Create a spring_booted ActiveSupport load hook

This load hook is run after the Application has been preloaded. It can be used to set additional paths for the file watcher.

Co-Authored-By: Gannon McGibbon <[email protected]>

lib/spring/application.rb

@@ -129,6 +129,8 @@ def preload
           watcher.add secrets_path
         end
       end
+
+      ActiveSupport.run_load_hooks(:spring_booted, self)
     end
 
     def eager_preload

test/support/acceptance_test.rb

@@ -715,6 +715,16 @@ def exec_name
       test "rails db:system:change" do
         assert_success "bin/rails db:system:change --to=sqlite3"
       end
+
+      test "spring_booted load hook is called when booting the application" do
+        File.write(app.path("config/initializers/spring_booted.rb"), <<~RUBY)
+          ActiveSupport.on_load(:spring_booted) do
+            raise "Argh!"
+          end
+        RUBY
+
+        assert_failure "bin/rails runner ''", stderr: "Argh!"
+      end
     end
   end
 end