Refactor AS::Callbacks halt config and fix the documentation

Move from `AS::Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
to `AS::Callbacks.halt_and_display_warning_on_return_false` base on
[this
discussion](https://github.com/rails/rails/pull/21218#discussion_r39354580)

Fix the documentation broken by 0a120a818d413c64ff9867125f0b03788fc306f8

4 files changed, 15 insertions, 17 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index a39344e464..19588d622c 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -291,18 +291,15 @@
     In the past, callbacks could only be halted by explicitly providing a
     terminator and by having a callback match the conditions of the terminator.
 
-*   Add `Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
+*   Add `ActiveSupport.halt_callback_chains_on_return_false`
 
-    Setting `Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
+    Setting `ActiveSupport.halt_callback_chains_on_return_false`
     to `true` will let an app support the deprecated way of halting Active Record,
-    Active Model and Active Model validations callback chains by returning `false`.
+    and Active Model callback chains by returning `false`.
 
     Setting the value to `false` will tell the app to ignore any `false` value
     returned by those 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
diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 588d6c49f9..63277a65b4 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -75,11 +75,11 @@ module ActiveSupport
   cattr_accessor :test_order # :nodoc:
 
   def self.halt_callback_chains_on_return_false
-    Callbacks::CallbackChain.halt_and_display_warning_on_return_false
+    Callbacks.halt_and_display_warning_on_return_false
   end
 
   def self.halt_callback_chains_on_return_false=(value)
-    Callbacks::CallbackChain.halt_and_display_warning_on_return_false = value
+    Callbacks.halt_and_display_warning_on_return_false = value
   end
 end
 
diff --git a/activesupport/lib/active_support/callbacks.rb b/activesupport/lib/active_support/callbacks.rb
index 3db9ea2ac0..252374e817 100644
--- a/activesupport/lib/active_support/callbacks.rb
+++ b/activesupport/lib/active_support/callbacks.rb
@@ -4,6 +4,7 @@ require 'active_support/core_ext/array/extract_options'
 require 'active_support/core_ext/class/attribute'
 require 'active_support/core_ext/kernel/reporting'
 require 'active_support/core_ext/kernel/singleton_class'
+require 'active_support/core_ext/module/attribute_accessors'
 require 'active_support/core_ext/string/filters'
 require 'active_support/deprecation'
 require 'thread'
@@ -66,6 +67,12 @@ module ActiveSupport
 
     CALLBACK_FILTER_TYPES = [:before, :after, :around]
 
+    # If true, Active Record and Active Model callbacks 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+.
+    mattr_accessor(:halt_and_display_warning_on_return_false) { true }
+
     # Runs the callbacks for the given event.
     #
     # Calls the before and around callbacks in the order they were set, yields
@@ -450,12 +457,6 @@ 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 = {
@@ -760,7 +761,7 @@ module ActiveSupport
           terminate = true
           catch(:abort) do
             result = result_lambda.call if result_lambda.is_a?(Proc)
-            if CallbackChain.halt_and_display_warning_on_return_false && result == false
+            if Callbacks.halt_and_display_warning_on_return_false && result == false
               display_deprecation_warning_for_false_terminator
             else
               terminate = false
diff --git a/activesupport/test/callbacks_test.rb b/activesupport/test/callbacks_test.rb
index 4f47e0519f..3b00ff87a0 100644
--- a/activesupport/test/callbacks_test.rb
+++ b/activesupport/test/callbacks_test.rb
@@ -776,7 +776,7 @@ module CallbacksTest
 
   class CallbackFalseTerminatorWithConfigTrueTest < ActiveSupport::TestCase
     def setup
-      ActiveSupport::Callbacks::CallbackChain.halt_and_display_warning_on_return_false = true
+      ActiveSupport::Callbacks.halt_and_display_warning_on_return_false = true
     end
 
     def test_returning_false_does_not_halt_callback_if_config_variable_is_true
@@ -789,7 +789,7 @@ module CallbacksTest
 
   class CallbackFalseTerminatorWithConfigFalseTest < ActiveSupport::TestCase
     def setup
-      ActiveSupport::Callbacks::CallbackChain.halt_and_display_warning_on_return_false = false
+      ActiveSupport::Callbacks.halt_and_display_warning_on_return_false = false
     end
 
     def test_returning_false_does_not_halt_callback_if_config_variable_is_false