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

2 files changed, 14 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/callbacks.rb b/activesupport/lib/active_support/callbacks.rb
index 7dd97eb1ab..0f1de8b076 100644
--- a/activesupport/lib/active_support/callbacks.rb
+++ b/activesupport/lib/active_support/callbacks.rb
@@ -516,6 +516,12 @@ module ActiveSupport
 
       attr_reader :name, :config
 
+      # If true, any callback returning +false+ will halt the entire callback
+      # chain and display a deprecation message. If false, callback chains will
+      # only be halted by calling +throw :abort+. Defaults to +true+.
+      class_attribute :halt_and_display_warning_on_return_false
+      self.halt_and_display_warning_on_return_false = true
+
       def initialize(name, config)
         @name = name
         @config = {
@@ -597,7 +603,7 @@ module ActiveSupport
           terminate = true
           catch(:abort) do
             result = result_lambda.call if result_lambda.is_a?(Proc)
-            if result == false
+            if halt_and_display_warning_on_return_false && result == false
               display_deprecation_warning_for_false_terminator
             else
               terminate = false
diff --git a/activesupport/lib/active_support/railtie.rb b/activesupport/lib/active_support/railtie.rb
index 133aa6a054..6eba24b569 100644
--- a/activesupport/lib/active_support/railtie.rb
+++ b/activesupport/lib/active_support/railtie.rb
@@ -13,6 +13,13 @@ module ActiveSupport
       end
     end
 
+    initializer "active_support.halt_callback_chains_on_return_false", after: :load_config_initializers do |app|
+      if app.config.active_support.key? :halt_callback_chains_on_return_false
+        ActiveSupport::Callbacks::CallbackChain.halt_and_display_warning_on_return_false = \
+          app.config.active_support.halt_callback_chains_on_return_false
+      end
+    end
+
     # Sets the default value for Time.zone
     # If assigned value cannot be matched to a TimeZone, an exception will be raised.
     initializer "active_support.initialize_time_zone" do |app|