IndexError: pop from an empty deque from _kernel_wake

[mwerezak]

Got this exception from inside curio/sched.py

Not sure yet as to the cause.

Traceback (most recent call last):
  File "...", line 72, in <module>
    curio.run(main)
  File "...\venv\Lib\site-packages\curio\kernel.py", line 824, in run
    return kernel.run(corofunc, *args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\venv\Lib\site-packages\curio\kernel.py", line 146, in run
    task = self._runner(coro)
           ^^^^^^^^^^^^^^^^^^
  File "...\venv\Lib\site-packages\curio\kernel.py", line 768, in kernel_run
    traps[trap[0]](*trap[1:])
  File "...\venv\Lib\site-packages\curio\kernel.py", line 519, in trap_sched_wake
    tasks = sched._kernel_wake(n)
            ^^^^^^^^^^^^^^^^^^^^^
  File "...\venv\Lib\site-packages\curio\sched.py", line 88, in _kernel_wake
    task, = self._queue.popleft()
            ^^^^^^^^^^^^^^^^^^^^^
IndexError: pop from an empty deque

ntasks is 1, but self._queue is empty.

__exception__ = {tuple: 3} (<class 'IndexError'>, pop from an empty deque, <traceback object at 0x00000261BE8AA500>)
ntasks = {int} 1
self = {SchedFIFO: 0} SchedFIFO<0 tasks waiting>
  _abc_impl = {_abc_data} <_abc._abc_data object at 0x000002619CFB9E80>
  _actual_len = {int} 0
  _queue = {deque: 0} deque([])
task = {NoneType} None
tasks = {list: 0} []
 __len__ = {int} 0

[mwerezak]

I have no idea where to even start looking for the cause of this. Any insight would be appreciated.

[mwerezak]

It seems related to creating a lot of timeouts using curio.timeout_after()

This appears to be the code in my application that triggers it:

        async with self._condition:
            while not self._bus_available():
                try:
                    async with curio.timeout_after(self.wait_interval):
                        await self._condition.wait()
                except curio.TaskTimeout:
                    _log.debug(f"{client}: wait.")
                    await client.send_message(WAIT_MSG)

I'm trying to wait for a condition but I also want to send regular messages back to the client so they know the connection isn't timed out

[dabeaz]

What version of curio? What version of Python? What operating system? Is the bug predictably reproducible or is it random?

[mwerezak]

Hi, sorry for forgetting to include that information.

curio version 1.6
Python version 3.11.5

From memory the bug was pretty predictable but depended on workload... that snippet above was being hit ~10-100/second, with the timeout value being about 5 seconds IIRC... I guess that means several hundred timeouts active at once. Under lesser loads the bug did not manifest, but I don't recall the exact number.

We implemented a workaround by frequently polling (and checking the timeout manually using time()) instead of blocking on a condition variable. In retrospect maybe a MPSC setup using a Queue might have been a better solution than either of these.