allow raising HTTPError on PRE/POST Request events

rw/httpbase.py

@@ -117,9 +117,17 @@ def start_request(self, server_conn, request_conn):
     def _handle_request(self, request_scope, request):
         handler = self.handler(self, request)
         request_scope['handler'] = handler
-        yield PRE_REQUEST()
-        yield handler._execute([])
-        yield POST_REQUEST()
+        try:
+            yield PRE_REQUEST()
+            yield handler._execute([])
+            yield POST_REQUEST()
+        except Exception as e:
+            # Ensure exceptions in PRE and POST_REQUEST are
+            # also forwarded to the exception handler.
+            # It is not just important for logging but making
+            # `raise HTTPError()` in event handlers work.
+            handler._transforms = []
+            handler._handle_request_exception(e)
 
     def _request_finished(self, request_future):
         # access result to throw exceptions that might have occurred during

test/test_httpbase.py

@@ -1,3 +1,4 @@
+import tornado.web
 from tornado.testing import AsyncHTTPTestCase, AsyncTestCase, ExpectLog, gen_test
 
 import rw.httpbase
@@ -15,3 +16,20 @@ def get_app(self):
     def test_hello_world(self):
         response = self.fetch('/')
         assert response.body.decode('utf-8') == u'Hello World'
+
+
+class HTTPServerEventTest(AsyncHTTPTestCase):
+    def get_app(self):
+        return rw.httpbase.Application(handler=HelloWorldHandler)
+
+    def test_hello_world(self):
+        rw.httpbase.PRE_REQUEST.add(self.raise_404)
+        response = self.fetch('/')
+        assert response.code == 404
+
+    def raise_404(self):
+        raise tornado.web.HTTPError(404)
+
+    @classmethod
+    def teardown_class(cls):
+        rw.httpbase.PRE_REQUEST.clear()