Celery task hangs after calling .delay() in Django

Question

While calling the .delay() method of an imported task from a django application, the process gets stuck and the request is never completed.

We also don't get any error on the console. Setting up a set_trace() with pdb results in the same thing.

The following questions were reviewed which didn't help resolve the issue:

Calling celery task hangs for delay and apply_async

celery.delay hangs (recent, not an auth problem)

Eg.:

backend/settings.py

CELERY_BROKER_URL = os.environ.get("CELERY_BROKER", RABBIT_URL)
CELERY_RESULT_BACKEND = os.environ.get("CELERY_BROKER", RABBIT_URL)

backend/celery.py

from __future__ import absolute_import, unicode_literals

import os

from celery import Celery

# set the default Django settings module for the 'celery' program.
os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'backend.settings')

app = Celery('backend')

app.config_from_object('django.conf:settings', namespace='CELERY')

# Load task modules from all registered Django app configs.
app.autodiscover_tasks()


@app.task(bind=True)
def debug_task(self):
    print('Request: {0!r}'.format(self.request))

app/tasks.py

import time
from celery import shared_task

@shared_task
def upload_file(request_id):
    time.sleep(request_id)
    return True

app/views.py

from rest_framework.views import APIView

from .tasks import upload_file

class UploadCreateAPIView(APIView):
    # other methods...

    def post(self, request, *args, **kwargs):
        id = request.data.get("id", None)
        # business logic ...
        print("Going to submit task.")
        import pdb; pdb.set_trace()
        upload_file.delay(id)                  # <- this hangs the runserver as well as the set_trace()
        print("Submitted task.")

Answer 1

The issue was with the setup of the celery application with Django. We need to make sure that the celery app is imported and initialized in the following file:

backend\\__init__.py

from __future__ import absolute_import, unicode_literals

# This will make sure the app is always imported when
# Django starts so that shared_task will use this app.
from .celery import app as celery_app

__all__ = ('celery_app',)

Answer 2

I've run into this issue that Celery calls through delay or apply_async may randomly hang the program indefinitely. I tried the all broker_transport_options and retry_policy options to let Celery to recover, but it still happens. Then I found this solution to enforce an execution time limit for an execution block/function by using underlying Python signal handlers.

@contextmanager
def time_limit(seconds):
    def signal_handler(signum, frame):
        raise TimeoutException("Timed out!")

    signal.signal(signal.SIGALRM, signal_handler)
    signal.alarm(seconds)
    try:
        yield
    finally:
        signal.alarm(0)

def my_function():
    with time_limit(3):
        celery_call.apply_sync(kwargs={"k1", "v1"}, expires=30)