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