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

• disable_sync_subtasks setting not being respected #5330

Possible Duplicates

• disable_sync_subtasks setting not being respected #5330

Environment & Settings

Celery version:

software -> celery:4.4.2 (cliffs) kombu:4.6.8 py:2.7.17
            billiard:3.6.3.0 py-amqp:2.5.2
platform -> system:Darwin arch:64bit
            kernel version:19.4.0 imp:CPython
loader   -> celery.loaders.default.Loader
settings -> transport:amqp results:disabled

Steps to Reproduce

Required Dependencies

• Minimal Python Version: N/A or Unknown
• Minimal Celery Version: N/A or Unknown
• Minimal Kombu Version: N/A or Unknown
• Minimal Broker Version: N/A or Unknown
• Minimal Result Backend Version: N/A or Unknown
• Minimal OS and/or Kernel Version: N/A or Unknown
• Minimal Broker Client Version: N/A or Unknown
• Minimal Result Backend Client Version: N/A or Unknown

Python Packages

amqp==2.5.2
billiard==3.6.3.0
celery==4.4.2
configparser==4.0.2
contextlib2==0.6.0.post1
importlib-metadata==1.6.0
kombu==4.6.8
pathlib2==2.3.5
pytz==2020.1
scandir==1.10.0
six==1.14.0
vine==1.3.0
zipp==1.2.0

Other Dependencies

Minimally Reproducible Test Case

@app.task
def task(s):
    signature(s).apply().get(disable_sync_subtasks=False)


@app.task
def task2():
    pass


@app.task
def task3():
    pass

# works
task.delay(task2.s())

# results in RuntimeError(E_WOULDBLOCK)
task.delay(task2.s() | task3.s())

Expected Behavior

Both scheduled tasks should succeed.

Actual Behavior

The second task raises in the following error:

[2020-05-04 16:42:29,038: ERROR/ForkPoolWorker-7] [-] Task foo.task[44640f0b-c9cc-45ba-b082-f100eec77ee0] raised unexpected: RuntimeError(u'Never call result.get() within a task!\nSee http://docs.celeryq.org/en/latest/userguide/tasks.html#task-synchronous-subtasks\n',)
Traceback (most recent call last):
  File "/env/lib/python2.7/site-packages/celery/app/trace.py", line 411, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/env/lib/python2.7/site-packages/celery/app/trace.py", line 680, in __protected_call__
    return self.run(*args, **kwargs)
  File "/foo/tasks/__init__.py", line 18, in task
    signature(s).apply().get(disable_sync_subtasks=False)
  File "/env/lib/python2.7/site-packages/celery/canvas.py", line 808, in apply
    last and (last.get(),), **dict(self.options, **options))
  File "/env/lib/python2.7/site-packages/celery/result.py", line 1027, in get
    assert_will_not_block()
  File "/env/lib/python2.7/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

As a side note, I'm not entirely sure why is it necessary to pass disable_sync_subtasks to EagerResult.get(). Since the code is executed in the same process there doesn't seem to be much potential for anything to block.