Merge payload for EventObject subscribers

When instrumenting a block of code like:

```ruby
ActiveSupport::Notifications.instrument("process_action.action_controller", raw_paylaod) do |payload|
 payload[:view_runtime] = render_view
end
```
If we use an evented subscriber like so:

``` ruby
ActiveSupport::Notifications.subscribe("process_action.action_controller", raw_payload) do |event|
 assert event.payload[:view_runtime]
end
```

The code breaks because the underlying EventObject's payload does not have the
`:view_runtime` key added during instrumentation.

This is because the `EventedObject` subscriber calls the `finish` method with the
`payload` of the event at the time it was pushed into the stack, before the
block executes, but we want to call `finish`  with the `payload` after the
instrument block executes this way if the `payload` was modified during the block
we have access to it. This is consistent with the other types of subscribers
who don't have this bug.

2 files changed, 3 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/notifications/fanout.rb b/activesupport/lib/active_support/notifications/fanout.rb
index c37bec4ee5..97fe5bb09b 100644
--- a/activesupport/lib/active_support/notifications/fanout.rb
+++ b/activesupport/lib/active_support/notifications/fanout.rb
@@ -218,6 +218,7 @@ module ActiveSupport
           def finish(name, id, payload)
             stack = Thread.current[:_event_stack]
             event = stack.pop
+            event.payload = payload
             event.finish!
             @delegate.call event
           end
diff --git a/activesupport/lib/active_support/notifications/instrumenter.rb b/activesupport/lib/active_support/notifications/instrumenter.rb
index 12546511a8..6ce5aa1af4 100644
--- a/activesupport/lib/active_support/notifications/instrumenter.rb
+++ b/activesupport/lib/active_support/notifications/instrumenter.rb
@@ -53,7 +53,8 @@ module ActiveSupport
     end
 
     class Event
-      attr_reader :name, :time, :end, :transaction_id, :payload, :children
+      attr_reader :name, :time, :end, :transaction_id, :children
+      attr_accessor :payload
 
       def self.clock_gettime_supported? # :nodoc:
         defined?(Process::CLOCK_PROCESS_CPUTIME_ID) &&