Merge pull request #17227 from claudiob/explicitly-abort-callbacks

Introduce explicit way of halting callback chains by throwing :abort. Deprecate current implicit behavior of halting callback chains by returning `false` in apps ported to Rails 5.0. Completely remove that behavior in brand new Rails 5.0 apps.

Conflicts:
	railties/CHANGELOG.md

2 files changed, 24 insertions, 0 deletions

diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 0a375d7cb8..5baab8a4b5 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -507,6 +507,8 @@ There are a few configuration options available in Active Support:
 
 * `config.active_support.time_precision` sets the precision of JSON encoded time values. Defaults to `3`.
 
+* `config.active_support.halt_callback_chains_on_return_false` specifies whether ActiveRecord, ActiveModel and ActiveModel::Validations callback chains can be halted by returning `false` in a 'before' callback. Defaults to `true`.
+
 * `ActiveSupport::Logger.silencer` is set to `false` to disable the ability to silence logging in a block. The default is `true`.
 
 * `ActiveSupport::Cache::Store.logger` specifies the logger to use within cache store operations.
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 0b9f59bb46..c0c94475e0 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -53,6 +53,28 @@ Don't forget to review the difference, to see if there were any unexpected chang
 Upgrading from Rails 4.2 to Rails 5.0
 -------------------------------------
 
+### Halting callback chains by returning `false`
+
+In Rails 4.2, when a 'before' callback returns `false` in ActiveRecord,
+ActiveModel and ActiveModel::Validations, then the entire callback chain
+is halted. In other words, successive 'before' callbacks are not executed,
+and neither is the action wrapped in callbacks.
+
+In Rails 5.0, returning `false` in a callback will not have this side effect
+of halting the callback chain. Instead, callback chains must be explicitly
+halted by calling `throw(:abort)`.
+
+When you upgrade from Rails 4.2 to Rails 5.0, returning `false` in a callback
+will still halt the callback chain, but you will receive a deprecation warning
+about this upcoming change.
+
+When you are ready, you can opt into the new behavior and remove the deprecation
+warning by adding the following configuration to your `config/application.rb`:
+
+    config.active_support.halt_callback_chains_on_return_false = false
+
+See [#17227](https://github.com/rails/rails/pull/17227) for more details.
+
 Upgrading from Rails 4.1 to Rails 4.2
 -------------------------------------