Use-after-free in OSError.__str__ when a formatting callback clears filename #160773

Crash report

The following segfaults on CPython 3.14.5 and 3.15.0 (at least):

class Text:
    def __repr__(self):
        error.filename = None
        return 'message'

error = OSError(2, Text(), Text())
str(error)

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.15.0 (main, Oct 9 2026, 15:11:47) [Clang 22.1.3 ]

Linked PRs

• gh-160773: Fix use-after-free in OSError.__str__ #160822