Improve deprecation message for enqueue returning false

And make sure new applications in Rails 6.0 has this config enabled.

Also, improve test coverage and add a CHANGELOG entry.

activejob/CHANGELOG.md

+*   Return false instead of the job instance when `enqueue` is aborted.
+
+    This will be the behavior in Rails 6.1 but it can be controlled now with
+    `config.active_job.return_false_on_aborted_enqueue`.
+
+    *Kir Shatrov*
+
 *   Keep executions for each specific declaration
 
     Each `retry_on` declaration has now its own specific executions counter. Before it was

activejob/lib/active_job/enqueuing.rb

@@ -49,21 +49,29 @@ def enqueue(options = {})
       self.queue_name   = self.class.queue_name_from_part(options[:queue]) if options[:queue]
       self.priority     = options[:priority].to_i if options[:priority]
       successfully_enqueued = false
+
       run_callbacks :enqueue do
         if scheduled_at
           self.class.queue_adapter.enqueue_at self, scheduled_at
         else
           self.class.queue_adapter.enqueue self
         end
+
         successfully_enqueued = true
       end
+
       if successfully_enqueued
         self
       else
         if self.class.return_false_on_aborted_enqueue
           false
         else
-          ActiveSupport::Deprecation.warn "this will return false, set config.active_job.return_false_on_aborted_enqueue = true to remove deprecation."
+          ActiveSupport::Deprecation.warn(
+            "Rails 6.0 will return false when the enqueing is aborted. Make sure your code doesn't depend on it" \
+            " returning the instance of the job and set `config.active_job.return_false_on_aborted_enqueue = true`" \
+            " to remove the deprecations."
+          )
+
           self
         end
       end

guides/source/configuring.md

@@ -782,6 +782,8 @@ There are a few configuration options available in Active Support:
 
 * `config.active_job.custom_serializers` allows to set custom argument serializers. Defaults to `[]`.
 
+* `config.active_job.return_false_on_aborted_enqueue` change the return value of `#enqueue` to false instead of the job instance when the enqueuing is aborted. Defaults to `false`.
+
 ### Configuring Action Cable
 
 * `config.action_cable.url` accepts a string for the URL for where

railties/lib/rails/application/configuration.rb

@@ -127,6 +127,10 @@ def load_defaults(target_version)
           if respond_to?(:action_dispatch)
             action_dispatch.use_cookies_with_metadata = true
           end
+
+          if respond_to?(:active_job)
+            active_job.return_false_on_aborted_enqueue = true
+          end
         else
           raise "Unknown version #{target_version.to_s.inspect}"
         end

railties/test/application/configuration_test.rb

@@ -2139,6 +2139,33 @@ class ::DummySerializer < ActiveJob::Serializers::ObjectSerializer; end
       assert_equal false, ActionView::Template.finalize_compiled_template_methods
     end
 
+    test "ActiveJob::Base.return_false_on_aborted_enqueue is true by default" do
+      app "development"
+
+      assert_equal true, ActiveJob::Base.return_false_on_aborted_enqueue
+    end
+
+    test "ActiveJob::Base.return_false_on_aborted_enqueue is false in the 5.x defaults" do
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "5.2"'
+
+      app "development"
+
+      assert_equal false, ActiveJob::Base.return_false_on_aborted_enqueue
+    end
+
+    test "ActiveJob::Base.return_false_on_aborted_enqueue can be configured in the new framework defaults" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
+        Rails.application.config.active_job.return_false_on_aborted_enqueue = true
+      RUBY
+
+      app "development"
+
+      assert_equal true, ActiveJob::Base.return_false_on_aborted_enqueue
+    end
+
     test "ActiveRecord::Base.filter_attributes should equal to filter_parameters" do
       app_file "config/initializers/filter_parameters_logging.rb", <<-RUBY
         Rails.application.config.filter_parameters += [ :password, :credit_card_number ]