Repository: celery/celery

Issue:
Pickling Retry instance is incorrect
# Checklist
- [x] I have verified that the issue exists against the `master` branch of Celery.
- [ ] This has already been asked to the [discussion group](https://groups.google.com/forum/#!forum/celery-users) first.
- [x] I have read the relevant section in the
  [contribution guide](http://docs.celeryproject.org/en/latest/contributing.html#other-bugs)
  on reporting bugs.
- [x] I have checked the [issues list](https://github.com/celery/celery/issues?q=is%3Aissue+label%3A%22Issue+Type%3A+Bug+Report%22+-label%3A%22Category%3A+Documentation%22)
  for similar or identical bug reports.
- [x] I have checked the [pull requests list](https://github.com/celery/celery/pulls?q=is%3Apr+label%3A%22PR+Type%3A+Bugfix%22+-label%3A%22Category%3A+Documentation%22)
  for existing proposed fixes.
- [x] I have checked the [commit log](https://github.com/celery/celery/commits/master)
  to find out if the bug was already fixed in the master branch.
- [x] 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).
- [x] I have verified that the issue exists against the `master` branch of Celery.
- [ ] I have included the contents of ``pip freeze`` in the issue.
- [x] I have included all the versions of all the external dependencies required
  to reproduce this bug.

## Optional Debugging Information
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Try some of the below if you think they are relevant.
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- [ ] 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.
- [x] 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.
- [x] I have tried reproducing the issue after downgrading
  and/or upgrading Celery and its dependencies.

## Related Issues and Possible Duplicates
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#### Related Issues

- None

#### Possible Duplicates

- None

## Environment & Settings
<!-- Include the contents of celery --version below -->
**Celery version**: 4.4.7, 5.0.5, master
<!-- Include the output of celery -A proj report below -->
<details>
<summary><b><code>celery report</code> Output:</b></summary>
<p>

```
```

</p>
</details>

# Steps to Reproduce

1. Run celery with default pool
2. Add `autoretry_for` or `raise Retry(exc=ValueError())`
2. Intercept `task-retried` event (i.e. for RabbitMQ it's published to `celeryev` exchange with routing key `task.retried`
3. Expecting "exception" argument to be not empty, getting "None" in it.

## Required Dependencies
<!-- Please fill the required dependencies to reproduce this issue -->
* **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
<!-- Please fill the contents of pip freeze below -->
<details>
<summary><b><code>pip freeze</code> Output:</b></summary>
<p>

```
```

</p>
</details>

### Other Dependencies
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<details>
<p>
N/A
</p>
</details>

## Minimally Reproducible Test Case
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<details>
<p>

```python

x = Retry('foo', KeyError(), when=datetime.utcnow())
y = pickle.loads(pickle.dumps(x))
assert repr(x) == repr(y)
```

</p>
</details>

# Expected Behavior
<!-- Describe in detail what you expect to happen -->
Retry instance after pickle/unpickle is the same as initial one

# Actual Behavior
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In addition, include the Celery daemon logs, the broker logs,
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After unpicklig Retry looses `self.exc`


Discussion:
Hey @tumb1er :wave:,
Thank you for opening an issue. We will get back to you as soon as we can.
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If you require immediate assistance please consider sponsoring us.


Resolving patch:
diff --git a/celery/exceptions.py b/celery/exceptions.py
--- a/celery/exceptions.py
+++ b/celery/exceptions.py
@@ -180,7 +180,7 @@ def __str__(self):
         return f'Retry {self.humanize()}'
 
     def __reduce__(self):
-        return self.__class__, (self.message, self.excs, self.when)
+        return self.__class__, (self.message, self.exc, self.when)
 
 
 RetryTaskError = Retry  # noqa: E305 XXX compat


Test patch:
diff --git a/t/unit/app/test_exceptions.py b/t/unit/app/test_exceptions.py
--- a/t/unit/app/test_exceptions.py
+++ b/t/unit/app/test_exceptions.py
@@ -12,7 +12,10 @@ def test_when_datetime(self):
 
     def test_pickleable(self):
         x = Retry('foo', KeyError(), when=datetime.utcnow())
-        assert pickle.loads(pickle.dumps(x))
+        y = pickle.loads(pickle.dumps(x))
+        assert x.message == y.message
+        assert repr(x.exc) == repr(y.exc)
+        assert x.when == y.when
 
 
 class test_Reject: