Add notifications to ActionDispatch::ShowExceptions, this can be used as hooks for plugins like ExceptionNotifier.

actionpack/lib/action_dispatch/middleware/show_exceptions.rb

@@ -1,7 +1,24 @@
 require 'active_support/core_ext/exception'
+require 'active_support/notifications'
 require 'action_dispatch/http/request'
 
 module ActionDispatch
+  # This middleware rescues any exception returned by the application and renders
+  # nice exception pages if it's being rescued locally.
+  #
+  # Every time an exception is caught, a notification is published, becoming a good API
+  # to deal with exceptions. So, if you want send an e-mail through ActionMailer
+  # everytime this notification is published, you just need to do the following:
+  #
+  #   ActiveSupport::Notifications.subscribe "action_dispatch.show_exception" do |name, start, end, instrumentation_id, payload|
+  #     ExceptionNotifier.deliver_exception(start, payload)
+  #   end
+  #
+  # The payload is a hash which has to pairs:
+  #
+  # * :env - Contains the rack env for the given request;
+  # * :exception - The exception raised;
+  #
   class ShowExceptions
     LOCALHOST = '127.0.0.1'.freeze
 
@@ -44,8 +61,11 @@ def initialize(app, consider_all_requests_local = false)
     def call(env)
       @app.call(env)
     rescue Exception => exception
-      raise exception if env['action_dispatch.show_exceptions'] == false
-      render_exception(env, exception)
+      ActiveSupport::Notifications.instrument 'action_dispatch.show_exception',
+        :env => env, :exception => exception do
+        raise exception if env['action_dispatch.show_exceptions'] == false
+        render_exception(env, exception)
+      end
     end
 
     private

actionpack/test/dispatch/show_exceptions_test.rb

@@ -104,4 +104,24 @@ class ShowExceptionsTest < ActionController::IntegrationTest
     assert_response 405
     assert_match /ActionController::MethodNotAllowed/, body
   end
+
+  test "publishes notifications" do
+    @app, event = ProductionApp, nil
+    self.remote_addr = '127.0.0.1'
+
+    ActiveSupport::Notifications.subscribe('action_dispatch.show_exception') do |*args|
+      event = args
+    end
+
+    get "/"
+    assert_response 500
+    assert_match /puke/, body
+
+    ActiveSupport::Notifications.notifier.wait
+
+    assert_equal 'action_dispatch.show_exception', event.first
+    assert_kind_of Hash, event.last[:env]
+    assert_equal 'GET', event.last[:env]["REQUEST_METHOD"]
+    assert_kind_of RuntimeError, event.last[:exception]
+  end
 end