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

I use mongo broker and mongo backend.
There is a decoding error while try to get task result with AsyncResult

File "task_result.py", line 9, in <module> print(res.result) ...... \python37-32\Lib\json\__init__.py", line 341, in loads raise TypeError(f'the JSON object must be str, bytes or bytearray, ' kombu.exceptions.DecodeError: the JSON object must be str, bytes or bytearray, not dict

tasks.py

> @celery.task(name='web.add', bind=True)
> def add_test(self, x):
>     time.sleep(6)
>     message = 'IN WORKER'
>     self.update_state(state='PROGRESS', meta={ 'current': 50, 'total': 100, 'status': message})
>     time.sleep(10)
>     message = 'END'
>     return { 'current': 100, 'total': 100, 'status': message, 'result': { 'video_url': 42, 'video_player_url' : 'https://invidza.com' } }

task_result.py

>res = add.AsyncResult('d6605146-9296-463f-9463-9795d6b87f37')
>print(res)
>print(res.result)

Possible Duplicates

• None

Environment & Settings

Celery version:

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

Other Dependencies

Minimally Reproducible Test Case

Expected Behavior

result = {'current': 50, 'total': 100, 'status': 'IN WORKER'}

Actual Behavior