Add ActiveSupport::Notifications hook to Broadcaster#broadcast

This addition of this notification hook will give users better visibility
into the messages being sent over the PubSub adapter.

actioncable/CHANGELOG.md

@@ -1,2 +1,5 @@
+*  Add ActiveSupport::Notifications hook to Broadcaster#broadcast
+
+   *Matthew Wear*
 
 Please check [5-0-stable](https://github.com/rails/rails/blob/5-0-stable/actioncable/CHANGELOG.md) for previous changes.

actioncable/lib/action_cable/server/broadcasting.rb

@@ -39,10 +39,14 @@ module ActionCable
 
           def broadcast(message)
             server.logger.info "[ActionCable] Broadcasting to #{broadcasting}: #{message.inspect}"
+
+            payload = { broadcasting: broadcasting, message: message, coder: coder }
+            ActiveSupport::Notifications.instrument("broadcast.action_cable", payload) do
               encoded = coder ? coder.encode(message) : message
               server.pubsub.broadcast broadcasting, encoded
             end
           end
         end
     end
+  end
 end

actioncable/test/server/broadcasting_test.rb

@@ -1,10 +1,7 @@
 require "test_helper"
+require "stubs/test_server"
 
 class BroadcastingTest < ActiveSupport::TestCase
-  class TestServer
-    include ActionCable::Server::Broadcasting
-  end
-
   test "fetching a broadcaster converts the broadcasting queue to a string" do
     broadcasting = :test_queue
     server = TestServer.new
@@ -12,4 +9,52 @@ class BroadcastingTest < ActiveSupport::TestCase
 
     assert_equal "test_queue", broadcaster.broadcasting
   end
+
+  test "broadcast generates notification" do
+    begin
+      server = TestServer.new
+
+      events = []
+      ActiveSupport::Notifications.subscribe "broadcast.action_cable" do |*args|
+        events << ActiveSupport::Notifications::Event.new(*args)
+      end
+
+      broadcasting = "test_queue"
+      message = { body: "test message" }
+      server.broadcast(broadcasting, message)
+
+      assert_equal 1, events.length
+      assert_equal "broadcast.action_cable", events[0].name
+      assert_equal broadcasting, events[0].payload[:broadcasting]
+      assert_equal message, events[0].payload[:message]
+      assert_equal ActiveSupport::JSON, events[0].payload[:coder]
+    ensure
+      ActiveSupport::Notifications.unsubscribe "broadcast.action_cable"
+    end
+  end
+
+  test "broadcaster from broadcaster_for generates notification" do
+    begin
+      server = TestServer.new
+
+      events = []
+      ActiveSupport::Notifications.subscribe "broadcast.action_cable" do |*args|
+        events << ActiveSupport::Notifications::Event.new(*args)
+      end
+
+      broadcasting = "test_queue"
+      message = { body: "test message" }
+
+      broadcaster = server.broadcaster_for(broadcasting)
+      broadcaster.broadcast(message)
+
+      assert_equal 1, events.length
+      assert_equal "broadcast.action_cable", events[0].name
+      assert_equal broadcasting, events[0].payload[:broadcasting]
+      assert_equal message, events[0].payload[:message]
+      assert_equal ActiveSupport::JSON, events[0].payload[:coder]
+    ensure
+      ActiveSupport::Notifications.unsubscribe "broadcast.action_cable"
+    end
+  end
 end