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.

2 files changed, 9 insertions, 4 deletions

diff --git a/activesupport/lib/active_support/notifications/fanout.rb b/activesupport/lib/active_support/notifications/fanout.rb
index 7798c7ec60..c53f9c1039 100644
--- a/activesupport/lib/active_support/notifications/fanout.rb
+++ b/activesupport/lib/active_support/notifications/fanout.rb
@@ -42,8 +42,8 @@ module ActiveSupport
         listeners_for(name).each { |s| s.start(name, id, payload) }
       end
 
-      def finish(name, id, payload)
-        listeners_for(name).each { |s| s.finish(name, id, payload) }
+      def finish(name, id, payload, listeners = listeners_for(name))
+        listeners.each { |s| s.finish(name, id, payload) }
       end
 
       def publish(name, *args)
diff --git a/activesupport/lib/active_support/notifications/instrumenter.rb b/activesupport/lib/active_support/notifications/instrumenter.rb
index 075ddc2382..67f2ee1a7f 100644
--- a/activesupport/lib/active_support/notifications/instrumenter.rb
+++ b/activesupport/lib/active_support/notifications/instrumenter.rb
@@ -15,14 +15,15 @@ module ActiveSupport
       # and publish it. Notice that events get sent even if an error occurs
       # in the passed-in block.
       def instrument(name, payload={})
-        start name, payload
+        # some of the listeners might have state
+        listeners_state = start name, payload
         begin
           yield payload
         rescue Exception => e
           payload[:exception] = [e.class.name, e.message]
           raise e
         ensure
-          finish name, payload
+          finish_with_state listeners_state, name, payload
         end
       end
 
@@ -36,6 +37,10 @@ module ActiveSupport
         @notifier.finish name, @id, payload
       end
 
+      def finish_with_state(listeners_state, name, payload)
+        @notifier.finish name, @id, payload, listeners_state
+      end
+
       private
 
       def unique_id