On the DST end that happened this weekend for Europe (the 2020-10-29, for CET/CEST or Europe/Paris), celery beat stopped sending tasks that should be running every minutes.

Note: AFAIK, Europe may stop DST change by 2021. so this bug may become irrelevant for Europe/CET/CEST. But other country may still encounter it.

On 26 March 2019, the European Parliament [...] [supports] a stop to the seasonal clock changes by 2021.
-- https://ec.europa.eu/transport/themes/summertime_en

Checklist

• I have verified that the issue exists against the master branch of Celery.
• This has already been asked to the discussion group first.
• I have read the relevant section in the
  contribution guide
  on reporting bugs.
• I have checked the issues list
  for similar or identical bug reports.
• I have checked the pull requests list
  for existing proposed fixes.
• I have checked the commit log
  to find out if the bug was already fixed in the master branch.
• I have included all related issues and possible duplicate issues
  in this issue (If there are none, check this box anyway).

Mandatory Debugging Information

• I have included the output of celery -A proj report in the issue.
  (if you are not able to do this, then at least specify the Celery
  version affected).
• I have verified that the issue exists against the master branch of Celery.
• I have included the contents of pip freeze in the issue.
• I have included all the versions of all the external dependencies required
  to reproduce this bug.

Optional Debugging Information

• I have tried reproducing the issue on more than one Python version
  and/or implementation.
• I have tried reproducing the issue on more than one message broker and/or
  result backend.
• I have tried reproducing the issue on more than one version of the message
  broker and/or result backend.
• I have tried reproducing the issue on more than one operating system.
• I have tried reproducing the issue on more than one workers pool.
• I have tried reproducing the issue with autoscaling, retries,
  ETA/Countdown & rate limits disabled.
• I have tried reproducing the issue after downgrading
  and/or upgrading Celery and its dependencies.

Related Issues and Possible Duplicates

Related Issues

This is not the first time DST change cause some issue. Example:

• Fix periodic tasks running twice the day after DST (Fix #1604) #4403

Possible Duplicates

• None

Environment & Settings

Celery version: 5.0.1 (singularity)

software -> celery:5.0.1 (singularity) kombu:5.0.2 py:3.8.5
            billiard:3.6.3.0 py-amqp:5.0.1
platform -> system:Linux arch:64bit, ELF
            kernel version:5.4.0-48-generic imp:CPython
loader   -> celery.loaders.app.AppLoader
settings -> transport:pyamqp results:disabled

broker_url: 'amqp://guest:********@localhost:5672//'
deprecated_settings: None
timezone: 'Europe/Paris'
enable_utc: True
beat_schedule: {
    'add-every-10-seconds': {   'args': (16, 16),
                                'schedule': 10.0,
                                'task': 'tasks.add'}}

Steps to Reproduce

The repository https://github.com/PierreF/celery-dst-issue contains code & README to reproduce steps.

But the idea is:

• Have any tasks running every minutes (I think anything more often than every hour).
• Start celery beat few minutes before DST end (at least for CET/CEST, probably because the "same" time happen twice. It is 2:00 CEST then one hour later it's 2:00 CET).
• See that minutely tasks stop running after DST change.

Required Dependencies

I believe version does not matter. I've tested with

• Minimal Python Version: 3.8.5
• Minimal Celery Version: master as of 2020-10-26, shown as 5.0.1
• Minimal Kombu Version: 5.0.2
• Minimal Broker Version: RabbitMQ 3.8.5
• Minimal Result Backend Version: none
• Minimal OS and/or Kernel Version: Ubuntu 20.04 with Linux 5.4.0-48-generic
• Minimal Broker Client Version: amqp==5.0.1
• Minimal Result Backend Client Version: none

Python Packages

amqp==5.0.1
appdirs==1.4.3
billiard==3.6.3.0
CacheControl==0.12.6
celery==5.0.1
certifi==2019.11.28
chardet==3.0.4
click==7.1.2
click-didyoumean==0.0.3
click-repl==0.1.6
colorama==0.4.3
contextlib2==0.6.0
distlib==0.3.0
distro==1.4.0
html5lib==1.0.1
idna==2.8
ipaddr==2.2.0
kombu==5.0.2
lockfile==0.12.2
msgpack==0.6.2
packaging==20.3
pep517==0.8.2
progress==1.5
prompt-toolkit==3.0.8
pyparsing==2.4.6
pytoml==0.1.21
pytz==2020.1
requests==2.22.0
retrying==1.3.3
six==1.14.0
urllib3==1.25.8
vine==5.0.0
wcwidth==0.2.5
webencodings==0.5.1

Other Dependencies

Minimally Reproducible Test Case

See https://github.com/PierreF/celery-dst-issue

sudo apt-get install faketime
git clone https://github.com/PierreF/celery-dst-issue
cd celery-dst-issue
mkvirtualenv -p /usr/bin/python3 celery
pip install celery
docker run -d --name celery-rabbitmq -p 127.0.0.1:5672:5672 rabbitmq:3
rm celerybeat-schedule
faketime '2020-10-25 00:58:30Z' celery -A tasks worker --loglevel=INFO -B

Expected Behavior

The tasks add-every-10-seconds runs every 10 seconds

Actual Behavior

The tasks add-every-10-seconds runs every 10 seconds before the DST change but stop after DST change.