Make connection handler per thread instead of per fiber

The connection handler was using the RuntimeRegistry which kind of
implies it's a per thread registry. But it's actually per fiber.

If you have an application that uses fibers and you're using multiple
databases, when you switch the connection handler to swap connections
new fibers running on the same thread used to get a different connection
id. This PR changes the code to actually use a thread so that we get
the same connection.

Fixes https://github.com/rails/rails/issues/30047

[Eileen M. Uchitelle, Aaron Patterson, & Arthur Neeves]

2 files changed, 39 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 50f3087c51..8f4d292a4b 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -129,11 +129,11 @@ module ActiveRecord
       self.filter_attributes = []
 
       def self.connection_handler
-        ActiveRecord::RuntimeRegistry.connection_handler || default_connection_handler
+        Thread.current.thread_variable_get("ar_connection_handler") || default_connection_handler
       end
 
       def self.connection_handler=(handler)
-        ActiveRecord::RuntimeRegistry.connection_handler = handler
+        Thread.current.thread_variable_set("ar_connection_handler", handler)
       end
 
       self.default_connection_handler = ConnectionAdapters::ConnectionHandler.new
diff --git a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
index a394430dfe..393b577c1d 100644
--- a/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
+++ b/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb
@@ -280,6 +280,43 @@ module ActiveRecord
         assert_nil @rw_handler.retrieve_connection_pool("foo")
         assert_nil @ro_handler.retrieve_connection_pool("foo")
       end
+
+      def test_connection_handlers_are_per_thread_and_not_per_fiber
+        original_handlers = ActiveRecord::Base.connection_handlers
+
+        ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler, reading: ActiveRecord::ConnectionAdapters::ConnectionHandler.new }
+
+        reading_handler = ActiveRecord::Base.connection_handlers[:reading]
+
+        reading = ActiveRecord::Base.with_handler(:reading) do
+          Person.connection_handler
+        end
+
+        assert_not_equal reading, ActiveRecord::Base.connection_handler
+        assert_equal reading, reading_handler
+      ensure
+        ActiveRecord::Base.connection_handlers = original_handlers
+      end
+
+      def test_connection_handlers_swapping_connections_in_fiber
+        original_handlers = ActiveRecord::Base.connection_handlers
+
+        ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler, reading: ActiveRecord::ConnectionAdapters::ConnectionHandler.new }
+
+        reading_handler = ActiveRecord::Base.connection_handlers[:reading]
+
+        enum = Enumerator.new do |r|
+          r << ActiveRecord::Base.connection_handler
+        end
+
+        reading = ActiveRecord::Base.with_handler(:reading) do
+          enum.next
+        end
+
+        assert_equal reading, reading_handler
+      ensure
+        ActiveRecord::Base.connection_handlers = original_handlers
+      end
     end
   end
 end