gh-156204: Guard recursion in PyErr_GivenExceptionMatches

[BHUVANSH855]

Fixes issue gh-156204.

PyErr_GivenExceptionMatches in Python/errors.c recursively unpacks tuple targets without recursion checks, causing native C stack exhaustion and a SIGSEGV when given deeply nested exception tuples.

This change:

• Guards recursion in PyErr_GivenExceptionMatches via Py_EnterRecursiveCall() and Py_LeaveRecursiveCall().
• Adds a _testcapi.err_givenexceptionmatches() helper and a regression test in Lib/test/test_exceptions.py.
• Adds a blurb NEWS entry.

Verified: with Python/errors.c reverted to the pre-fix version the new test segfaults; with the fix it passes.

[iritkatriel]

Can the test be written without ctypes?

[BHUVANSH855]

Switched to _testcapi. Pure Python can't reach it: _PyEval_CheckExceptTypeValid() and get_matcher_type() both reject subtuples one level deep, so nested tuples only arrive via the C API.

Also found the old test asserted assertEqual(res, 0) - true with and without the fix. It now asserts RecursionError via support.run_with_limited_c_stack(); verified it segfaults on reverted errors.c.

One question: _Py_CheckRecursiveCall() replaces the current exception rather than chaining it, which matters for PyErr_ExceptionMatches() callers. Happy to use _Py_ReachedRecursionLimit() instead if you'd prefer.

[read-the-docs-community]

Documentation build overview

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2 files changed

± c-api/exceptions.html
± whatsnew/changelog.html