concurrent.interpreters: crash (NULL dereference) when an unpickled AttributeError has a non-string arg

Python/crossinterp.c's check_missing___main___attr() passes the result of PyUnicode_AsUTF8() straight to strncmp() without a NULL check:

const char *err = PyUnicode_AsUTF8(msgobj);
// Check if it's a missing __main__ attr.
int cmp = strncmp(err, "module '__main__' has no attribute '", 36);

msgobj is args[0] of an AttributeError raised on the receive side of the cross-interpreter pickle fallback, reached when an object is moved between interpreters through a concurrent.interpreters queue or channel and its unpickling in the receiving interpreter fails.

PyUnicode_AsUTF8() returns NULL (and sets an exception) when args[0] is not a str — e.g. AttributeError(42), AttributeError(b'x'), AttributeError(None) — or is a str containing lone surrogates (AttributeError('\ud800')). When err == NULL, strncmp(NULL, ...) dereferences NULL and the interpreter crashes (SIGSEGV).

Reproduction

Put an object into a concurrent.interpreters queue whose unpickling on the receive side raises an AttributeError with a non-string (or surrogate) first argument, then get() it — the receiving interpreter segfaults instead of raising NotShareableError. A self-contained reproduction is added as a regression test (test_get_unpickle_fails_with_bad_attributeerror_arg in Lib/test/test_interpreters/test_queues.py), exercising args 42, b'x', None, and '\ud800'.

Scope

This is the check_missing___main___attr() NULL dereference reached through the queue/channel receive path. It does not by itself make every cross-interpreter unpickle failure safe — Interpreter.call() has a separate crash on its result-preserve path that I will report independently.

Fix

Add a NULL guard immediately after PyUnicode_AsUTF8, matching the correctly-checked sibling helper _copy_string_obj_raw(): on NULL, clear the exception (the function asserts !PyErr_Occurred() on entry), Py_DECREF(msgobj), and return 0 (not a missing-__main__ attribute).

Affected versions

concurrent.interpreters (PEP 734) is public in 3.14+; this reproduces on main and 3.14.

Linked PR

A PR follows.

Linked PRs

• gh-151863

The issue points to Python/crossinterp.c and check_missing___main___attr(); compare its PyUnicode_AsUTF8() handling with the sibling _copy_string_obj_raw() helper. Run Lib/test/test_interpreters/test_queues.py, especially test_get_unpickle_fails_with_bad_attributeerror_arg, and confirm the listed malformed AttributeError arguments raise NotShareableError without a crash. A linked PR, gh-151863, indicates the work is already underway.