Issue 1:
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. N/A

I have included all the versions of all the external dependencies required

to reproduce this bug. N/A

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

#4653

Possible Duplicates

None

Environment & Settings

Celery version: 4.4.7 (cliffs)

celery report Output:

software -> celery:4.4.7 (cliffs) kombu:4.6.11 py:3.8.2
billiard:3.6.3.0 py-amqp:2.6.1
platform -> system:Darwin arch:64bit
kernel version:16.7.0 imp:CPython
loader -> celery.loaders.app.AppLoader
settings -> transport:amqp results:db+postgresql://<redacted>

include: ['redacted']
accept_content: ['redacted-custom']
database_table_names: {
'group': 'celery_group', 'task': 'celery_task'}
result_serializer: 'redacted-custom'
task_serializer: 'redacted-custom'
task_track_started: True
broker_url: 'amqp://<redacted>'
result_backend: 'db+postgresql://<redacted>'

Steps to Reproduce

When celery uses a database result backend, the following line can be called multiple times from different processes:

celery/celery/backends/database/session.py

Line 56
in
9a6c292

ResultModelBase.metadata.create_all(engine)

This is a race condition because SQLAlchemy first checks if the tables/sequences exist and then tries to create them. It causes errors like this (at least on PostgreSQL):

Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/redacted.py", line 168, in _redacted
result = async_result.get()
File "/usr/local/lib/python3.7/site-packages/celery/result.py", line 226, in get
self.maybe_throw(callback=callback)
File "/usr/local/lib/python3.7/site-packages/celery/result.py", line 342, in maybe_throw
self.throw(value, self._to_remote_traceback(tb))
File "/usr/local/lib/python3.7/site-packages/celery/result.py", line 335, in throw
self.on_ready.throw(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/vine/promises.py", line 244, in throw
reraise(type(exc), exc, tb)
File "/usr/local/lib/python3.7/site-packages/vine/five.py", line 195, in reraise
raise value
Exception: <class 'sqlalchemy.exc.IntegrityError'>(('(psycopg2.errors.UniqueViolation) duplicate key value violates unique constraint "pg_type_typname_nsp_index"\nDETAIL: Key (typname, typnamespace)=(taskset_id_sequence, 2200) already exists.\n',))

One workaround is to force the table creation ahead of time as was proposed by a user in the issue I linked: #4653 (comment).

I think Celery should handle this itself. A possible solution would catch IntegrityError and try again until create_all succeeds. (Perhaps with a limited number of retries and with sleeps compared to this snippet):

def prepare_models(self, engine):
from sqlalchemy.exc import IntegrityError
if not self.prepared:
while True:
try:
ResultModelBase.metadata.create_all(engine)
except IntegrityError:
continue
else:
break
self.prepared = True

Minimally Reproducible Test Case

This example doesn't use celery at all, but shows that calling create_all in multiple processes can cause the error. It's a race condition, so you might need to try it multiple times or play around with the number of processes:

Requires a local postgres, and this database must be created:

createdb racetest

from concurrent.futures import ProcessPoolExecutor, as_completed

from sqlalchemy import Column, Integer, Table, MetaData, create_engine

metadata = MetaData()

tbl1 = Table('tbl1', metadata, Column('id', Integer, primary_key=True))

def create_all(url):
engine = create_engine(url)
metadata.create_all(bind=engine)

def main():
url = 'postgresql:///racetest'
engine = create_engine(url)

# Make sure schema is empty before we start
metadata.drop_all(bind=engine)

with ProcessPoolExecutor(max_workers=50) as executor:
futures = []
for _ in range(50):
future = executor.submit(create_all, url)
futures.append(future)

for fut in as_completed(futures):
fut.result()

if __name__ == '__main__':
main()

Issue 2:
Checklist

I have checked the issues list

for similar or identical enhancement to an existing feature.

I have checked the pull requests list

for existing proposed enhancements.

I have checked the commit log

to find out if the if the same enhancement was already implemented in the

main branch.

I have included all related issues and possible duplicate issues in this issue

(If there are none, check this box anyway).

Related Issues and Possible Duplicates

Related Issues

None

Possible Duplicates

None

Brief Summary

Considering pytz is deprecating in Django and will be removed in 5.0, suggest follow approaches in Django

use zoneinfo to replace pytz

backport by backports.zoneinfo

Ref: https://docs.djangoproject.com/en/4.1/releases/4.0/

Design

Architectural Considerations

None

Proposed Behavior

No changes

Proposed UI/UX

No changes

Diagrams

N/A

Alternatives

None