Revert "Revert "Reduce allocations when running AR callbacks.""

This reverts commit bdc1d329d4eea823d07cf010064bd19c07099ff3.

Before:
Calculating -------------------------------------
                        22.000  i/100ms
-------------------------------------------------
                        229.700  (± 0.4%) i/s -      1.166k
Total Allocated Object: 9939

After:
Calculating -------------------------------------
                        24.000  i/100ms
-------------------------------------------------
                        246.443  (± 0.8%) i/s -      1.248k
Total Allocated Object: 7939

```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # gem 'rails', github: 'rails/rails', ref: 'bdc1d329d4eea823d07cf010064bd19c07099ff3'
  gem 'rails', github: 'rails/rails', ref: 'd2876141d08341ec67cf6a11a073d1acfb920de7'
  gem 'arel', github: 'rails/arel'
  gem 'sqlite3'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('sqlite3::memory:')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.boolean :admin
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base
  default_scope { where(admin: true) }
end

admin = true

1000.times do
  attributes = {
    name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
    email: "foobar@email.com",
    admin: admin
  }

  User.create!(attributes)

  admin = !admin
end

GC.disable

Benchmark.ips(5, 3) do |x|
  x.report { User.all.to_a }
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.all.to_a
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

1 files changed, 11 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/callbacks.rb b/activesupport/lib/active_support/callbacks.rb
index e8ab3a7db5..f35e1f5098 100644
--- a/activesupport/lib/active_support/callbacks.rb
+++ b/activesupport/lib/active_support/callbacks.rb
@@ -80,8 +80,12 @@ module ActiveSupport
     #     save
     #   end
     def run_callbacks(kind, &block)
-      callbacks = send("_#{kind}_callbacks")
+      send "_run_#{kind}_callbacks", &block
+    end
+
+    private
 
+    def __run_callbacks__(callbacks, &block)
       if callbacks.empty?
         yield if block_given?
       else
@@ -91,8 +95,6 @@ module ActiveSupport
       end
     end
 
-    private
-
     # A hook invoked every time a before callback is halted.
     # This can be overridden in AS::Callback implementors in order
     # to provide better debugging/logging.
@@ -806,6 +808,12 @@ module ActiveSupport
         names.each do |name|
           class_attribute "_#{name}_callbacks"
           set_callbacks name, CallbackChain.new(name, options)
+
+          module_eval <<-RUBY, __FILE__, __LINE__ + 1
+            def _run_#{name}_callbacks(&block)
+              __run_callbacks__(_#{name}_callbacks, &block)
+            end
+          RUBY
         end
       end