signature.link could raise AttributeError if signature is built with from_dict class method

[AnthonyDeroche]

Checklist

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• 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

• Complex canvas might raise AttributeError: 'dict' object has no attribute 'clone' #5265

Possible Duplicates

• None

Environment & Settings

Celery version: 4.3.0

celery report Output:

Steps to Reproduce

Required Dependencies

• Minimal Python Version: N/A or Unknown
• Minimal Celery Version: 4.3.0
• 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

pip freeze Output:

Other Dependencies

N/A

Minimally Reproducible Test Case

Expected Behavior

The signature.link method could raise AttributeError if signature is built with from_dict class method. It should call the from_dict class method if this happens to transform the dict into a Signature instance. The call should be recursive to rebuild all sub-signatures if they are dict instances instead of Signature.

Actual Behavior

[2019-10-31 15:35:07,882: ERROR/ForkPoolWorker-8] Task cancel[5eaedd7c-9d11-42e6-88fd-952b2526ab2a] raised unexpected: AttributeError("'dict' object has no attribute 'link'")
Traceback (most recent call last):
  File "/home/[...]/python3.7/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/home/[...]/venv/lib/python3.7/site-packages/celery/app/trace.py", line 650, in __protected_call__
    return self.run(*args, **kwargs)
[...]
  File "/home/[...]/venv/lib/python3.7/site-packages/celery/canvas.py", line 1351, in link
    self.body.link(callback)
  File "/home/[...]/venv/lib/python3.7/site-packages/celery/canvas.py", line 1041, in link
    return self.tasks[0].link(sig)
AttributeError: 'dict' object has no attribute 'link'

I'm not an expert in Celery code byt maybe it should be possible to replace:

def link(self, sig):
        # Simply link to first task
        sig = sig.clone().set(immutable=True)
        return self.tasks[0].link(sig)

by:

def link(self, sig):
        # Simply link to first task
        sig = sig.clone().set(immutable=True)
        try:
            return self.tasks[0].link(sig)
        except AttributeError:
             return Signature.from_dict(self.tasks[0]).link(sig)

The same applies to link_error.

[auvipy]

so #5297 didn't fix it fully? can you provide fix with test?

[auvipy]

?