Fix flaky test ActionControllerBaseRenderTest#test_direct_render_to_string

actionpack/test/controller/render_test.rb

@@ -4,6 +4,11 @@ require "abstract_unit"
 require "controller/fake_models"
 
 class TestControllerWithExtraEtags < ActionController::Base
+  self.view_paths = [ActionView::FixtureResolver.new(
+    "test/with_implicit_template.erb" => "Hello explicitly!",
+    "test/hello_world.erb" => "Hello world!"
+  )]
+
   def self.controller_name; "test"; end
   def self.controller_path; "test"; end
 
@@ -37,6 +42,11 @@ class TestControllerWithExtraEtags < ActionController::Base
 end
 
 class ImplicitRenderTestController < ActionController::Base
+  self.view_paths = [ActionView::FixtureResolver.new(
+    "implicit_render_test/hello_world.erb" => "Hello world!",
+    "implicit_render_test/empty_action_with_template.html.erb" => "<h1>Empty action rendered this implicitly.</h1>\n"
+  )]
+
   def empty_action
   end
 
@@ -46,6 +56,10 @@ end
 
 module Namespaced
   class ImplicitRenderTestController < ActionController::Base
+    self.view_paths = [ActionView::FixtureResolver.new(
+      "namespaced/implicit_render_test/hello_world.erb" => "Hello world!"
+    )]
+
     def hello_world
       fresh_when(etag: "abc")
     end
@@ -293,13 +307,15 @@ end
 module TemplateModificationHelper
   private
     def modify_template(name)
-      path = File.expand_path("../fixtures/#{name}.erb", __dir__)
-      original = File.read(path)
-      File.write(path, "#{original} Modified!")
+      hash = @controller.view_paths.first.instance_variable_get(:@hash)
+      key = name + ".erb"
+      original = hash[key]
+      hash[key] = "#{original} Modified!"
       ActionView::LookupContext::DetailsKey.clear
       yield
     ensure
-      File.write(path, original)
+      hash[key] = original
+      ActionView::LookupContext::DetailsKey.clear
     end
 end