Unittest: self-referencing explicit exception cause results in infinite loop

[AlexTate]

Bug report

If an exception is raised with a self-referencing __cause__ or __context__ then TestResult._clean_tracebacks() in result.py enters an infinite loop.

Minimal example 1:

try:
    raise Exception()
except Exception as e:
    raise e from e

Minimal example 2:

try:
    e = Exception()
    raise e from e
except Exception as e:
    raise e

Identified Cause

Self-references are not checked while unwinding the chained exception on line 216 of result.py

Your environment

• CPython versions tested on: 3.9, 3.10
• Operating system and architecture: macOS

Linked PRs

• gh-98458: unittest: bugfix for infinite loop while handling chained exceptions that contain cycles #98459
• [3.11] gh-98458: unittest: bugfix for infinite loop while handling chained exceptions that contain cycles (GH-98459) #99995
• [3.10] gh-98458: unittest: bugfix for infinite loop while handling chained exceptions that contain cycles (GH-98459) #99996

[gpshead]

Thanks! A bit of an unusual issue but no doubt a pain to debug when something winds up with this happening.
The backport PRs should automerge after CI runs.