neutron should forbid configuring agent_down_time that is known to crash due to CPython epoll limitation

Hi,

Thanks for the report! I'm not really sure if we need to protect ourselves against an admin maintaining configuration, but at the same time I don't see a problem with the validation either. By the way let me link your patch here:

https://review.opendev.org/c/openstack/neutron/+/889373

@Bence, admin may not be aware that a particular config option gets passed down to epoll that has this (undocumented) limit. But we are aware, and so oslo.service (and neutron) could help admins by failing early - by enforcing that insane intervals raise exceptions.

The admins in the sceanario of this bug were setting `agent_down_time` to a really high value because they wanted to effectively disable the periodic checks / updates of the agent status in neutron. While it may be argued that they shouldn't have done it for other reasons (there's a good reason these checks should happen periodically and not be disabled), still that's what they did, and nothing stopped them. This bug is to make sure they are stopped at some limit that we know is broken.

Perhaps neutron may also want to tighten the option down even more to make sure that the periodic is executed more often, but that's a different discussion. This bug is for theoretical limit defined by libc common implementation.

@Ihar: Thanks for the explanation!

Reviewed: https://review.opendev.org/c/openstack/neutron/+/889373
Committed: https://opendev.org/openstack/neutron/commit/6fef1e65250dbda057206e1c2ee64f59b21d490f
Submitter: "Zuul (22348)"
Branch: master

commit 6fef1e65250dbda057206e1c2ee64f59b21d490f
Author: Lewis Denny <email address hidden>
Date: Mon Jul 31 16:38:22 2023 +1000

    Add max limit to agent_down_time

    The agent_down_time ends up being passed to an eventlet green-thread;
    under the hood, this uses a CPython C-types interface with a limitation
    of (2^32 / 2 - 1) INT_MAX (as defined in C) where int is usually 32 bits

    I have set the max value to (2^32 / 2 - 1)/1000 as agent_down_time
    configured in seconds, this ends up being 2147483.

    This patch is required as passing a larger number
    causes this error: OverflowError: timeout is too large

    If a user currently has a value larger than (2^32 / 2 - 1)/1000 set,
    Neutron Server will fail to start and will print out a very helpful
    error message.

    Closes-Bug: #2028724
    Change-Id: Ib5b943344cddbd468c00768461ba1ee00a2b4c58

This issue was fixed in the openstack/neutron 23.0.0.0b3 development milestone.