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, 50 insertions, 0 deletions

diff --git a/activesupport/test/callbacks_test.rb b/activesupport/test/callbacks_test.rb
index 73b00e80f6..f6abef8cee 100644
--- a/activesupport/test/callbacks_test.rb
+++ b/activesupport/test/callbacks_test.rb
@@ -571,6 +571,17 @@ module CallbacksTest
     set_save_callbacks
   end
 
+  class CallbackFalseTerminator < AbstractCallbackTerminator
+    define_callbacks :save
+
+    def second
+      @history << "second"
+      false
+    end
+
+    set_save_callbacks
+  end
+
   class CallbackObject
     def before(caller)
       caller.record << "before"
@@ -754,6 +765,45 @@ module CallbacksTest
     end
   end
 
+  class CallbackFalseTerminatorWithoutConfigTest < ActiveSupport::TestCase
+    def test_returning_false_halts_callback_if_config_variable_is_not_set
+      obj = CallbackFalseTerminator.new
+      assert_deprecated do
+        obj.save
+        assert_equal :second, obj.halted
+        assert !obj.saved
+      end
+    end
+  end
+
+  class CallbackFalseTerminatorWithConfigTrueTest < ActiveSupport::TestCase
+    def setup
+      ActiveSupport::Callbacks::CallbackChain.halt_and_display_warning_on_return_false = true
+    end
+
+    def test_returning_false_halts_callback_if_config_variable_is_true
+      obj = CallbackFalseTerminator.new
+      assert_deprecated do
+        obj.save
+        assert_equal :second, obj.halted
+        assert !obj.saved
+      end
+    end
+  end
+
+  class CallbackFalseTerminatorWithConfigFalseTest < ActiveSupport::TestCase
+    def setup
+      ActiveSupport::Callbacks::CallbackChain.halt_and_display_warning_on_return_false = false
+    end
+
+    def test_returning_false_does_not_halt_callback_if_config_variable_is_false
+      obj = CallbackFalseTerminator.new
+      obj.save
+      assert_equal nil, obj.halted
+      assert obj.saved
+    end
+  end
+
   class HyphenatedKeyTest < ActiveSupport::TestCase
     def test_save
       obj = HyphenatedCallbacks.new