Fix #3541 breaking on reload with parent_controller set

lib/rails_admin.rb

@@ -32,7 +32,7 @@ def self.config(entity = nil, &block)
     if entity
       RailsAdmin::Config.model(entity, &block)
     elsif block_given?
-      suppress_const_load { yield(RailsAdmin::Config) }
+      RailsAdmin::Config::ConstLoadSuppressor.suppressing { yield(RailsAdmin::Config) }
     else
       RailsAdmin::Config
     end

lib/rails_admin/config.rb

@@ -270,10 +270,12 @@ def parent_controller=(name)
         @parent_controller = name
 
         if defined?(RailsAdmin::ApplicationController) || defined?(RailsAdmin::MainController)
-          RailsAdmin.send(:remove_const, :ApplicationController)
-          RailsAdmin.send(:remove_const, :MainController)
-          load RailsAdmin::Engine.root.join('app/controllers/rails_admin/application_controller.rb')
-          load RailsAdmin::Engine.root.join('app/controllers/rails_admin/main_controller.rb')
+          RailsAdmin::Config::ConstLoadSuppressor.allowing do
+            RailsAdmin.send(:remove_const, :ApplicationController)
+            RailsAdmin.send(:remove_const, :MainController)
+            load RailsAdmin::Engine.root.join('app/controllers/rails_admin/application_controller.rb')
+            load RailsAdmin::Engine.root.join('app/controllers/rails_admin/main_controller.rb')
+          end
         end
       end
 

lib/rails_admin/config/const_load_suppressor.rb

@@ -3,14 +3,42 @@
 module RailsAdmin
   module Config
     module ConstLoadSuppressor
-      def suppress_const_load
-        original = Object.method(:const_missing)
-        Object.define_singleton_method(:const_missing) do |name|
-          ConstProxy.new(name.to_s)
+      class << self
+        @original_const_missing = nil
+
+        def suppressing
+          raise 'Constant Loading is already suppressed' if @original_const_missing
+
+          begin
+            @original_const_missing = Object.method(:const_missing)
+            intercept_const_missing
+            yield
+          ensure
+            Object.define_singleton_method(:const_missing, @original_const_missing)
+            @original_const_missing = nil
+          end
+        end
+
+        def allowing
+          if @original_const_missing
+            begin
+              Object.define_singleton_method(:const_missing, @original_const_missing)
+              yield
+            ensure
+              intercept_const_missing
+            end
+          else
+            yield
+          end
+        end
+
+      private
+
+        def intercept_const_missing
+          Object.define_singleton_method(:const_missing) do |name|
+            ConstProxy.new(name.to_s)
+          end
         end
-        yield
-      ensure
-        Object.define_singleton_method(:const_missing, original)
       end
 
       class ConstProxy < BasicObject

spec/rails_admin/config/const_load_suppressor_spec.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe RailsAdmin::Config::ConstLoadSuppressor do
+  describe '.suppressing' do
+    it 'suppresses constant loading' do
+      expect do
+        subject.suppressing { UnknownConstant }
+      end.not_to raise_error
+    end
+
+    it 'raises the error on recursion' do
+      expect do
+        subject.suppressing do
+          subject.suppressing {}
+        end
+      end.to raise_error(/already suppressed/)
+    end
+  end
+
+  describe '.allowing' do
+    it 'suspends constant loading suppression' do
+      expect do
+        subject.suppressing do
+          subject.allowing { UnknownConstant }
+        end
+      end.to raise_error NameError
+    end
+
+    it 'does not break when suppression is disabled' do
+      expect do
+        subject.allowing {}
+      end.not_to raise_error
+    end
+  end
+end