Add `action_view.finalize_compiled_template_methods` config option

ActionView::Template instances compile their source to methods on the
ActionView::CompiledTemplates module. To prevent leaks in development
mode, where templates can frequently change, a finalizer is added that
undefines these methods[1] when the templates are garbage-collected.

This is undesirable in the test environment, however, as templates don't
change during the life of the test. Moreover, the cost of undefining a
method is proportional to the number of descendants a class or module
has, since the method cache must be cleared for all descendant classes.

As ActionView::CompiledTemplates is mixed into every
ActionView::TestCase (or in RSpec suites, every view spec example
group), it can end up with a very large number of descendants, and
undefining its methods can become very expensive.

In large test suites, this results in a long delay at the end of the
test suite as all template finalizers are run, only for the process to
then exit.

To avoid this unnecessary cost, this change adds a config option,
`action_view.finalize_compiled_template_methods`, defaulting to true,
and sets it to false in the test environment only.

[1] https://github.com/rails/rails/blob/09b2348f7fc8d4e7191e70e06608c5909067e2aa/actionview/lib/action_view/template.rb#L118-L126

5 files changed, 33 insertions, 1 deletions

diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index 393de562a2..918ae48bed 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,12 @@
+*   Disable `ActionView::Template` finalizers in test environment
+
+    Template finalization can be expensive in large view test suites.
+    Add a configuration option,
+    `action_view.finalize_compiled_template_methods`, and turn it off in
+    the test environment.
+
+    *Simon Coffey*
+
 *   Extract the `confirm` call in its own, overridable method in `rails_ujs`.
     Example :
         Rails.confirm = function(message, element) {
diff --git a/actionview/lib/action_view/railtie.rb b/actionview/lib/action_view/railtie.rb
index 12bdc642d4..12d06bf376 100644
--- a/actionview/lib/action_view/railtie.rb
+++ b/actionview/lib/action_view/railtie.rb
@@ -10,6 +10,7 @@ module ActionView
     config.action_view.embed_authenticity_token_in_remote_forms = nil
     config.action_view.debug_missing_translation = true
     config.action_view.default_enforce_utf8 = nil
+    config.action_view.finalize_compiled_template_methods = true
 
     config.eager_load_namespaces << ActionView
 
@@ -45,6 +46,13 @@ module ActionView
       end
     end
 
+    initializer "action_view.finalize_compiled_template_methods" do |app|
+      ActiveSupport.on_load(:action_view) do
+        ActionView::Template.finalize_compiled_template_methods =
+          app.config.action_view.delete(:finalize_compiled_template_methods)
+      end
+    end
+
     initializer "action_view.logger" do
       ActiveSupport.on_load(:action_view) { self.logger ||= Rails.logger }
     end
diff --git a/actionview/lib/action_view/template.rb b/actionview/lib/action_view/template.rb
index 0c4bb73acb..1c8713d6e3 100644
--- a/actionview/lib/action_view/template.rb
+++ b/actionview/lib/action_view/template.rb
@@ -9,6 +9,9 @@ module ActionView
   class Template
     extend ActiveSupport::Autoload
 
+    mattr_accessor :finalize_compiled_template_methods
+    self.finalize_compiled_template_methods = true
+
     # === Encodings in ActionView::Template
     #
     # ActionView::Template is one of a few sources of potential
@@ -307,7 +310,9 @@ module ActionView
         end
 
         mod.module_eval(source, identifier, 0)
-        ObjectSpace.define_finalizer(self, Finalizer[method_name, mod])
+        if finalize_compiled_template_methods
+          ObjectSpace.define_finalizer(self, Finalizer[method_name, mod])
+        end
       end
 
       def handle_render_error(view, e)
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 8bdba4b3de..7d5ca4b8a7 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -600,6 +600,13 @@ Defaults to `'signed cookie'`.
 
 * `config.action_view.default_enforce_utf8` determines whether forms are generated with a hidden tag that forces older versions of Internet Explorer to submit forms encoded in UTF-8. This defaults to `false`.
 
+* `config.action_view.finalize_compiled_template_methods` determines
+  whether the methods on `ActionView::CompiledTemplates` that templates
+  compile themselves to are removed when template instances are
+  destroyed by the garbage collector. This helps prevent memory leaks in
+  development mode, but for large test suites, disabling this option in
+  the test environment can improve performance. This defaults to `true`.
+
 ### Configuring Action Mailer
 
 There are a number of settings available on `config.action_mailer`:
diff --git a/railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt
index ff4c89219a..fe01b1b0c6 100644
--- a/railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt
@@ -47,4 +47,7 @@ Rails.application.configure do
 
   # Raises error for missing translations
   # config.action_view.raise_on_missing_translations = true
+
+  # Prevent expensive template finalization at end of test suite runs.
+  config.action_view.finalize_compiled_template_methods = false
 end