RuntimeError: Unexpected ASGI message 'websocket.send', after sending 'websocket.close' or response already completed.

[MarcSkovMadsen]

I've created an Panel application which basically works as described below.

We've defined a global instance of

class DataStore(param.Parameterized):
     value = param.DataFrame()

global_data_store = DataStore()

The global_data_store.value is updated every 3 seconds from a loop running in a separate thread. Every application then displays the DataFrame in a table Tabulator(value=global_data_store.param.value, ....). The Tabulator table is inside a layout in the main area of a FastListTemplate.

When ever I reload the page I see

RuntimeError: Unexpected ASGI message 'websocket.send', after sending 'websocket.close' or response already completed.
2025-08-17 17:28:27,445 | ERROR | tornado.application | Exception in callback functools.partial(<bound method IOLoop._discard_future_result of <tornado.platform.asyncio.AsyncIOMainLoop object at 0x7f57735b1710>>, <Task finished name='Task-49128' coro=<ServerSession.with_document_locked() done, defined at /home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/bokeh/server/session.py:77> exception=RuntimeError("Unexpected ASGI message 'websocket.send', after sending 'websocket.close' or response already completed.")>)
Traceback (most recent call last):
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/tornado/ioloop.py", line 758, in _run_callback
    ret = callback()
          ^^^^^^^^^^
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/tornado/ioloop.py", line 782, in _discard_future_result
    future.result()
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/bokeh/server/session.py", line 106, in _needs_document_lock_wrapper
    await p
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/bokeh_fastapi/handler.py", line 355, in send_message
    await self._socket.send_text(message.header_json)
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/starlette/websockets.py", line 166, in send_text
    await self.send({"type": "websocket.send", "text": data})
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/starlette/websockets.py", line 86, in send
    await self._send(message)
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 39, in sender
    await send(message)
  File "/home/jovyan/repos/revenue-price-gui/.venv/lib/python3.11/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 360, in asgi_send
    raise RuntimeError(msg % message_type)
RuntimeError: Unexpected ASGI message 'websocket.send', after sending 'websocket.close' or response already completed.

I will try to provide minimum, reproducible code when/ if I get the time. But I believe it could be possible from this explanation to find the place where messages continues to be send after websocket.close has been sent.

[MarcSkovMadsen]

If I update WSHandler.send_message to check for websocket connection closed, no exception is raised and the app seems to work just fine before and after reload. I.e. a working method could look like below:

    async def send_message(self, message: Message) -> int:
        sent = 0

        from starlette.websockets import WebSocketState
        if self._socket.client_state != WebSocketState.CONNECTED:
            log.debug("WebSocket state is %s, skipping message send", self._socket.client_state.name)
            return sent
    
        try:
            await self._socket.send_text(message.header_json)
            sent += len(message.header_json)

            await self._socket.send_text(message.metadata_json)
            sent += len(message.metadata_json)

            await self._socket.send_text(message.content_json)
            sent += len(message.content_json)

            for buff in message._buffers:
                header = json.dumps(buff.ref)
                payload = buff.to_bytes()
                await self._socket.send_text(header)
                await self._socket.send_bytes(payload)
                sent += len(header) + len(payload)
        except WebSocketDisconnect as e:
            self.on_close(e.code, e.reason)
        return sent

[MarcSkovMadsen]

I've created a reproducible example here https://github.com/MarcSkovMadsen/panel-live-prices.

I can recreate this problem when installing as described in the README.md file and then running

cd src/panel_live_prices
python main.py

I can trigger the RuntimeError: Cannot call "send" once a close message has been sent. when I open multiple tabs and refresh the tabs multiple times quickly after each other.

I explain and reproduce the problem in this 3 min video https://youtu.be/6GQYPgNRp3s.

[MarcSkovMadsen]

Related to holoviz/panel#8137 @philippjfr . Did you experience this issue while fixing #8137?