Introduce ActiveSupport::IsolatedExecutionState for internal use

Many places in Active Support and Rails in general use `Thread.current#[]`
to store "request (or job) local data". This often cause problems with
`Enumerator` because it runs in a different fiber.

On the other hand, some places migrated to `Thread#thread_variable_get`
which cause issues with fiber based servers (`falcon`).

Based on this, I believe the isolation level should be an application
configuration.

For backward compatibility it could ship with `:fiber` isolation as a default
but longer term :thread would make more sense as it would work fine for
all deployment targets except falcon.

Ref: https://github.com/rails/rails/pull/38905
Ref: https://github.com/rails/rails/pull/39428
Ref: https://github.com/rails/rails/pull/34495
(and possibly many others)

activesupport/lib/active_support.rb

@@ -47,6 +47,7 @@ module ActiveSupport
   autoload :EventedFileUpdateChecker
   autoload :ForkTracker
   autoload :LogSubscriber
+  autoload :IsolatedExecutionState
   autoload :Notifications
   autoload :Reloader
   autoload :SecureCompareRotator
@@ -115,10 +116,6 @@ module ActiveSupport
     DateAndTime::Compatibility.utc_to_local_returns_utc_offset_times = value
   end
 
-  def self.current_attributes_use_thread_variables=(value)
-    CurrentAttributes._use_thread_variables = value
-  end
-
   @has_native_class_descendants = Class.method_defined?(:descendants) # RUBY_VERSION >= "3.1"
 end
 

activesupport/lib/active_support/current_attributes.rb

@@ -155,24 +155,13 @@ module ActiveSupport
         current_instances.clear
       end
 
-      def _use_thread_variables=(value) # :nodoc:
-        clear_all
-        @@use_thread_variables = value
-      end
-      @@use_thread_variables = false
-
       private
         def generated_attribute_methods
           @generated_attribute_methods ||= Module.new.tap { |mod| include mod }
         end
 
         def current_instances
-          if @@use_thread_variables
-            Thread.current.thread_variable_get(:current_attributes_instances) ||
-              Thread.current.thread_variable_set(:current_attributes_instances, {})
-          else
-            Thread.current[:current_attributes_instances] ||= {}
-          end
+          IsolatedExecutionState[:current_attributes_instances] ||= {}
         end
 
         def current_instances_key

activesupport/lib/active_support/isolated_execution_state.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+require "fiber"
+
+module ActiveSupport
+  module IsolatedExecutionState # :nodoc:
+    @isolation_level = :thread
+
+    Thread.attr_accessor :active_support_execution_state
+    Fiber.attr_accessor :active_support_execution_state
+
+    class << self
+      attr_reader :isolation_level
+
+      def isolation_level=(level)
+        unless %i(thread fiber).include?(level)
+          raise ArgumentError, "isolation_level must be `:thread` or `:fiber`, got: `#{level.inspect}`"
+        end
+
+        if level != isolation_level
+          clear
+          singleton_class.alias_method(:current, "current_#{level}")
+          singleton_class.send(:private, :current)
+          @isolation_level = level
+        end
+      end
+
+      def [](key)
+        current[key]
+      end
+
+      def []=(key, value)
+        current[key] = value
+      end
+
+      def clear
+        current.clear
+      end
+
+      private
+        def current_thread
+          Thread.current.active_support_execution_state ||= {}
+        end
+
+        def current_fiber
+          Fiber.current.active_support_execution_state ||= {}
+        end
+
+        alias_method :current, :current_thread
+    end
+  end
+end

activesupport/lib/active_support/railtie.rb

@@ -9,6 +9,12 @@ module ActiveSupport
 
     config.eager_load_namespaces << ActiveSupport
 
+    initializer "active_support.isolation_level" do |app|
+      if level = app.config.active_support.delete(:isolation_level)
+        ActiveSupport::IsolatedExecutionState.isolation_level = level
+      end
+    end
+
     initializer "active_support.remove_deprecated_time_with_zone_name" do |app|
       config.after_initialize do
         if app.config.active_support.remove_deprecated_time_with_zone_name

activesupport/test/current_attributes_test.rb

@@ -177,21 +177,28 @@ class CurrentAttributesTest < ActiveSupport::TestCase
   end
 
   test "CurrentAttributes use fiber-local variables" do
+    previous_level = ActiveSupport::IsolatedExecutionState.isolation_level
+    ActiveSupport::IsolatedExecutionState.isolation_level = :fiber
+
     Session.current = 42
     enumerator = Enumerator.new do |yielder|
       yielder.yield Session.current
     end
     assert_nil enumerator.next
+  ensure
+    ActiveSupport::IsolatedExecutionState.isolation_level = previous_level
   end
 
   test "CurrentAttributes can use thread-local variables" do
-    ActiveSupport::CurrentAttributes._use_thread_variables = true
+    previous_level = ActiveSupport::IsolatedExecutionState.isolation_level
+    ActiveSupport::IsolatedExecutionState.isolation_level = :thread
+
     Session.current = 42
     enumerator = Enumerator.new do |yielder|
       yielder.yield Session.current
     end
     assert_equal 42, enumerator.next
   ensure
-    ActiveSupport::CurrentAttributes._use_thread_variables = false
+    ActiveSupport::IsolatedExecutionState.isolation_level = previous_level
   end
 end

activesupport/test/isolated_execution_state_test.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+require_relative "abstract_unit"
+
+class IsolatedExecutionStateTest < ActiveSupport::TestCase
+  setup do
+    ActiveSupport::IsolatedExecutionState.clear
+    @original_isolation_level = ActiveSupport::IsolatedExecutionState.isolation_level
+  end
+
+  teardown do
+    ActiveSupport::IsolatedExecutionState.clear
+    ActiveSupport::IsolatedExecutionState.isolation_level = @original_isolation_level
+  end
+
+  test "#[] when isolation level is :fiber" do
+    ActiveSupport::IsolatedExecutionState.isolation_level = :fiber
+
+    ActiveSupport::IsolatedExecutionState[:test] = 42
+    assert_equal 42, ActiveSupport::IsolatedExecutionState[:test]
+    enumerator = Enumerator.new do |yielder|
+      yielder.yield ActiveSupport::IsolatedExecutionState[:test]
+    end
+    assert_nil enumerator.next
+
+    assert_nil Thread.new { ActiveSupport::IsolatedExecutionState[:test] }.value
+  end
+
+  test "#[] when isolation level is :thread" do
+    ActiveSupport::IsolatedExecutionState.isolation_level = :thread
+
+    ActiveSupport::IsolatedExecutionState[:test] = 42
+    assert_equal 42, ActiveSupport::IsolatedExecutionState[:test]
+    enumerator = Enumerator.new do |yielder|
+      yielder.yield ActiveSupport::IsolatedExecutionState[:test]
+    end
+    assert_equal 42, enumerator.next
+
+    assert_nil Thread.new { ActiveSupport::IsolatedExecutionState[:test] }.value
+  end
+
+  test "changing the isolation level clear the old store" do
+    original = ActiveSupport::IsolatedExecutionState.isolation_level
+    other = ActiveSupport::IsolatedExecutionState.isolation_level == :fiber ? :thread : :fiber
+
+    ActiveSupport::IsolatedExecutionState[:test] = 42
+    ActiveSupport::IsolatedExecutionState.isolation_level = original
+    assert_equal 42, ActiveSupport::IsolatedExecutionState[:test]
+
+    ActiveSupport::IsolatedExecutionState.isolation_level = other
+    assert_nil ActiveSupport::IsolatedExecutionState[:test]
+
+    ActiveSupport::IsolatedExecutionState.isolation_level = original
+    assert_nil ActiveSupport::IsolatedExecutionState[:test]
+  end
+end

guides/source/configuring.md

@@ -1384,6 +1384,11 @@ Configures deprecation warnings that the Application considers disallowed. This
 
 Allows you to disable all deprecation warnings (including disallowed deprecations); it makes `ActiveSupport::Deprecation.warn` a no-op. This is enabled by default in production.
 
+#### `active_support.isolation_level`
+
+Configures the locality of most of Rails internal state. If you use a fiber based server or job processor (e.g. `falcon`), you should set it to `:fiber`.
+Otherwise it is best to use `:thread` locality.
+
 #### `config.active_support.use_rfc4122_namespaced_uuids`
 
 Specifies whether generated namespaced UUIDs follow the RFC 4122 standard for namespace IDs provided as a `String` to `Digest::UUID.uuid_v3` or `Digest::UUID.uuid_v5` method calls.
@@ -1816,6 +1821,7 @@ Accepts a string for the HTML tag used to wrap attachments. Defaults to `"action
 - `config.active_support.key_generator_hash_digest_class`: `OpenSSL::Digest::SHA1`
 - `config.active_support.cache_format_version`: `6.1`
 - `config.active_support.executor_around_test_case`: `false`
+- `active_support.isolation_level`: `:thread`
 - ``config.active_support.use_rfc4122_namespaced_uuids``: `false`
 - `config.action_dispatch.return_only_request_media_type_on_content_type`: `true`
 - `ActiveSupport.utc_to_local_returns_utc_offset_times`: `false`

railties/lib/rails/application/configuration.rb

@@ -215,6 +215,7 @@ module Rails
             active_support.cache_format_version = 7.0
             active_support.use_rfc4122_namespaced_uuids = true
             active_support.executor_around_test_case = true
+            active_support.isolation_level = :thread
           end
 
           if respond_to?(:action_mailer)

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_0.rb.tt

@@ -51,6 +51,11 @@
 # and asynchronous queries will then be enabled.
 # Rails.application.config.active_support.executor_around_test_case = true
 
+# Define the isolation level of most of Rails internal state.
+# If you use a fiber based server or job processor, you should set it to `:fiber`.
+# Otherwise the default of `:thread` if preferable.
+# Rails.application.config.active_support.isolation_level = :thread
+
 # Set both the `:open_timeout` and `:read_timeout` values for `:smtp` delivery method.
 # Rails.application.config.action_mailer.smtp_timeout = 5