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}"
```

8 files changed, 32 insertions, 23 deletions

diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index 72094a26c0..5f1dde4aa3 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -404,7 +404,7 @@ module ActiveModel
   protected
 
     def run_validations! #:nodoc:
-      run_callbacks :validate
+      _run_validate_callbacks
       errors.empty?
     end
 
diff --git a/activemodel/lib/active_model/validations/callbacks.rb b/activemodel/lib/active_model/validations/callbacks.rb
index b4301c23e4..4b58ef66e3 100644
--- a/activemodel/lib/active_model/validations/callbacks.rb
+++ b/activemodel/lib/active_model/validations/callbacks.rb
@@ -109,7 +109,7 @@ module ActiveModel
 
       # Overwrite run validations to include callbacks.
       def run_validations! #:nodoc:
-        run_callbacks(:validation) { super }
+        _run_validation_callbacks { super }
       end
     end
   end
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index cd79266952..1aa6a2ca74 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -133,7 +133,7 @@ module ActiveRecord
             if scope.klass.primary_key
               count = scope.destroy_all.length
             else
-              scope.each { |record| record.run_callbacks :destroy }
+              scope.each(&:_run_destroy_callbacks)
 
               arel = scope.arel
 
diff --git a/activerecord/lib/active_record/callbacks.rb b/activerecord/lib/active_record/callbacks.rb
index 3027ce928e..19f0dca5a6 100644
--- a/activerecord/lib/active_record/callbacks.rb
+++ b/activerecord/lib/active_record/callbacks.rb
@@ -289,24 +289,25 @@ module ActiveRecord
     end
 
     def destroy #:nodoc:
-      run_callbacks(:destroy) { super }
+      _run_destroy_callbacks { super }
     end
 
     def touch(*) #:nodoc:
-      run_callbacks(:touch) { super }
+      _run_touch_callbacks { super }
     end
 
   private
+
     def create_or_update(*) #:nodoc:
-      run_callbacks(:save) { super }
+      _run_save_callbacks { super }
     end
 
     def _create_record #:nodoc:
-      run_callbacks(:create) { super }
+      _run_create_callbacks { super }
     end
 
     def _update_record(*) #:nodoc:
-      run_callbacks(:update) { super }
+      _run_update_callbacks { super }
     end
   end
 end
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index 6535121075..282af220fb 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -508,7 +508,7 @@ module ActiveRecord
         synchronize do
           remove_connection_from_thread_cache conn
 
-          conn.run_callbacks :checkin do
+          conn._run_checkin_callbacks do
             conn.expire
           end
 
@@ -764,7 +764,7 @@ module ActiveRecord
       end
 
       def checkout_and_verify(c)
-        c.run_callbacks :checkout do
+        c._run_checkout_callbacks do
           c.verify!
         end
         c
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 8a014e682e..b82488a59c 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -303,7 +303,7 @@ module ActiveRecord
       assign_attributes(attributes) if attributes
 
       yield self if block_given?
-      run_callbacks :initialize
+      _run_initialize_callbacks
     end
 
     # Initialize an empty model object from +coder+. +coder+ should be
@@ -330,8 +330,8 @@ module ActiveRecord
 
       self.class.define_attribute_methods
 
-      run_callbacks :find
-      run_callbacks :initialize
+      _run_find_callbacks
+      _run_initialize_callbacks
 
       self
     end
@@ -367,7 +367,7 @@ module ActiveRecord
       @attributes = @attributes.dup
       @attributes.reset(self.class.primary_key)
 
-      run_callbacks(:initialize)
+      _run_initialize_callbacks
 
       @new_record  = true
       @destroyed   = false
diff --git a/activerecord/lib/active_record/transactions.rb b/activerecord/lib/active_record/transactions.rb
index 6f2def0df1..3131723828 100644
--- a/activerecord/lib/active_record/transactions.rb
+++ b/activerecord/lib/active_record/transactions.rb
@@ -319,8 +319,8 @@ module ActiveRecord
     end
 
     def before_committed! # :nodoc:
-      run_callbacks :before_commit_without_transaction_enrollment
-      run_callbacks :before_commit
+      _run_before_commit_without_transaction_enrollment_callbacks
+      _run_before_commit_callbacks
     end
 
     # Call the +after_commit+ callbacks.
@@ -329,8 +329,8 @@ module ActiveRecord
     # but call it after the commit of a destroyed object.
     def committed!(should_run_callbacks: true) #:nodoc:
       if should_run_callbacks && destroyed? || persisted?
-        run_callbacks :commit_without_transaction_enrollment
-        run_callbacks :commit
+        _run_commit_without_transaction_enrollment_callbacks
+        _run_commit_callbacks
       end
     ensure
       force_clear_transaction_record_state
@@ -340,8 +340,8 @@ module ActiveRecord
     # state should be rolled back to the beginning or just to the last savepoint.
     def rolledback!(force_restore_state: false, should_run_callbacks: true) #:nodoc:
       if should_run_callbacks
-        run_callbacks :rollback
-        run_callbacks :rollback_without_transaction_enrollment
+        _run_rollback_callbacks
+        _run_rollback_without_transaction_enrollment_callbacks
       end
     ensure
       restore_transaction_record_state(force_restore_state)
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