[QUESTION] execute a long running subprocess inside celery task is not starting

[sivaguru-nathan]

celery failed to run the command in subprocess. when restarting the celery worker the command is running in the background.
the subprocess is working without calling as celery task

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

• None

Possible Duplicates

• subprocess command call inside a task is not executing #891

Environment & Settings

Celery version:

celery report Output:

software -> celery:4.4.0 (cliffs) kombu:4.6.11 py:3.6.11
billiard:3.6.3.0 py-amqp:2.6.1
platform -> system:Linux arch:64bit, ELF
kernel version:5.3.0-51-generic imp:CPython
loader -> celery.loaders.app.AppLoader
settings -> transport:pyamqp results:rpc:///

broker_url: 'amqp://guest:********@192.168.43.150:5672//'
result_backend: 'rpc:///'
task_queue: <unbound Queue test_subprocess -> <unbound Exchange test_subprocess(direct)> -> model_builder.#>
task_routes: {
'*': {'queue': 'test_subprocess'}}

Python Packages

pip freeze Output:

absl-py==0.9.0 affine==2.3.0 amqp==2.6.1 astor==0.8.1 attrs==20.1.0 Babel==2.8.0 billiard==3.6.3.0 celery==4.4.0 certifi==2020.6.20 chardet==3.0.4 click==7.1.2 click-plugins==1.1.1 cligj==0.5.0 cycler==0.10.0 Cython==3.0a6 Flask==1.1.1 Flask-Cors==3.0.8 flower==0.9.3 gast==0.3.3 gevent==1.4.0 google-pasta==0.2.0 greenlet==0.4.16 grpcio==1.25.0 grpcio-tools==1.25.0 h5py==2.10.0 idna==2.10 importlib-metadata==1.7.0 itsdangerous==1.1.0 Jinja2==2.11.2 Keras-Applications==1.0.8 Keras-Preprocessing==1.1.2 kiwisolver==1.2.0 kombu==4.6.11 Markdown==3.2.2 MarkupSafe==1.1.1 matplotlib==3.3.1 minio==5.0.6 numpy==1.19.1 object-detection==0.1 opencv-python==4.1.2.30 pandas==1.1.0 pika==1.1.0 Pillow==7.0.0 protobuf==3.12.2 pyparsing==2.4.7 python-dateutil==2.8.1 python-dotenv==0.11.0 pytz==2020.1 rasterio==1.1.3 redis==3.3.11 requests==2.23.0 scipy==1.5.2 simplification==0.4.4 six==1.15.0 snuggs==1.4.7 tensorboard==1.14.0 tensorflow==1.14.0 tensorflow-estimator==1.14.0 termcolor==1.1.0 tf-slim==1.1.0 tornado==5.1.1 urllib3==1.25.10 vine==1.3.0 Werkzeug==1.0.1 wrapt==1.12.1 zipp==3.1.0

reference subprocess code

    train_obj = subprocess.Popen(
    cmd,
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
    shell=True
    )
    for line in iter(train_obj.stdout.read(1), ''):  # replace '' with b'' for Python 3
        sys.stdout.write(line)

Expected Behavior

subprocess need to be start and stdout can able to see in the logs

Actual Behavior

stopped before the line of subprocess
restarting the worker. the command is running in the background. not able to see in the logs

similar question in stack overflow

https://stackoverflow.com/questions/36995868/how-to-execute-a-long-running-subprocess-inside-celery-task

[thedrow]

Unfortunately it's currently not supported by Celery to run subprocesses when you use the prefork workers pool.