1 files changed, 60 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/per_thread_registry.rb b/activesupport/lib/active_support/per_thread_registry.rb
new file mode 100644
index 0000000000..eb92fb4371
--- /dev/null
+++ b/activesupport/lib/active_support/per_thread_registry.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require "active_support/core_ext/module/delegation"
+
+module ActiveSupport
+  # NOTE: This approach has been deprecated for end-user code in favor of {thread_mattr_accessor}[rdoc-ref:Module#thread_mattr_accessor] and friends.
+  # Please use that approach instead.
+  #
+  # This module is used to encapsulate access to thread local variables.
+  #
+  # Instead of polluting the thread locals namespace:
+  #
+  #   Thread.current[:connection_handler]
+  #
+  # you define a class that extends this module:
+  #
+  #   module ActiveRecord
+  #     class RuntimeRegistry
+  #       extend ActiveSupport::PerThreadRegistry
+  #
+  #       attr_accessor :connection_handler
+  #     end
+  #   end
+  #
+  # and invoke the declared instance accessors as class methods. So
+  #
+  #   ActiveRecord::RuntimeRegistry.connection_handler = connection_handler
+  #
+  # sets a connection handler local to the current thread, and
+  #
+  #   ActiveRecord::RuntimeRegistry.connection_handler
+  #
+  # returns a connection handler local to the current thread.
+  #
+  # This feature is accomplished by instantiating the class and storing the
+  # instance as a thread local keyed by the class name. In the example above
+  # a key "ActiveRecord::RuntimeRegistry" is stored in <tt>Thread.current</tt>.
+  # The class methods proxy to said thread local instance.
+  #
+  # If the class has an initializer, it must accept no arguments.
+  module PerThreadRegistry
+    def self.extended(object)
+      object.instance_variable_set "@per_thread_registry_key", object.name.freeze
+    end
+
+    def instance
+      Thread.current[@per_thread_registry_key] ||= new
+    end
+
+    private
+      def method_missing(name, *args, &block)
+        # Caches the method definition as a singleton method of the receiver.
+        #
+        # By letting #delegate handle it, we avoid an enclosure that'll capture args.
+        singleton_class.delegate name, to: :instance
+
+        send(name, *args, &block)
+      end
+  end
+end