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

[tgxworld]

😿 Reverting my own revert of 796cab4.

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}"

View full graphs:
http://rubybench.org/rails/rails/commits?result_type=activerecord/postgres_finders_all_with_default_scope
http://rubybench.org/rails/rails/commits?result_type=activerecord/sqlite3_finders_all_with_default_scope

Side note: MySQL is alot slower...
http://rubybench.org/rails/rails/commits?result_type=activerecord/mysql2_finders_all_with_default_scope&display_count=200

[tgxworld]

Renamed to __run_callbacks__ from _run_callbacks to avoid breaking #19405.

[rafaelfranca]

Thank you for the work!

[tgxworld]

@rafaelfranca Updated 😄

Thank you for reviewing and sorry for the ping pong with the reverts 😝