Make legacy_connection_handling a module instance variable

Ref: https://github.com/rails/rails/pull/42237
Ref: https://bugs.ruby-lang.org/issues/17763

Ruby class variables are slow, and get slower the more ancestors the class has.
And it doesn't seem likely to get faster any time soon. Overall I'm under the
impression that ruby-core consider them more or less deprecated.

But in many cases where `cattr_accessor` is used, a class instance variable
could work just fine, and would be much faster.

For Active Record this means most of the `cattr_accessor` on `ActiveRecord::Base`
could actually be `attr_accessor` on `ActiveRecord`.

I started with `legacy_connection_handling` as a proof of concept.

activerecord/lib/active_record.rb

@@ -169,6 +169,9 @@ module ActiveRecord
   autoload :TestDatabases, "active_record/test_databases"
   autoload :TestFixtures, "active_record/fixtures"
 
+  singleton_class.attr_accessor :legacy_connection_handling
+  self.legacy_connection_handling = true
+
   def self.eager_load!
     super
     ActiveRecord::Locking.eager_load!

activerecord/lib/active_record/connection_adapters/abstract/connection_handler.rb

@@ -100,7 +100,7 @@ module ActiveRecord
       # See +READ_QUERY+ for the queries that are blocked by this
       # method.
       def while_preventing_writes(enabled = true)
-        unless ActiveRecord::Base.legacy_connection_handling
+        unless ActiveRecord.legacy_connection_handling
           raise NotImplementedError, "`while_preventing_writes` is only available on the connection_handler with legacy_connection_handling"
         end
 
@@ -143,7 +143,7 @@ module ActiveRecord
           payload[:config] = db_config.configuration_hash
         end
 
-        if ActiveRecord::Base.legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           owner_to_pool_manager[pool_config.connection_specification_name] ||= LegacyPoolManager.new
         else
           owner_to_pool_manager[pool_config.connection_specification_name] ||= PoolManager.new

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -140,7 +140,7 @@ module ActiveRecord
       # will return true based on +current_preventing_writes+.
       def preventing_writes?
         return true if replica?
-        return ActiveRecord::Base.connection_handler.prevent_writes if ActiveRecord::Base.legacy_connection_handling
+        return ActiveRecord::Base.connection_handler.prevent_writes if ActiveRecord.legacy_connection_handling
         return false if connection_klass.nil?
 
         connection_klass.current_preventing_writes

activerecord/lib/active_record/connection_handling.rb

@@ -135,7 +135,7 @@ module ActiveRecord
     #     Dog.first # finds first Dog record stored on the shard one replica
     #   end
     def connected_to(role: nil, shard: nil, prevent_writes: false, &blk)
-      if legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         if self != Base
           raise NotImplementedError, "`connected_to` can only be called on ActiveRecord::Base with legacy connection handling."
         end
@@ -176,7 +176,7 @@ module ActiveRecord
     def connected_to_many(*classes, role:, shard: nil, prevent_writes: false)
       classes = classes.flatten
 
-      if legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         raise NotImplementedError, "connected_to_many is not available with legacy connection handling"
       end
 
@@ -200,7 +200,7 @@ module ActiveRecord
     # It is not recommended to use this method in a request since it
     # does not yield to a block like +connected_to+.
     def connecting_to(role: default_role, shard: default_shard, prevent_writes: false)
-      if legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         raise NotImplementedError, "`connecting_to` is not available with `legacy_connection_handling`."
       end
 
@@ -221,7 +221,7 @@ module ActiveRecord
     # See +READ_QUERY+ for the queries that are blocked by this
     # method.
     def while_preventing_writes(enabled = true, &block)
-      if legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         connection_handler.while_preventing_writes(enabled, &block)
       else
         connected_to(role: current_role, prevent_writes: enabled, &block)
@@ -239,7 +239,7 @@ module ActiveRecord
     end
 
     def lookup_connection_handler(handler_key) # :nodoc:
-      if ActiveRecord::Base.legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         handler_key ||= ActiveRecord::Base.writing_role
         connection_handlers[handler_key] ||= ActiveRecord::ConnectionAdapters::ConnectionHandler.new
       else
@@ -249,7 +249,7 @@ module ActiveRecord
 
     # Clears the query cache for all connections associated with the current thread.
     def clear_query_caches_for_current_thread
-      if ActiveRecord::Base.legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         ActiveRecord::Base.connection_handlers.each_value do |handler|
           clear_on_handler(handler)
         end
@@ -358,7 +358,7 @@ module ActiveRecord
       def with_role_and_shard(role, shard, prevent_writes)
         prevent_writes = true if role == reading_role
 
-        if ActiveRecord::Base.legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           with_handler(role.to_sym) do
             connection_handler.while_preventing_writes(prevent_writes) do
               self.connected_to_stack << { shard: shard, klasses: [self] }

activerecord/lib/active_record/core.rb

@@ -164,8 +164,6 @@ module ActiveRecord
 
       class_attribute :default_shard, instance_writer: false
 
-      mattr_accessor :legacy_connection_handling, instance_writer: false, default: true
-
       mattr_accessor :application_record_class, instance_accessor: false, default: nil
 
       # Sets the async_query_executor for an application. By default the thread pool executor
@@ -225,7 +223,7 @@ module ActiveRecord
       end
 
       def self.connection_handlers
-        if legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
         else
           raise NotImplementedError, "The new connection handling does not support accessing multiple connection handlers."
         end
@@ -234,7 +232,7 @@ module ActiveRecord
       end
 
       def self.connection_handlers=(handlers)
-        if legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           ActiveSupport::Deprecation.warn(<<~MSG)
             Using legacy connection handling is deprecated. Please set
             `legacy_connection_handling` to `false` in your application.
@@ -270,7 +268,7 @@ module ActiveRecord
       #     ActiveRecord::Base.current_role #=> :reading
       #   end
       def self.current_role
-        if ActiveRecord::Base.legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           connection_handlers.key(connection_handler) || default_role
         else
           connected_to_stack.reverse_each do |hash|
@@ -311,7 +309,7 @@ module ActiveRecord
       #     ActiveRecord::Base.current_preventing_writes #=> false
       #   end
       def self.current_preventing_writes
-        if legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           connection_handler.prevent_writes
         else
           connected_to_stack.reverse_each do |hash|

activerecord/lib/active_record/query_cache.rb

@@ -28,7 +28,7 @@ module ActiveRecord
     def self.run
       pools = []
 
-      if ActiveRecord::Base.legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         ActiveRecord::Base.connection_handlers.each do |key, handler|
           pools.concat(handler.connection_pool_list.reject { |p| p.query_cache_enabled }.each { |p| p.enable_query_cache! })
         end
@@ -42,7 +42,7 @@ module ActiveRecord
     def self.complete(pools)
       pools.each { |pool| pool.disable_query_cache! }
 
-      if ActiveRecord::Base.legacy_connection_handling
+      if ActiveRecord.legacy_connection_handling
         ActiveRecord::Base.connection_handlers.each do |_, handler|
           handler.connection_pool_list.each do |pool|
             pool.release_connection if pool.active_connection? && !pool.connection.transaction_open?

activerecord/lib/active_record/railtie.rb

@@ -200,11 +200,22 @@ To keep using the current cache store, you can turn off cache versioning entirel
     end
 
     initializer "active_record.set_configs" do |app|
-      ActiveSupport.on_load(:active_record) do
-        configs = app.config.active_record
+      configs = app.config.active_record
 
+      configs.each do |k, v|
+        next if k == :encryption
+        setter = "#{k}="
+        if ActiveRecord.respond_to?(setter)
+          ActiveRecord.send(setter, v)
+        end
+      end
+
+      ActiveSupport.on_load(:active_record) do
         configs.each do |k, v|
-          send "#{k}=", v if k != :encryption
+          next if k == :encryption
+          setter = "#{k}="
+          next if ActiveRecord.respond_to?(setter)
+          send(setter, v)
         end
       end
     end
@@ -213,7 +224,7 @@ To keep using the current cache store, you can turn off cache versioning entirel
     # and then establishes the connection.
     initializer "active_record.initialize_database" do
       ActiveSupport.on_load(:active_record) do
-        if ActiveRecord::Base.legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           self.connection_handlers = { writing_role => ActiveRecord::Base.default_connection_handler }
         end
         self.configurations = Rails.application.config.database_configuration

activerecord/lib/active_record/test_fixtures.rb

@@ -193,7 +193,7 @@ module ActiveRecord
       # need to share a connection pool so that the reading connection
       # can see data in the open transaction on the writing connection.
       def setup_shared_connection_pool
-        if ActiveRecord::Base.legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           writing_handler = ActiveRecord::Base.connection_handlers[ActiveRecord::Base.writing_role]
 
           ActiveRecord::Base.connection_handlers.values.each do |handler|
@@ -236,7 +236,7 @@ module ActiveRecord
       end
 
       def teardown_shared_connection_pool
-        if ActiveRecord::Base.legacy_connection_handling
+        if ActiveRecord.legacy_connection_handling
           @legacy_saved_pool_configs.each_pair do |handler, names|
             names.each_pair do |name, shards|
               shards.each_pair do |shard_name, pool_config|

activerecord/test/cases/adapter_prevent_writes_test.rb

@@ -127,8 +127,8 @@ module ActiveRecord
 
   class AdapterPreventWritesLegacyTest < ActiveRecord::TestCase
     def setup
-      @old_value = ActiveRecord::Base.legacy_connection_handling
-      ActiveRecord::Base.legacy_connection_handling = true
+      @old_value = ActiveRecord.legacy_connection_handling
+      ActiveRecord.legacy_connection_handling = true
 
       @connection = ActiveRecord::Base.connection
       @connection_handler = ActiveRecord::Base.connection_handler
@@ -136,7 +136,7 @@ module ActiveRecord
 
     def teardown
       clean_up_legacy_connection_handlers
-      ActiveRecord::Base.legacy_connection_handling = @old_value
+      ActiveRecord.legacy_connection_handling = @old_value
     end
 
     def test_preventing_writes_predicate_legacy

activerecord/test/cases/adapters/mysql2/mysql2_adapter_prevent_writes_test.rb

@@ -105,15 +105,15 @@ class Mysql2AdapterPreventWritesLegacyTest < ActiveRecord::Mysql2TestCase
   include DdlHelper
 
   def setup
-    @old_value = ActiveRecord::Base.legacy_connection_handling
-    ActiveRecord::Base.legacy_connection_handling = true
+    @old_value = ActiveRecord.legacy_connection_handling
+    ActiveRecord.legacy_connection_handling = true
 
     @conn = ActiveRecord::Base.connection
     @connection_handler = ActiveRecord::Base.connection_handler
   end
 
   def teardown
-    ActiveRecord::Base.legacy_connection_handling = @old_value
+    ActiveRecord.legacy_connection_handling = @old_value
   end
 
   def test_errors_when_an_insert_query_is_called_while_preventing_writes

activerecord/test/cases/adapters/postgresql/postgresql_adapter_prevent_writes_test.rb

@@ -106,15 +106,15 @@ module ActiveRecord
       include ConnectionHelper
 
       def setup
-        @old_value = ActiveRecord::Base.legacy_connection_handling
-        ActiveRecord::Base.legacy_connection_handling = true
+        @old_value = ActiveRecord.legacy_connection_handling
+        ActiveRecord.legacy_connection_handling = true
 
         @connection = ActiveRecord::Base.connection
         @connection_handler = ActiveRecord::Base.connection_handler
       end
 
       def teardown
-        ActiveRecord::Base.legacy_connection_handling = @old_value
+        ActiveRecord.legacy_connection_handling = @old_value
       end
 
       def test_errors_when_an_insert_query_is_called_while_preventing_writes

activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_prevent_writes_test.rb

@@ -95,8 +95,8 @@ module ActiveRecord
       self.use_transactional_tests = false
 
       def setup
-        @old_value = ActiveRecord::Base.legacy_connection_handling
-        ActiveRecord::Base.legacy_connection_handling = true
+        @old_value = ActiveRecord.legacy_connection_handling
+        ActiveRecord.legacy_connection_handling = true
 
         @conn = ActiveRecord::Base.connection
 
@@ -104,7 +104,7 @@ module ActiveRecord
       end
 
       def teardown
-        ActiveRecord::Base.legacy_connection_handling = @old_value
+        ActiveRecord.legacy_connection_handling = @old_value
       end
 
       def test_errors_when_an_insert_query_is_called_while_preventing_writes

activerecord/test/cases/base_prevent_writes_test.rb

@@ -97,15 +97,15 @@ class BasePreventWritesTest < ActiveRecord::TestCase
 
   class BasePreventWritesLegacyTest < ActiveRecord::TestCase
     def setup
-      @old_value = ActiveRecord::Base.legacy_connection_handling
-      ActiveRecord::Base.legacy_connection_handling = true
+      @old_value = ActiveRecord.legacy_connection_handling
+      ActiveRecord.legacy_connection_handling = true
       ActiveRecord::Base.establish_connection :arunit
       ARUnit2Model.establish_connection :arunit2
     end
 
     def teardown
       clean_up_legacy_connection_handlers
-      ActiveRecord::Base.legacy_connection_handling = @old_value
+      ActiveRecord.legacy_connection_handling = @old_value
     end
 
     if !in_memory_db?

activerecord/test/cases/base_test.rb

@@ -1682,8 +1682,8 @@ class BasicsTest < ActiveRecord::TestCase
   end
 
   test "cannot call connected_to on subclasses of ActiveRecord::Base with legacy connection handling" do
-    old_value = ActiveRecord::Base.legacy_connection_handling
-    ActiveRecord::Base.legacy_connection_handling = true
+    old_value = ActiveRecord.legacy_connection_handling
+    ActiveRecord.legacy_connection_handling = true
 
     error = assert_raises(NotImplementedError) do
       Bird.connected_to(role: :reading) { }
@@ -1692,7 +1692,7 @@ class BasicsTest < ActiveRecord::TestCase
     assert_equal "`connected_to` can only be called on ActiveRecord::Base with legacy connection handling.", error.message
   ensure
     clean_up_legacy_connection_handlers
-    ActiveRecord::Base.legacy_connection_handling = old_value
+    ActiveRecord.legacy_connection_handling = old_value
   end
 
   test "cannot call connected_to with role and shard on non-abstract classes" do
@@ -1744,25 +1744,25 @@ class BasicsTest < ActiveRecord::TestCase
   end
 
   test "#connecting_to doesn't work with legacy connection handling" do
-    old_value = ActiveRecord::Base.legacy_connection_handling
-    ActiveRecord::Base.legacy_connection_handling = true
+    old_value = ActiveRecord.legacy_connection_handling
+    ActiveRecord.legacy_connection_handling = true
 
     assert_raises NotImplementedError do
       SecondAbstractClass.connecting_to(role: :writing, prevent_writes: true)
     end
   ensure
-    ActiveRecord::Base.legacy_connection_handling = old_value
+    ActiveRecord.legacy_connection_handling = old_value
   end
 
   test "#connected_to_many doesn't work with legacy connection handling" do
-    old_value = ActiveRecord::Base.legacy_connection_handling
-    ActiveRecord::Base.legacy_connection_handling = true
+    old_value = ActiveRecord.legacy_connection_handling
+    ActiveRecord.legacy_connection_handling = true
 
     assert_raises NotImplementedError do
       ActiveRecord::Base.connected_to_many([SecondAbstractClass], role: :writing)
     end
   ensure
-    ActiveRecord::Base.legacy_connection_handling = old_value
+    ActiveRecord.legacy_connection_handling = old_value
   end
 
   test "#connected_to_many cannot be called on anything but ActiveRecord::Base" do

activerecord/test/cases/connection_adapters/legacy_connection_handlers_multi_db_test.rb

@@ -11,8 +11,8 @@ module ActiveRecord
       fixtures :people
 
       def setup
-        @old_value = ActiveRecord::Base.legacy_connection_handling
-        ActiveRecord::Base.legacy_connection_handling = true
+        @old_value = ActiveRecord.legacy_connection_handling
+        ActiveRecord.legacy_connection_handling = true
         assert_deprecated do
           ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
         end
@@ -29,7 +29,7 @@ module ActiveRecord
 
       def teardown
         clean_up_legacy_connection_handlers
-        ActiveRecord::Base.legacy_connection_handling = @old_value
+        ActiveRecord.legacy_connection_handling = @old_value
       end
 
       class SecondaryBase < ActiveRecord::Base

activerecord/test/cases/connection_adapters/legacy_connection_handlers_sharding_db_test.rb

@@ -11,8 +11,8 @@ module ActiveRecord
       fixtures :people
 
       def setup
-        @legacy_setting = ActiveRecord::Base.legacy_connection_handling
-        ActiveRecord::Base.legacy_connection_handling = true
+        @legacy_setting = ActiveRecord.legacy_connection_handling
+        ActiveRecord.legacy_connection_handling = true
         assert_deprecated do
           ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
         end
@@ -29,7 +29,7 @@ module ActiveRecord
 
       def teardown
         clean_up_legacy_connection_handlers
-        ActiveRecord::Base.legacy_connection_handling = @legacy_setting
+        ActiveRecord.legacy_connection_handling = @legacy_setting
       end
 
       unless in_memory_db?

activerecord/test/cases/fixtures_test.rb

@@ -1541,8 +1541,8 @@ if current_adapter?(:SQLite3Adapter) && !in_memory_db?
     fixtures :dogs
 
     def setup
-      @old_value = ActiveRecord::Base.legacy_connection_handling
-      ActiveRecord::Base.legacy_connection_handling = true
+      @old_value = ActiveRecord.legacy_connection_handling
+      ActiveRecord.legacy_connection_handling = true
 
       @old_handler = ActiveRecord::Base.connection_handler
       @prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, config
@@ -1565,7 +1565,7 @@ if current_adapter?(:SQLite3Adapter) && !in_memory_db?
       ActiveRecord::Base.configurations = @prev_configs
       ActiveRecord::Base.connection_handler = @old_handler
       clean_up_legacy_connection_handlers
-      ActiveRecord::Base.legacy_connection_handling = false
+      ActiveRecord.legacy_connection_handling = false
     end
 
     def test_uses_writing_connection_for_fixtures

activerecord/test/cases/query_cache_test.rb

@@ -75,8 +75,8 @@ class QueryCacheTest < ActiveRecord::TestCase
   end
 
   def test_query_cache_is_applied_to_legacy_connections_in_all_handlers
-    old_value = ActiveRecord::Base.legacy_connection_handling
-    ActiveRecord::Base.legacy_connection_handling = true
+    old_value = ActiveRecord.legacy_connection_handling
+    ActiveRecord.legacy_connection_handling = true
 
     assert_deprecated do
       ActiveRecord::Base.connection_handlers = {
@@ -101,7 +101,7 @@ class QueryCacheTest < ActiveRecord::TestCase
     mw.call({})
   ensure
     clean_up_legacy_connection_handlers
-    ActiveRecord::Base.legacy_connection_handling = old_value
+    ActiveRecord.legacy_connection_handling = old_value
   end
 
   def test_query_cache_is_applied_to_all_connections
@@ -126,8 +126,8 @@ class QueryCacheTest < ActiveRecord::TestCase
 
   if Process.respond_to?(:fork) && !in_memory_db?
     def test_query_cache_with_multiple_handlers_and_forked_processes_legacy_handling
-      old_value = ActiveRecord::Base.legacy_connection_handling
-      ActiveRecord::Base.legacy_connection_handling = true
+      old_value = ActiveRecord.legacy_connection_handling
+      ActiveRecord.legacy_connection_handling = true
 
       assert_deprecated do
         ActiveRecord::Base.connection_handlers = {
@@ -190,7 +190,7 @@ class QueryCacheTest < ActiveRecord::TestCase
       rd.close
     ensure
       clean_up_legacy_connection_handlers
-      ActiveRecord::Base.legacy_connection_handling = old_value
+      ActiveRecord.legacy_connection_handling = old_value
     end
 
     def test_query_cache_with_forked_processes
@@ -665,8 +665,8 @@ class QueryCacheTest < ActiveRecord::TestCase
 
   def test_clear_query_cache_is_called_on_all_legacy_connections
     skip "with in memory db, reading role won't be able to see database on writing role" if in_memory_db?
-    old_value = ActiveRecord::Base.legacy_connection_handling
-    ActiveRecord::Base.legacy_connection_handling = true
+    old_value = ActiveRecord.legacy_connection_handling
+    ActiveRecord.legacy_connection_handling = true
 
     assert_deprecated do
       ActiveRecord::Base.connection_handlers = {
@@ -704,7 +704,7 @@ class QueryCacheTest < ActiveRecord::TestCase
   ensure
     unless in_memory_db?
       clean_up_legacy_connection_handlers
-      ActiveRecord::Base.legacy_connection_handling = old_value
+      ActiveRecord.legacy_connection_handling = old_value
     end
   end
 

activerecord/test/cases/test_fixtures_test.rb

@@ -23,8 +23,8 @@ class TestFixturesTest < ActiveRecord::TestCase
 
   unless in_memory_db?
     def test_doesnt_rely_on_active_support_test_case_specific_methods_with_legacy_connection_handling
-      old_value = ActiveRecord::Base.legacy_connection_handling
-      ActiveRecord::Base.legacy_connection_handling = true
+      old_value = ActiveRecord.legacy_connection_handling
+      ActiveRecord.legacy_connection_handling = true
 
       tmp_dir = Dir.mktmpdir
       File.write(File.join(tmp_dir, "zines.yml"), <<~YML)
@@ -59,7 +59,7 @@ class TestFixturesTest < ActiveRecord::TestCase
       clean_up_legacy_connection_handlers
       ActiveRecord::Base.connection_handler = old_handler
       FileUtils.rm_r(tmp_dir)
-      ActiveRecord::Base.legacy_connection_handling = old_value
+      ActiveRecord.legacy_connection_handling = old_value
     end
 
     def test_doesnt_rely_on_active_support_test_case_specific_methods

activerecord/test/support/connection.rb

@@ -19,7 +19,7 @@ module ARTest
   end
 
   def self.connect
-    ActiveRecord::Base.legacy_connection_handling = false
+    ActiveRecord.legacy_connection_handling = false
     ActiveRecord::Base.async_query_executor = :global_thread_pool
     puts "Using #{connection_name}"
     ActiveRecord::Base.logger = ActiveSupport::Logger.new("debug.log", 0, 100 * 1024 * 1024)