Fix unbounded recursion when traceback contains an exception that has a reference to itself in its cause chain

[leonardo-panseri]

Summary

Fixes #726.

This pull request aims to close issue #726 (as pull request #727 aims to do), with an approach that considers also the problem highlighted by pull request #729.

I have written a more in depth comment explaining the two problems and my proposed solutions in #727, here a short summary:

• fix RecursionError when logging re-raised Exception from an ExceptionGroup #727 solves only the case when an ExceptionGroup is catched and a single exception from that group is re-raised.
• Handle exception cycles #729 solves only the case when there is a direct loop in the exception cause chain, but not when the loop is caused by an ExceptionGroup.

My approach solves both problems, I have included tests from both pull requests in the test suite.

The only problem with this approach is that in the Stack for the ExceptionGroup there will be an empty Trace for the exception that has been re-raised (as exemplified by the test case).

Stack(
            exc_type="ExceptionGroup",
            # ...
            exceptions=[
                # ...
                tracebacks.Trace(stacks=[])
            ]
)

If the maintainers want this handled in another way I can easily do it.

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[hynek]

Hi @leonardo-panseri, thanks for you patience!

I think this should do, I've even tried to literally copy over the tests from #727 and #729 and they passed so it should be fine.

Since your solution is based on their work, I've co-authored @leiyi2000 & @hinthornw to give credit where credit is due.

I hope this settles it now.

[jonashaag]

I'd like to use this, can we please get a release? :)