clearing autoloaded constants triggers routes reloading [Fixes #10685]

Conflicts:
	railties/test/application/loading_test.rb

1 files changed, 27 insertions, 8 deletions

diff --git a/railties/lib/rails/application/finisher.rb b/railties/lib/rails/application/finisher.rb
index 3ae60312c5..7a1bb1e25c 100644
--- a/railties/lib/rails/application/finisher.rb
+++ b/railties/lib/rails/application/finisher.rb
@@ -62,17 +62,28 @@ module Rails
         ActiveSupport.run_load_hooks(:after_initialize, self)
       end
 
-      # Set app reload just after the finisher hook to ensure
-      # routes added in the hook are still loaded.
+      # Set routes reload after the finisher hook to ensure routes added in
+      # the hook are taken into account.
       initializer :set_routes_reloader_hook do
         reloader = routes_reloader
         reloader.execute_if_updated
         self.reloaders << reloader
-        ActionDispatch::Reloader.to_prepare { reloader.execute_if_updated }
+        ActionDispatch::Reloader.to_prepare do
+          # We configure #execute rather than #execute_if_updated because if
+          # autoloaded constants are cleared we need to reload routes also in
+          # case any was used there, as in
+          #
+          #   mount MailPreview => 'mail_view'
+          #
+          # This means routes are also reloaded if i18n is updated, which
+          # might not be necessary, but in order to be more precise we need
+          # some sort of reloaders dependency support, to be added.
+          reloader.execute
+        end
       end
 
-      # Set app reload just after the finisher hook to ensure
-      # paths added in the hook are still loaded.
+      # Set clearing dependencies after the finisher hook to ensure paths
+      # added in the hook are taken into account.
       initializer :set_clear_dependencies_hook, group: :all do
         callback = lambda do
           ActiveSupport::DescendantsTracker.clear
@@ -82,9 +93,17 @@ module Rails
         if config.reload_classes_only_on_change
           reloader = config.file_watcher.new(*watchable_args, &callback)
           self.reloaders << reloader
-          # We need to set a to_prepare callback regardless of the reloader result, i.e.
-          # models should be reloaded if any of the reloaders (i18n, routes) were updated.
-          ActionDispatch::Reloader.to_prepare(prepend: true){ reloader.execute }
+
+          # Prepend this callback to have autoloaded constants cleared before
+          # any other possible reloading, in case they need to autoload fresh
+          # constants.
+          ActionDispatch::Reloader.to_prepare(prepend: true) do
+            # In addition to changes detected by the file watcher, if routes
+            # or i18n have been updated we also need to clear constants,
+            # that's why we run #execute rather than #execute_if_updated, this
+            # callback has to clear autoloaded constants after any update.
+            reloader.execute
+          end
         else
           ActionDispatch::Reloader.to_cleanup(&callback)
         end