warn about late responses

which where dropped silently before

nicegui/page.py

@@ -12,6 +12,7 @@
 from .client import Client
 from .favicon import create_favicon_route
 from .language import Language
+from .logging import log
 
 if TYPE_CHECKING:
     from .api_router import APIRouter
@@ -87,6 +88,10 @@ def __call__(self, func: Callable[..., Any]) -> Callable[..., Any]:
         core.app.remove_route(self.path)  # NOTE make sure only the latest route definition is used
         parameters_of_decorated_func = list(inspect.signature(func).parameters.keys())
 
+        def warn_about_late_returns(task: asyncio.Task) -> None:
+            if task.result():
+                log.error(f'ignoring {task.result()}; it was returned after the html was delivered to the client')
+
         async def decorated(*dec_args, **dec_kwargs) -> Response:
             request = dec_kwargs['request']
             # NOTE cleaning up the keyword args so the signature is consistent with "func" again
@@ -105,7 +110,7 @@ async def wait_for_result() -> None:
                     if time.time() > deadline:
                         raise TimeoutError(f'Response not ready after {self.response_timeout} seconds')
                     await asyncio.sleep(0.1)
-                result = task.result() if task.done() else None
+                result = task.result() if task.done() else task.add_done_callback(warn_about_late_returns)  # type: ignore
             if isinstance(result, Response):  # NOTE if setup returns a response, we don't need to render the page
                 return result
             binding._refresh_step()  # pylint: disable=protected-access

tests/test_page.py

@@ -1,4 +1,5 @@
 import asyncio
+import re
 from uuid import uuid4
 
 from fastapi.responses import PlainTextResponse
@@ -286,6 +287,18 @@ async def page(plain: bool = False):
     screen.should_not_contain('normal NiceGUI page')
 
 
+def test_warning_about_to_late_responses(screen: Screen):
+    @ui.page('/')
+    async def page(client: Client):
+        await client.connected()
+        ui.label('NiceGUI page')
+        return PlainTextResponse('custom response')
+
+    screen.open('/')
+    screen.should_contain('NiceGUI page')
+    screen.assert_py_logger('ERROR', re.compile('it was returned after the html was delivered to the client'))
+
+
 def test_reconnecting_without_page_reload(screen: Screen):
     @ui.page('/', reconnect_timeout=3.0)
     def page():