Add config to halt callback chain on return false

This stems from [a comment](rails#17227 (comment)) by @dhh.
In summary:

* New Rails 5.0 apps will not accept `return false` as a way to halt callback chains, and will not display a deprecation warning.
* Existing apps ported to Rails 5.0 will still accept `return false` as a way to halt callback chains, albeit with a deprecation warning.

For this purpose, this commit introduces a Rails configuration option:

```ruby
config.active_support.halt_callback_chains_on_return_false
```

For new Rails 5.0 apps, this option will be set to `false` by a new initializer
`config/initializers/callback_terminator.rb`:

```ruby
Rails.application.config.active_support.halt_callback_chains_on_return_false = false
```

For existing apps ported to Rails 5.0, the initializers above will not exist.
Even running `rake rails:update` will not create this initializer.

Since the default value of `halt_callback_chains_on_return_false` is set to
`true`, these apps will still accept `return true` as a way to halt callback
chains, displaying a deprecation warning.

Developers will be able to switch to the new behavior (and stop the warning)
by manually adding the line above to their `config/application.rb`.

A gist with the suggested release notes to add to Rails 5.0 after this
commit is available at https://gist.github.com/claudiob/614c59409fb7d11f2931

1 files changed, 32 insertions, 0 deletions

diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index ac8f735cec..e30c9a7b2f 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -160,6 +160,38 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_file("#{app_root}/config/initializers/cookies_serializer.rb", /Rails\.application\.config\.action_dispatch\.cookies_serializer = :json/)
   end
 
+  def test_rails_update_does_not_create_callback_terminator_initializer
+    app_root = File.join(destination_root, 'myapp')
+    run_generator [app_root]
+
+    FileUtils.rm("#{app_root}/config/initializers/callback_terminator.rb")
+
+    Rails.application.config.root = app_root
+    Rails.application.class.stubs(:name).returns("Myapp")
+    Rails.application.stubs(:is_a?).returns(Rails::Application)
+
+    generator = Rails::Generators::AppGenerator.new ["rails"], { with_dispatchers: true }, destination_root: app_root, shell: @shell
+    generator.send(:app_const)
+    quietly { generator.send(:update_config_files) }
+    assert_no_file "#{app_root}/config/initializers/callback_terminator.rb"
+  end
+
+  def test_rails_update_does_not_remove_callback_terminator_initializer_if_already_present
+    app_root = File.join(destination_root, 'myapp')
+    run_generator [app_root]
+
+    FileUtils.touch("#{app_root}/config/initializers/callback_terminator.rb")
+
+    Rails.application.config.root = app_root
+    Rails.application.class.stubs(:name).returns("Myapp")
+    Rails.application.stubs(:is_a?).returns(Rails::Application)
+
+    generator = Rails::Generators::AppGenerator.new ["rails"], { with_dispatchers: true }, destination_root: app_root, shell: @shell
+    generator.send(:app_const)
+    quietly { generator.send(:update_config_files) }
+    assert_file "#{app_root}/config/initializers/callback_terminator.rb"
+  end
+
   def test_rails_update_set_the_cookie_serializer_to_marchal_if_it_is_not_already_configured
     app_root = File.join(destination_root, 'myapp')
     run_generator [app_root]