Correct prestreaming controller response status.

if the controller action has not yet streamed any data, actions should
process as normal, and errors should trigger the appropriate behavior
(500, or in the case of ActionController::BadRequest, a 400 Bad Request)

actionpack/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Set stream status to 500 (or 400 on BadRequest) when an error is thrown
+    before commiting.
+
+    Fixes #12552.
+
+    *Kevin Casey*
+
 *   Add new config option `config.action_dispatch.cookies_serializer` for
     specifying a serializer for the signed and encrypted cookie jars.
 

actionpack/lib/action_controller/metal/live.rb

@@ -205,6 +205,8 @@ module ActionController
         begin
           super(name)
         rescue => e
+          @_response.status = 500 unless @_response.committed?
+          @_response.status = 400 if e.class == ActionController::BadRequest
           begin
             @_response.stream.write(ActionView::Base.streaming_completion_on_exception) if request.format == :html
             @_response.stream.call_on_error

actionpack/test/controller/live_stream_test.rb

@@ -156,6 +156,14 @@ module ActionController
         raise 'An exception occurred...'
       end
 
+      def exception_in_controller
+        raise 'Exception in controller'
+      end
+
+      def bad_request_error
+        raise ActionController::BadRequest
+      end
+
       def exception_in_exception_callback
         response.headers['Content-Type'] = 'text/event-stream'
         response.stream.on_error do
@@ -275,6 +283,16 @@ module ActionController
       end
     end
 
+    def test_exception_in_controller_before_streaming
+      response = get :exception_in_controller, format: 'text/event-stream'
+      assert_equal 500, response.status
+    end
+
+    def test_bad_request_in_controller_before_streaming
+      response = get :bad_request_error, format: 'text/event-stream'
+      assert_equal 400, response.status
+    end
+
     def test_exceptions_raised_handling_exceptions
       capture_log_output do |output|
         get :exception_in_exception_callback, format: 'text/event-stream'