Client has been deleted but is still being used. This is most likely a bug in your application code

[Smug246]

Question

i cant really minimalise the code because the error doesnt tell me where its coming from but for context im hosting an app of fly.io and randomly i will see this error in the live logs but nothing is actually affected. so ive no idea how to actually fix this or if anyone has come across it before. or is it just a harmless warning?

NiceGUI ready to go on http://localhost:8080, http://172.19.136.58:8080, and http://172.19.136.59:8080
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] Client has been deleted but is still being used. This is most likely a bug in your application code. See https://github.com/zauberzeug/nicegui/issues/3028 for more information.
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] Stack (most recent call last):
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "<string>", line 1, in <module>
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/multiprocessing/spawn.py", line 122, in spawn_main
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] exitcode = _main(fd, parent_sentinel)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/multiprocessing/spawn.py", line 135, in _main
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] return self._bootstrap(parent_sentinel)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/multiprocessing/process.py", line 314, in _bootstrap
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] self.run()
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/multiprocessing/process.py", line 108, in run
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] self._target(*self._args, **self._kwargs)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/uvicorn/_subprocess.py", line 80, in subprocess_started
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] target(sockets=sockets)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/nicegui/server.py", line 36, in run
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] super().run(sockets=sockets)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/uvicorn/server.py", line 65, in run
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] return asyncio.run(self.serve(sockets=sockets))
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/asyncio/runners.py", line 194, in run
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] return runner.run(main)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/asyncio/runners.py", line 118, in run
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] return self._loop.run_until_complete(task)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/nicegui/awaitable_response.py", line 29, in _fire
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] self.fire_and_forget()
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/nicegui/client.py", line 214, in send_and_forget
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] self.outbox.enqueue_message('run_javascript', {'code': code}, target_id)
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/nicegui/outbox.py", line 51, in enqueue_message
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] self.client.check_existence()
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] File "/usr/local/lib/python3.12/site-packages/nicegui/client.py", line 330, in check_existence
2024-06-11T11:10:46.976 app[0801967b166738] lhr [info] log.warning('Client has been deleted but is still being used. This is most likely a bug in your application code. '

i notice this #3028 but i dont have a ui.log and i dont explicitly delete anything. so im guessing it could be coming from a ui.refreshable func that is being deleted and updated maybe?

[falkoschindler]

Hi @Smug246,

The problem isn't that an element is deleted. In case of ui.log this was only the operation that caused an exception, unveiling the underlying issue. The newly implemented warning is triggered as soon as some UI element is updated, which doesn't have a connected client. So when some of your application code holds a reference to, say, a label in some global variable and regularly updates its text, you will get a warning as soon as the client, which displays this label, disconnects. Like any other UI element, the label holds a reference to its client. But if your code holds a reference to the label, its disconnected client can't be removed from memory, causing a memory leak. This is a very subtle but serious bug, which is why we added this warning. But it seems like it is hard to pinpoint the root cause, because the warning is triggered by the outbox which sends the update to the client, not by the code that actually causes the update.

How to proceed: I recommend reproducing the warning by opening and closing pages (and waiting several seconds, the reconnect_timeout, until the client actually gets disconnected). Once you have a reproduction, try to remove code until the warning doesn't appear anymore. Look out for code that collects UI elements in long-living lists or dictionaries, i.e. that are still used after a client disconnects. In #3028 this happened in form of log handlers that held a reference to ui.log.

For reference, here is my original explanation of the problem: #3028 (comment).

[v479038280]

The issue also occurs when the page corresponding to the /path path is opened in field_Friend.

[falkoschindler]

@v479038280

the page corresponding to the /path path

I don't quite understand. But if the warning appears within the Field Friend project, I'd suggest to open an issue over there.

[v479038280]

The error message shows that it's the problem of /nicegui/client.py

[falkoschindler]

client.py is where the problem is detected and the warning triggered. But the root cause is probably "in your application code", which could be the Field Friend repository in this case.

[v479038280]

I opend one discussion within Field Friend git repo

[Buruxianian]

I have encountered a similar problem， #3231 . After testing, lowering the Nicegui version to 1.4.25 can solve this problem.

[rodja]

@Buruxianian the problem is not solved when lowering the version. You just don't see the warning anymore. The memory leak is still there.