まず thread safe にする設定は config/environments/production.rb にある

# Enable threaded mode
# config.threadsafe!

からなので，これを調べると railties/lib/initializer.rb の

def threadsafe!
  self.cache_classes = true
  self.dependency_loading = false
  self.action_controller.allow_concurrency = true
  self
end

なので，actionpack/lib/action_controller/base.rb を見ると

@@allow_concurrency = false
cattr_accessor :allow_concurrency

とあるだけ．ならばと rak ‘allow_concurrency’ とかやってみると

activerecord/test/cases/pooled_connections_test.rb
  79|  def test_allow_concurrency_is_deprecated
  80|    assert_deprecated('ActiveRecord::Base.allow_concurrency') do
  81|      ActiveRecord::Base.allow_concurrency
  83|    assert_deprecated('ActiveRecord::Base.allow_concurrency=') do
  84|      ActiveRecord::Base.allow_concurrency = true

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
 181|    # * <tt>:allow_concurrency</tt> - If true, use async query methods so Ruby threads don't deadlock; otherwise, use blocking query methods.
 935|          @async = @config[:allow_concurrency] && @connection.respond_to?(:async_exec)

activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb
  90|      def allow_concurrency
  91|        ActiveSupport::Deprecation.warn("ActiveRecord::Base.allow_concurrency has been deprecated and no longer has any effect. Please remove all references to allow_concurrency.")
  95|      def allow_concurrency=(flag)
  96|        ActiveSupport::Deprecation.warn("ActiveRecord::Base.allow_concurrency= has been deprecated and no longer has any effect. Please remove all references to allow_concurrency=.")

activerecord/lib/active_record/test_case.rb
  48|      setup :connection_allow_concurrency_setup
  49|      teardown :connection_allow_concurrency_teardown
  52|    def connection_allow_concurrency_setup
  54|      ActiveRecord::Base.establish_connection(@connection.merge({:allow_concurrency => true}))
  57|    def connection_allow_concurrency_teardown

actionpack/lib/action_controller/dispatcher.rb
 116|      if ActionController::Base.allow_concurrency

actionpack/lib/action_controller/base.rb
 290|    @@allow_concurrency = false
 291|    cattr_accessor :allow_concurrency

railties/doc/guides/source/2_2_release_notes.txt
 408|* +ActiveRecord::Base.allow_concurrency+ no longer has any effect.

railties/doc/guides/html/2_2_release_notes.html
1098|<tt>ActiveRecord::Base.allow_concurrency</tt> no longer has any effect.

railties/lib/webrick_server.rb
  43|# can change this behavior by setting ActionController::Base.allow_concurrency

railties/lib/initializer.rb
 790|      self.action_controller.allow_concurrency = true

と言うことは，

• actionpack/lib/action_controller/dispatcher.rb
• activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb

このあたりが怪しいので，次はここを見てみよう．続くかも．

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