Clear all current instances before a reload.

If users added an attribute or otherwise changed a CurrentAttributes subclass
they'd see exceptions on the next page load.

Because `ActiveSupport::CurrentAttributes.current_instances` would keep
references to the old instances from the previous request.

We can fix this by clearing out the `current_attributes` before we unload
constants. Then any change to the model can be autoloaded again since its
slot isn't taken by an old instance.

We'll still have to call reset before we clear so external collaborators,
like Time.zone, won't linger with their current value throughout other code.

2 files changed, 8 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/current_attributes.rb b/activesupport/lib/active_support/current_attributes.rb
index 837471e97d..872b0663c7 100644
--- a/activesupport/lib/active_support/current_attributes.rb
+++ b/activesupport/lib/active_support/current_attributes.rb
@@ -126,6 +126,11 @@ module ActiveSupport
         current_instances.each_value(&:reset)
       end
 
+      def clear_all # :nodoc:
+        reset_all
+        current_instances.clear
+      end
+
       private
         def generated_attribute_methods
           @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
diff --git a/activesupport/lib/active_support/railtie.rb b/activesupport/lib/active_support/railtie.rb
index 39c83f65a3..1b4ecf4d72 100644
--- a/activesupport/lib/active_support/railtie.rb
+++ b/activesupport/lib/active_support/railtie.rb
@@ -8,8 +8,9 @@ module ActiveSupport
     config.eager_load_namespaces << ActiveSupport
 
     initializer "active_support.reset_all_current_attributes_instances" do |app|
-      app.executor.to_run      { ActiveSupport::CurrentAttributes.reset_all }
-      app.executor.to_complete { ActiveSupport::CurrentAttributes.reset_all }
+      app.reloader.before_class_unload { ActiveSupport::CurrentAttributes.clear_all }
+      app.executor.to_run              { ActiveSupport::CurrentAttributes.reset_all }
+      app.executor.to_complete         { ActiveSupport::CurrentAttributes.reset_all }
     end
 
     initializer "active_support.deprecation_behavior" do |app|