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, 91 insertions, 7 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 30e2bdd1e1..f59ad476ae 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,8 +1,30 @@
-*   Deprecate returning `false` as a way to halt callback chains.
+*   Change the way in which callback chains can be halted.
 
-    Returning `false` in a callback will display a deprecation warning
-    explaining that the preferred method to halt a callback chain is to
-    explicitly `throw(:abort)`.
+    The preferred method to halt a callback chain from now on is to explicitly
+    `throw(:abort)`.
+    In the past, returning `false` in an ActiveSupport callback had the side
+    effect of halting the callback chain. This is not recommended anymore and,
+    depending on the value of
+    `Callbacks::CallbackChain.halt_and_display_warning_on_return_false`, will
+    either not work at all or display a deprecation warning.
+
+
+*   Add Callbacks::CallbackChain.halt_and_display_warning_on_return_false
+
+    Setting `Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
+    to true will let an app support the deprecated way of halting callback
+    chains by returning `false`.
+
+    Setting the value to false will tell the app to ignore any `false` value
+    returned by callbacks, and only halt the chain upon `throw(:abort)`.
+
+    The value can also be set with the Rails configuration option
+    `config.active_support.halt_callback_chains_on_return_false`.
+
+    When the configuration option is missing, its value is `true`, so older apps
+    ported to Rails 5.0 will not break (but display a deprecation warning).
+    For new Rails 5.0 apps, its value is set to `false` in an initializer, so
+    these apps will support the new behavior by default.
 
     *claudiob*
 
@@ -13,8 +35,7 @@
 
     Chains of callbacks defined with a `:terminator` option will maintain their
     existing behavior of halting as soon as a `before_` callback matches the
-    terminator's expectation. For instance, ActiveModel's callbacks will still
-    halt the chain when a `before_` callback returns `false`.
+    terminator's expectation.
 
     *claudiob*
 
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|
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