`anext(A(), default)` crashes when `StopAsyncIteration` is raised by a synchronous `__anext__`

[picnixz]

Crash report

What happened?

Python 3.14.0a6+ (heads/main:0e53038ea82, Mar 24 2025, 14:56:34) [GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class A:
...     def __anext__(self):
...         raise StopAsyncIteration
...
>>> anext(A(), "a")
Segmentation fault (core dumped)

There is no crash with anext(A()) or when directly calling __anext__(). Or when __anext__ is declared with async def, it's fine. Or when __anext__() doesn't raise, it's also fine.

NB: I haven't bisected the change but it might be related to what I wrote in 76ffaef.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

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

No response

Linked PRs

• gh-131670: Fix crash in anext() when __anext__ is sync and raises #131682
• [3.13] gh-131670: Fix crash in anext() when __anext__ is sync and raises (GH-131682) #131686
• [3.12] gh-131670: Fix crash in anext() when __anext__ is sync and raises (GH-131682) #131687

[picnixz]

cc @kumaraditya303

[sobolevn]

This happens because awaitable here is NULL:

cpython/Python/bltinmodule.c

Lines 1839 to 1842 in d16f455

	awaitable = (*t->tp_as_async->am_anext)(aiterator);
	if (default_value == NULL) {
	return awaitable;
	}

Looks like simple if (awaitable == NULL) check might solve the issue:

» PYTHONFAULTHANDLER=1 ./python.exe ex.py
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/cpython/ex.py", line 5, in <module>
    print(anext(A(), "a"))
          ~~~~~^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython/ex.py", line 3, in __anext__
    raise StopAsyncIteration
StopAsyncIteration

[sobolevn]

And not just StopAsyncIteration, but any exception really.

[picnixz]

Yes, that's what I observed (note that I actually found this bug because it was a typo I made when creating tests for another PR...)