Eagerly preload framework base classes

CHANGELOG.md

@@ -1,7 +1,16 @@
-## Next Release
+## Unreleased
 
 * Fixed crashes when a client disconnects mid-handshake (e.g. on connect timeout). Previously, `Errno::EPIPE` raised in `Spring::Server#serve` or `Spring::Application#serve` would propagate up through the accept loop and kill the process, leaving a stale socket that broke every subsequent client. Both crash sites are now rescued, including writes that happen inside the `rescue Exception` handler in `Application#serve` while reporting an earlier failure to the gone client.
 
+* Eagerly autoload framework base classes (`ActionMailer::Base`,
+  `ActionController::Base`, `ActionController::API`) at the end of preload
+  so their `ActiveSupport.on_load` hooks fire in the parent process.
+  Without this, the reloader probe in `#serve` materializes Rails
+  internals (notably ActionView's `CacheExpiry::ViewReloader`) in a
+  half-initialized state and triggers an expensive `FileUpdateChecker`
+  rebuild on every `prepend_view_path` inside each fork. See
+  rails/rails#51308 for the lazy-init contract this aligns with.
+
 ## 4.4.0
 
 * Revert the removal of UTF-8 force encoding in JSON loading.

lib/spring/application.rb

@@ -117,6 +117,7 @@ def preload
       require Spring.application_root_path.join("config", "environment")
 
       invoke_after_environment_load_callbacks
+      preload_framework_base_classes
 
       disconnect_database
 
@@ -144,6 +145,19 @@ def preload
       end
     end
 
+    # Eagerly autoload framework base classes
+    FRAMEWORK_BASE_CLASSES = %w[
+      ActionMailer::Base
+      ActionController::Base
+      ActionController::API
+    ].freeze
+
+    def preload_framework_base_classes
+      FRAMEWORK_BASE_CLASSES.each do |const|
+        Object.const_get(const) if Object.const_defined?(const)
+      end
+    end
+
     def eager_preload
       with_pty do
         # we can't see stderr and there could be issues when it's overflown