Remove child event tracking from AS::Subscriber

Previously ActiveSupport::Subscriber would manage its own events, keep
them in a thread-local stack, and track "child" events of the same
subscriber.

I don't think this was particularly useful, since it only considered
notifications delivered to the same subscriber, and I can't find
evidence of this being used in the wild. I think this was intended to
support tracing, but any uch tool would want to do this itself (at
minimum in order to support multiple namespaces).

Additionally, this has caused a few users to OOM in production
environments, notably when queueing many jobs from another job.

activesupport/lib/active_support/log_subscriber.rb

@@ -107,14 +107,10 @@ module ActiveSupport
       LogSubscriber.logger
     end
 
-    def start(name, id, payload)
+    def call(event)
-      super if logger
-    end
-
-    def finish(name, id, payload)
       super if logger
     rescue => e
-      log_exception(name, e)
+      log_exception(event.name, e)
     end
 
     def publish_event(event)

activesupport/lib/active_support/subscriber.rb

@@ -126,26 +126,12 @@ module ActiveSupport
     attr_reader :patterns # :nodoc:
 
     def initialize
-      @queue_key = [self.class.name, object_id].join "-"
       @patterns  = {}
       super
     end
 
-    def start(name, id, payload)
+    def call(event)
-      event = ActiveSupport::Notifications::Event.new(name, nil, nil, id, payload)
+      method = event.name.split(".").first
-      event.start!
-      parent = event_stack.last
-      parent << event if parent
-
-      event_stack.push event
-    end
-
-    def finish(name, id, payload)
-      event = event_stack.pop
-      event.finish!
-      event.payload.merge!(payload)
-
-      method = name.split(".").first
       send(method, event)
     end
 
@@ -153,11 +139,5 @@ module ActiveSupport
       method = event.name.split(".").first
       send(method, event)
     end
-
-    private
-      def event_stack
-        registry = ActiveSupport::IsolatedExecutionState[:active_support_subscriber_queue_registry] ||= {}
-        registry[@queue_key] ||= []
-      end
   end
 end

activesupport/test/log_subscriber_test.rb

@@ -77,7 +77,9 @@ class SyncLogSubscriberTest < ActiveSupport::TestCase
 
   def test_event_attributes
     ActiveSupport::LogSubscriber.attach_to :my_log_subscriber, @log_subscriber
-    instrument "some_event.my_log_subscriber"
+    instrument "some_event.my_log_subscriber" do
+      [] # Make an allocation
+    end
     wait
     event = @log_subscriber.event
     if defined?(JRUBY_VERSION)