Update the guide in `configuration.md`.
+
+        # To remove configurable deprecated behavior, follow these steps:
+        # 1. Update or remove the entry in the guides.
+        # 2. Remove the references below.
+        # 3. Remove the legacy code paths and config check.
+        # 4. Remove the config accessor.
+
         case target_version.to_s
         when "5.0"
           if respond_to?(:action_controller)
@@ -279,7 +293,7 @@ def load_defaults(target_version)
           end
 
           if respond_to?(:active_job)
-            active_job.use_big_decimal_serializer = false
+            active_job.use_big_decimal_serializer = true
           end
 
           if respond_to?(:active_support)

railties/test/application/configuration_test.rb

@@ -2656,6 +2656,43 @@ class ::DummySerializer < ActiveJob::Serializers::ObjectSerializer; end
       assert_includes ActiveJob::Serializers.serializers, DummySerializer
     end
 
+    test "use_big_decimal_serializer is enabled in new apps" do
+      app "development"
+
+      # When loaded, ActiveJob::Base triggers the :active_job load hooks, which is where config is attached.
+      # Referencing the constant auto-loads it.
+      ActiveJob::Base
+
+      assert ActiveJob.use_big_decimal_serializer, "use_big_decimal_serializer should be enabled in new apps"
+    end
+
+    test "use_big_decimal_serializer is disabled if using defaults prior to 7.1" do
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "7.0"'
+      app "development"
+
+      # When loaded, ActiveJob::Base triggers the :active_job load hooks, which is where config is attached.
+      # Referencing the constant auto-loads it.
+      ActiveJob::Base
+
+      assert_not ActiveJob.use_big_decimal_serializer, "use_big_decimal_serializer should be disabled in defaults prior to 7.1"
+    end
+
+    test "use_big_decimal_serializer can be enabled in config" do
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "7.0"'
+      app_file "config/initializers/new_framework_defaults_7_1.rb", <<-RUBY
+        Rails.application.config.active_job.use_big_decimal_serializer = true
+      RUBY
+      app "development"
+
+      # When loaded, ActiveJob::Base triggers the :active_job load hooks, which is where config is attached.
+      # Referencing the constant auto-loads it.
+      ActiveJob::Base
+
+      assert ActiveJob.use_big_decimal_serializer, "use_big_decimal_serializer should be enabled if set in config"
+    end
+
     test "active record job queue is set" do
       app "development"