Add config to determine Rails initialization order

Starting the agent after :load_config_initializers prevents the dynamic
initializer for roadie-rails from running, making the gem moot. The customer
who used the bugfix also had an error pop up with one of their
initializers. Though this change solves the problem with configs getting
applied, it may interact unpredictably with existing initializers.
We won't turn on this behavior by default until a major version release.

lib/new_relic/agent/configuration/default_source.rb

@@ -1850,6 +1850,15 @@ def self.enforce_fallback(allowed_values: nil, fallback: nil)
           :allowed_from_server => false,
           :description => 'Specify a custom host name for [display in the New Relic UI](/docs/apm/new-relic-apm/maintenance/add-rename-remove-hosts#display_name).'
         },
+        # Rails
+        :'defer_rails_initialization' => {
+          :default => false,
+          :public => true,
+          :type => Boolean,
+          :allowed_from_server => true,
+          :description => 'If `true`, when the agent is in an application using Ruby on Rails, it will start after ' \
+            'config/initializers are run.'
+        },
         # Rake
         :'rake.tasks' => {
           :default => [],

lib/new_relic/control/instance_methods.rb

@@ -65,7 +65,7 @@ def init_plugin(options = {})
         # An artifact of earlier implementation, we put both #add_method_tracer and #trace_execution
         # methods in the module methods.
         # Rails applications load the next two lines before any other initializers are run
-        unless defined?(Rails::VERSION)
+        unless defined?(Rails::VERSION) && NewRelic::Agent.config[:defer_rails_initialization]
           Module.send(:include, NewRelic::Agent::MethodTracer::ClassMethods)
           Module.send(:include, NewRelic::Agent::MethodTracer)
         end

lib/newrelic_rpm.rb

@@ -20,13 +20,19 @@
 if defined?(Rails::VERSION)
   module NewRelic
     class Railtie < Rails::Railtie
-      initializer "newrelic_rpm.include_method_tracers", before: :load_config_initializers do |app|
-        Module.send(:include, NewRelic::Agent::MethodTracer::ClassMethods)
-        Module.send(:include, NewRelic::Agent::MethodTracer)
-      end
+      if NewRelic::Agent.config[:defer_rails_initialization]
+        initializer "newrelic_rpm.include_method_tracers", before: :load_config_initializers do |app|
+          Module.send(:include, NewRelic::Agent::MethodTracer::ClassMethods)
+          Module.send(:include, NewRelic::Agent::MethodTracer)
+        end
 
-      initializer "newrelic_rpm.start_plugin", after: :load_config_initializers do |app|
-        NewRelic::Control.instance.init_plugin(config: app.config)
+        initializer "newrelic_rpm.start_plugin", after: :load_config_initializers do |app|
+          NewRelic::Control.instance.init_plugin(config: app.config)
+        end
+      else
+        initializer "newrelic_rpm.start_plugin", before: :load_config_initializers do |app|
+          NewRelic::Control.instance.init_plugin(config: app.config)
+        end
       end
     end
   end

test/new_relic/newrelic_rpm_test.rb

@@ -10,38 +10,76 @@ class NewRelicRpmTest < Minitest::Test
 
   # We have documentation recommending customers call add_method_tracer
   # in their initializers, let's make sure this works
-  def test_add_method_tracer_in_initializer_gets_traced
+  def test_add_method_tracer_in_initializer_gets_traced_when_agent_initialized_before_config_initializers
     skip unless defined?(Rails::VERSION)
     skip 'MySQL error for Rails 3.2' if Rails::VERSION::MAJOR == 3
 
-    assert Bloodhound.newrelic_method_exists?('sniff'),
-      'Bloodhound#sniff not found by' \
-      'NewRelic::Agent::MethodTracer::ClassMethods::AddMethodTracer.newrelic_method_exists?'
-    assert Bloodhound.method_traced?('sniff'),
-      'Bloodhound#sniff not found by' \
-      'NewRelic::Agent::MethodTracer::ClassMethods::AddMethodTracer.method_traced?'
+    with_config(defer_rails_initialization: false) do
+      assert Bloodhound.newrelic_method_exists?('sniff'),
+        'Bloodhound#sniff not found by' \
+        'NewRelic::Agent::MethodTracer::ClassMethods::AddMethodTracer.newrelic_method_exists?'
+      assert Bloodhound.method_traced?('sniff'),
+        'Bloodhound#sniff not found by' \
+        'NewRelic::Agent::MethodTracer::ClassMethods::AddMethodTracer.method_traced?'
+    end
+  end
+
+  def test_add_method_tracer_in_initializer_gets_traced_when_agent_initialized_after_config_initializers
+    skip unless defined?(Rails::VERSION)
+    skip 'MySQL error for Rails 3.2 if we define an initializer in config/initializers' if Rails::VERSION::MAJOR == 3
+
+    with_config(defer_rails_initialization: true) do
+      assert Bloodhound.newrelic_method_exists?('sniff'),
+        'Bloodhound#sniff not found by' \
+        'NewRelic::Agent::MethodTracer::ClassMethods::AddMethodTracer.newrelic_method_exists?'
+      assert Bloodhound.method_traced?('sniff'),
+        'Bloodhound#sniff not found by' \
+        'NewRelic::Agent::MethodTracer::ClassMethods::AddMethodTracer.method_traced?'
+    end
   end
 
   # All supported Rails versions have the default value for
   # timestamped_migrations as true. Our initializer sets it to false.
   # Test to resolve: https://github.com/newrelic/newrelic-ruby-agent/issues/662
-  def test_active_record_initializer_config_change_saved
+
+  def test_active_record_initializer_config_change_saved_when_agent_initialized_after_config_initializers
     skip unless defined?(Rails::VERSION)
     # TODO: This test passes in a Rails console in a playground app and in the customer's environment
     # but fails in this unit test context
     skip if Gem::Version.new(NewRelic::VERSION::STRING) >= Gem::Version.new('8.13.0')
     skip 'MySQL error for Rails 3.2' if Rails::VERSION::MAJOR == 3
 
-    # Verify the configuration value was set to the initializer value
-    refute Rails.application.config.active_record.timestamped_migrations,
-      "Rails.application.config.active_record.timestamped_migrations equals true, expected false"
+    with_config(defer_rails_initialization: true) do
+      # Verify the configuration value was set to the initializer value
+      refute Rails.application.config.active_record.timestamped_migrations,
+        "Rails.application.config.active_record.timestamped_migrations equals true, expected false"
+
+      # Verify the configuration value was applied to the ActiveRecord class variable
+      if Gem::Version.new(Rails::VERSION::STRING) >= Gem::Version.new('7.1')
+        refute ActiveRecord.timestamped_migrations, "ActiveRecord.timestamped_migrations equals true, expected false"
+      else
+        refute ActiveRecord::Base.timestamped_migrations,
+          "ActiveRecord::Base.timestamped_migrations equals true, expected false"
+      end
+    end
+  end
+
+  def test_active_record_initializer_config_change_not_saved_when_agent_initialized_before_config_initializers
+    skip unless defined?(Rails::VERSION)
+    skip 'MySQL error for Rails 3.2' if Rails::VERSION::MAJOR == 3
+
+    with_config(defer_rails_initialization: false) do
+      # Verify the configuration value was set to the initializer value
+      refute Rails.application.config.active_record.timestamped_migrations,
+        "Rails.application.config.active_record.timestamped_migrations equals true, expected false"
 
-    # Verify the configuration value was applied to the ActiveRecord class variable
-    if Gem::Version.new(Rails::VERSION::STRING) >= Gem::Version.new('7.1')
-      refute ActiveRecord.timestamped_migrations, "ActiveRecord.timestamped_migrations equals true, expected false"
-    else
-      refute ActiveRecord::Base.timestamped_migrations,
-        "ActiveRecord::Base.timestamped_migrations equals true, expected false"
+      # Rails.application.config value should not be applied (this is the state of the original bug)
+      if Gem::Version.new(Rails::VERSION::STRING) >= Gem::Version.new('7.1')
+        assert ActiveRecord.timestamped_migrations, "ActiveRecord.timestamped_migrations equals false, expected true"
+      else
+        refute ActiveRecord::Base.timestamped_migrations,
+          "ActiveRecord::Base.timestamped_migrations equals false, expected true"
+      end
     end
   end
 end