response return value runtime check (#3321)

aiohttp/web_protocol.py

@@ -18,7 +18,7 @@
 from .tcp_helpers import tcp_cork, tcp_keepalive, tcp_nodelay
 from .web_exceptions import HTTPException
 from .web_request import BaseRequest
-from .web_response import Response
+from .web_response import Response, StreamResponse
 
 
 __all__ = ('RequestHandler', 'RequestPayloadError', 'PayloadAccessError')
@@ -346,6 +346,7 @@ async def start(self):
         handler = self._task_handler
         manager = self._manager
         keepalive_timeout = self._keepalive_timeout
+        resp = None
 
         while not self._force_close:
             if not self._messages:
@@ -389,6 +390,13 @@ async def start(self):
                             "please raise the exception instead",
                             DeprecationWarning)
 
+                if self.debug:
+                    if not isinstance(resp, StreamResponse):
+                        self.log_debug("Possibly missing return "
+                                       "statement on request handler")
+                        raise RuntimeError("Web-handler should return "
+                                           "a response instance, "
+                                           "got {!r}".format(resp))
                 await resp.prepare(request)
                 await resp.write_eof()
 
@@ -438,7 +446,7 @@ async def start(self):
                 self.log_exception('Unhandled exception', exc_info=exc)
                 self.force_close()
             finally:
-                if self.transport is None:
+                if self.transport is None and resp is not None:
                     self.log_debug('Ignored premature client disconnection.')
                 elif not self._force_close:
                     if self._keepalive and not self._close:

tests/test_web_protocol.py

@@ -517,6 +517,32 @@ async def cancel():
     assert log.debug.called
 
 
+async def test_handle_none_response(make_srv, transport, request_handler):
+    loop = asyncio.get_event_loop()
+    log = mock.Mock()
+
+    srv = make_srv(logger=log, debug=True)
+    srv.connection_made(transport)
+
+    handle = mock.Mock()
+    handle.return_value = loop.create_future()
+    handle.return_value.set_result(None)
+    request_handler.side_effect = handle
+
+    srv.data_received(
+        b'GET / HTTP/1.0\r\n'
+        b'Content-Length: 10\r\n'
+        b'Host: example.com\r\n\r\n')
+
+    assert srv._task_handler
+
+    await asyncio.sleep(0, loop=loop)
+    await srv._task_handler
+    assert request_handler.called
+    log.debug.assert_called_with("Possibly missing return "
+                                 "statement on request handler")
+
+
 async def test_handle_cancelled(make_srv, transport) -> None:
     log = mock.Mock()