[BUG] celery/restart bug

[ben-bay]

🐛 Bug Report

Describe the bug
Looks like this:

ERROR:celery.app.trace:Task merlin.common.tasks.merlin_step[31704a45-84d0-4b5a-8c71-4d9aa55cd9f2] raised unexpected: RuntimeError('Never call result.get() within a tas
k!\nSee http://docs.celeryq.org/en/latest/userguide/tasks.html#task-synchronous-subtasks\n',)
Traceback (most recent call last):
  File "/Users/bay1/merlin/venv_merlin/lib/python3.6/site-packages/celery/app/base.py", line 487, in run
    return task._orig_run(*args, **kwargs)
  File "/Users/bay1/merlin/merlin/common/tasks.py", line 130, in merlin_step
    raise RestartException
merlin.exceptions.RestartException

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/bay1/merlin/venv_merlin/lib/python3.6/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/Users/bay1/merlin/venv_merlin/lib/python3.6/site-packages/celery/app/base.py", line 500, in run
    raise task.retry(exc=exc, **retry_kwargs)
  File "/Users/bay1/merlin/venv_merlin/lib/python3.6/site-packages/celery/app/task.py", line 716, in retry
    S.apply().get()
  File "/Users/bay1/merlin/venv_merlin/lib/python3.6/site-packages/celery/result.py", line 1027, in get
    assert_will_not_block()
  File "/Users/bay1/merlin/venv_merlin/lib/python3.6/site-packages/celery/result.py", line 43, in assert_will_not_block
    raise RuntimeError(E_WOULDBLOCK)
RuntimeError: Never call result.get() within a task!
See http://docs.celeryq.org/en/latest/userguide/tasks.html#task-synchronous-subtasks

To Reproduce
Steps to reproduce the behavior:

1. Run a single-step DAG with this as the step:

study:
    - name: step1
      description: step 1
      run:
         cmd: exit $(MERLIN_RESTART)
         restart: echo "restarted :)"

• Found in merlin run --local mode.
• Reproduced on local Mac and Quartz.

[koning]

This is a known issue in celery:
celery/celery#4661

If you run with out --local it will work.