Subscribing to notifications while inside the said instrumented section.

The issue is that on the exit from Instrumenter#instrument section,
an Evented listener will run into an error because its thread local
(Thread.current[:_timestack]) has not been set up by the #start
method (this obviously happens because the Evented listeners didn't
exist at the time, since no subscribtion to that section was made yet).

Note: support for subscribing to instrumented sections, while being
inside those instrumented sections, might be removed in the future.

Maybe fixes #21873.

1 files changed, 15 insertions, 0 deletions

diff --git a/activesupport/test/notifications_test.rb b/activesupport/test/notifications_test.rb
index f729f0a95b..d9cc392ac9 100644
--- a/activesupport/test/notifications_test.rb
+++ b/activesupport/test/notifications_test.rb
@@ -42,6 +42,21 @@ module Notifications
       ActiveSupport::Notifications.instrument(name)
       assert_equal expected, events
     end
+
+    def test_subsribing_to_instrumentation_while_inside_it
+      # the repro requires that there are no evented subscribers for the "foo" event,
+      # so we have to duplicate some of the setup code
+      old_notifier = ActiveSupport::Notifications.notifier
+      ActiveSupport::Notifications.notifier = ActiveSupport::Notifications::Fanout.new
+
+      ActiveSupport::Notifications.subscribe('foo', TestSubscriber.new)
+
+      ActiveSupport::Notifications.instrument('foo') do
+        ActiveSupport::Notifications.subscribe('foo') {}
+      end
+    ensure
+      ActiveSupport::Notifications.notifier = old_notifier
+    end
   end
 
   class UnsubscribeTest < TestCase