[Mahalaxmibejugam]

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #18946 🦕

Problem

RetryError.__init__(self, message, cause) requires cause, but only message is passed to Exception.__init__, so self.args == (message,). The default BaseException.__reduce__ rebuilds the exception as RetryError(message), which fails:

TypeError: RetryError.__init__() missing 1 required positional argument: 'cause'

As a result, a RetryError raised in a worker process can't be returned to the parent. multiprocessing.Pool result handling crashes and AsyncResult.get() blocks, and ProcessPoolExecutor raises BrokenProcessPool. copy.copy() / copy.deepcopy() of a RetryError fail the same way.

Fix

Define RetryError.__reduce__ so the exception is rebuilt with both message and cause, and return self.__dict__ as the state (as BaseException does by default) so other instance attributes, such as notes added with add_note(), are preserved.

Testing

Added unit tests in tests/unit/test_exceptions.py:

• pickle round-trip for every pickle protocol (message, cause and str() preserved);
• round-trip with cause=None;
• instance attributes preserved across a round-trip;
• copy.copy and copy.deepcopy.

The new tests fail without the fix and pass with it. tests/unit/retry and tests/unit/test_exceptions.py pass (198 tests), and ruff format --check / ruff check are clean.

[gemini-code-assist]

Code Review

This pull request implements the reduce method for exceptions to support proper pickling and copying across processes, and adds corresponding unit tests. The reviewer noted that returning only a 2-tuple from reduce will cause any additional instance attributes to be lost, and suggested returning self.dict as the third element to preserve them.