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

4 files changed, 58 insertions, 0 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index d583be5e73..9165019d0d 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,3 +1,18 @@
+*   Add `config/initializers/callback_terminator.rb`
+
+    Newly generated Rails apps have a new initializer called
+    `callback_terminator.rb` which sets the value of the configuration option
+    `config.active_support.halt_callback_chains_on_return_false` to `false`.
+
+    As a result, new Rails apps do not halt callback chains when a callback
+    returns `false`; only when they are explicitly halted with `throw(:abort)`.
+
+    The terminator is *not* added when running `rake rails:update`, so returning
+    `false` will still work on old apps ported to Rails 5, displaying a
+    deprecation warning to prompt users to update their code to the new syntax.
+
+    *claudiob*
+
 *   Add `--skip-action-mailer` option to the app generator.
 
     *claudiob*
diff --git a/railties/lib/rails/generators/rails/app/app_generator.rb b/railties/lib/rails/generators/rails/app/app_generator.rb
index 1ff1f970b5..c235fbc7f7 100644
--- a/railties/lib/rails/generators/rails/app/app_generator.rb
+++ b/railties/lib/rails/generators/rails/app/app_generator.rb
@@ -88,12 +88,19 @@ module Rails
 
     def config_when_updating
       cookie_serializer_config_exist = File.exist?('config/initializers/cookies_serializer.rb')
+      callback_terminator_config_exist = File.exist?('config/initializers/callback_terminator.rb')
 
       config
 
+      unless callback_terminator_config_exist
+        remove_file 'config/initializers/callback_terminator.rb'
+      end
+
       unless cookie_serializer_config_exist
         gsub_file 'config/initializers/cookies_serializer.rb', /json/, 'marshal'
       end
+
+
     end
 
     def database_yml
diff --git a/railties/lib/rails/generators/rails/app/templates/config/initializers/callback_terminator.rb b/railties/lib/rails/generators/rails/app/templates/config/initializers/callback_terminator.rb
new file mode 100644
index 0000000000..e63022da91
--- /dev/null
+++ b/railties/lib/rails/generators/rails/app/templates/config/initializers/callback_terminator.rb
@@ -0,0 +1,4 @@
+# Be sure to restart your server when you modify this file.
+
+# Do not halt callback chains when a callback returns false.
+Rails.application.config.active_support.halt_callback_chains_on_return_false = false
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]