Merge branch 'tim/SessionClientDisconnectedError' into tim/StreamlitR…

…untime

* tim/SessionClientDisconnectedError:
  better docstring
  typo
  SessionClientDisconnectedError, with test
  WIP
  * Remove staticmethod decorator from __call__ method of SingletonAPI to avoid mypy bug python/mypy#7781 (streamlit#4981)

lib/streamlit/caching/singleton_decorator.py

@@ -122,22 +122,24 @@ class SingletonAPI:
 
     # Bare decorator usage
     @overload
-    @staticmethod
-    def __call__(func: F) -> F:
+    def __call__(self, func: F) -> F:
         ...
 
     # Decorator with arguments
     @overload
-    @staticmethod
     def __call__(
+        self,
         *,
         show_spinner: bool = True,
         suppress_st_warning=False,
     ) -> Callable[[F], F]:
         ...
 
-    @staticmethod
+    # __call__ should be a static method, but there's a mypy bug that
+    # breaks type checking for overloaded static functions:
+    # https://github.com/python/mypy/issues/7781
     def __call__(
+        self,
         func: Optional[F] = None,
         *,
         show_spinner: bool = True,

lib/streamlit/web/server/server.py

@@ -119,11 +119,21 @@
 SCRIPT_RUN_CHECK_TIMEOUT = 60
 
 
+class SessionClientDisconnectedError(Exception):
+    """Raised by operations on a disconnected SessionClient."""
+
+    pass
+
+
 class SessionClient(Protocol):
     """Interface for sending data to a session's client."""
 
     def write_forward_msg(self, msg: ForwardMsg) -> None:
-        """Deliver a ForwardMsg to the client."""
+        """Deliver a ForwardMsg to the client.
+
+        If the SessionClient has been disconnected, it should raise a
+        SessionClientDisconnectedError.
+        """
 
 
 class SessionInfo:
@@ -515,7 +525,7 @@ async def _loop_coroutine(
                         for msg in msg_list:
                             try:
                                 self._send_message(session_info, msg)
-                            except tornado.websocket.WebSocketClosedError:
+                            except SessionClientDisconnectedError:
                                 self._close_app_session(session_info.session.id)
                             await asyncio.sleep(0)
                         await asyncio.sleep(0)
@@ -722,7 +732,10 @@ def check_origin(self, origin: str) -> bool:
 
     def write_forward_msg(self, msg: ForwardMsg) -> None:
         """Send a ForwardMsg to the browser."""
-        self.write_message(serialize_forward_msg(msg), binary=True)
+        try:
+            self.write_message(serialize_forward_msg(msg), binary=True)
+        except tornado.websocket.WebSocketClosedError as e:
+            raise SessionClientDisconnectedError from e
 
     def open(self, *args, **kwargs) -> Optional[Awaitable[None]]:
         # Extract user info from the X-Streamlit-User header

lib/tests/streamlit/web/server/server_test.py

@@ -372,6 +372,48 @@ async def test_orphaned_upload_file_deletion(self):
                 [],
             )
 
+    @tornado.testing.gen_test
+    async def test_send_message_to_disconnected_websocket(self):
+        """Sending a message to a disconnected SessionClient raises an error.
+        We should gracefully handle the error by cleaning up the session.
+        """
+        with patch(
+            "streamlit.web.server.server.LocalSourcesWatcher"
+        ), self._patch_app_session():
+            await self.start_server_loop()
+            await self.ws_connect()
+
+            # Get the server's socket and session for this client
+            session_info = list(self.server._session_info_by_id.values())[0]
+
+            with patch.object(
+                session_info.session, "flush_browser_queue"
+            ) as flush_browser_queue, patch.object(
+                session_info.client, "write_message"
+            ) as ws_write_message:
+                # Patch flush_browser_queue to simulate a pending message.
+                flush_browser_queue.return_value = [_create_dataframe_msg([1, 2, 3])]
+
+                # Patch the session's WebsocketHandler to raise a
+                # WebSocketClosedError when we write to it.
+                ws_write_message.side_effect = tornado.websocket.WebSocketClosedError()
+
+                # Tick the server. Our session's browser_queue will be flushed,
+                # and the Websocket client's write_message will be called,
+                # raising our WebSocketClosedError.
+                while not flush_browser_queue.called:
+                    self.server._need_send_data.set()
+                    await asyncio.sleep(0)
+
+                flush_browser_queue.assert_called_once()
+                ws_write_message.assert_called_once()
+
+                # Our session should have been removed from the server as
+                # a result of the WebSocketClosedError.
+                self.assertIsNone(
+                    self.server._get_session_info(session_info.session.id)
+                )
+
 
 class ServerUtilsTest(unittest.TestCase):
     def test_is_url_from_allowed_origins_allowed_domains(self):