Crash in _interpreters.create() when config string has an unpaired surrogate

[rajkripal]

Crash Report

Crash description

_interpreters.create() segfaults when the config object exposes a string attribute (e.g. gil) containing an unpaired surrogate. The C helper _config_dict_copy_str calls PyUnicode_AsUTF8() and passes the result straight to strncpy() without a NULL check. When the string can't be UTF-8 encoded, PyUnicode_AsUTF8() returns NULL and sets UnicodeEncodeError, but the NULL then reaches strncpy and the interpreter crashes.

Lone surrogates are reachable from pure Python ('\udc80', chr(0xDC80)), and also show up naturally via surrogateescape — e.g. filenames, env vars, or argv with non-UTF-8 bytes that get forwarded into a config dict.

Related precedent: gh-126221 (same module, same module crash class from pure Python input).

Reproducer

import _interpreters

class BadConfig:
 use_main_obmalloc = False
 allow_fork = False
 allow_exec = False
 allow_threads = False
 allow_daemon_threads = False
 check_multi_interp_extensions = False
 own_gil = True
 gil = 'own\udc80'

_interpreters.create(BadConfig())

Expected: UnicodeEncodeError.
Actual: segfault (exit 139).

Reproduced on 3.14.3 and current main.

Error messages

zsh: segmentation fault python3 repro.py

Your environment

• CPython 3.14.3 and main (Python/interpconfig.c unchanged since gh-76785: Add PyInterpreterConfig Helpers #117170)
• macOS / Linux (encoding-independent; the bug is in C)

Linked PRs

(fix ready, will link once this is filed)

Linked PRs

• gh-148798: Fix crash in _interpreters.create() on non-UTF-8 string in config #148799