Do not memoize auto/eager load paths in engines

railties/lib/rails/engine.rb

@@ -578,7 +578,7 @@ def load_seed
     end
 
     initializer :set_eager_load_paths, before: :bootstrap_hook do
-      ActiveSupport::Dependencies._eager_load_paths.merge(config.eager_load_paths)
+      ActiveSupport::Dependencies._eager_load_paths.merge(config.all_eager_load_paths)
       config.eager_load_paths.freeze
     end
 
@@ -705,14 +705,14 @@ def default_middleware_stack
       end
 
       def _all_autoload_once_paths
-        config.autoload_once_paths.uniq
+        config.all_autoload_once_paths.uniq
       end
 
       def _all_autoload_paths
         @_all_autoload_paths ||= begin
-          autoload_paths  = config.autoload_paths
-          autoload_paths += config.eager_load_paths
-          autoload_paths -= config.autoload_once_paths
+          autoload_paths  = config.all_autoload_paths
+          autoload_paths += config.all_eager_load_paths
+          autoload_paths -= config.all_autoload_once_paths
           autoload_paths.uniq
         end
       end

railties/lib/rails/engine/configuration.rb

@@ -9,12 +9,45 @@ class Configuration < ::Rails::Railtie::Configuration
       attr_accessor :middleware, :javascript_path
       attr_writer :eager_load_paths, :autoload_once_paths, :autoload_paths
 
+      # An array of custom autoload paths to be added to the ones defined
+      # automatically by Rails. These won't be eager loaded, unless you push
+      # them to +eager_load_paths+ too, which is recommended.
+      #
+      # This collection is empty by default, it accepts strings and +Pathname+
+      # objects.
+      #
+      # If you'd like to add +lib+ to it, please see +autoload_lib+.
+      attr_reader :autoload_paths
+
+      # An array of custom autoload once paths. These won't be eager loaded
+      # unless you push them to +eager_load_paths+ too, which is recommended.
+      #
+      # This collection is empty by default, it accepts strings and +Pathname+
+      # objects.
+      #
+      # If you'd like to add +lib+ to it, please see +autoload_lib_once+.
+      attr_reader :autoload_once_paths
+
+      # An array of custom eager load paths to be added to the ones defined
+      # automatically by Rails. Anything in this collection is considered to be
+      # an autoload path regardless of whether it was added to +autoload_paths+.
+      #
+      # This collection is empty by default, it accepts strings and +Pathname+
+      # objects.
+      #
+      # If you'd like to add +lib+ to it, please see +autoload_lib+.
+      attr_reader :eager_load_paths
+
       def initialize(root = nil)
         super()
         @root = root
         @generators = app_generators.dup
         @middleware = Rails::Configuration::MiddlewareStackProxy.new
         @javascript_path = "javascript"
+
+        @autoload_paths = []
+        @autoload_once_paths = []
+        @eager_load_paths = []
       end
 
       # Holds generators configuration:
@@ -81,16 +114,22 @@ def root=(value)
         @root = paths.path = Pathname.new(value).expand_path
       end
 
-      def eager_load_paths
-        @eager_load_paths ||= paths.eager_load
+      # Private method that adds custom autoload paths to the ones defined by
+      # +paths+.
+      def all_autoload_paths # :nodoc:
+        autoload_paths + paths.autoload_paths
       end
 
-      def autoload_once_paths
-        @autoload_once_paths ||= paths.autoload_once
+      # Private method that adds custom autoload once paths to the ones defined
+      # by +paths+.
+      def all_autoload_once_paths # :nodoc:
+        autoload_once_paths + paths.autoload_once
       end
 
-      def autoload_paths
-        @autoload_paths ||= paths.autoload_paths
+      # Private method that adds custom eager load paths to the ones defined by
+      # +paths+.
+      def all_eager_load_paths # :nodoc:
+        eager_load_paths + paths.eager_load
       end
     end
   end

railties/test/application/configuration_test.rb

@@ -662,6 +662,30 @@ def assert_utf8
       assert_utf8
     end
 
+    # Regression test for https://github.com/rails/rails/issues/49629.
+    test "config.paths can be mutated after accessing auto/eager load paths" do
+      app_dir "vendor/auto"
+      app_dir "vendor/once"
+      app_dir "vendor/eager"
+
+      add_to_config <<~RUBY
+        # Reading the collections is enough, no need to modify them.
+        config.autoload_paths
+        config.autoload_once_paths
+        config.eager_load_paths
+
+        config.paths.add("vendor/auto", autoload: true)
+        config.paths.add("vendor/once", autoload_once: true)
+        config.paths.add("vendor/eager", eager_load: true)
+      RUBY
+
+      app "development"
+
+      assert_includes ActiveSupport::Dependencies.autoload_paths, "#{Rails.root}/vendor/auto"
+      assert_includes ActiveSupport::Dependencies.autoload_once_paths, "#{Rails.root}/vendor/once"
+      assert_includes ActiveSupport::Dependencies._eager_load_paths, "#{Rails.root}/vendor/eager"
+    end
+
     test "config.paths.public sets Rails.public_path" do
       add_to_config <<-RUBY
         config.paths["public"] = "somewhere"