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

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

@kumaraditya303

cc @kumaraditya303

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

And not just StopAsyncIteration, but any exception really.

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...)

pythongh-131670: Fix crash in `anext()` when `__anext__` is sync and …

gh-131670: Fix crash in `anext()` when `__anext__` is sync and raises (…

pythongh-131670: Fix crash in `anext()` when `__anext__` is sync and …

[3.12] gh-131670: Fix crash in `anext()` when `__anext__` is sync and…

Thanks for the quick fix Nikita!

[3.13] gh-131670: Fix crash in `anext()` when `__anext__` is sync and…

pythongh-131670: Fix crash in `anext()` when `__anext__` is sync and …

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`anext(A(), default)` crashes when `StopAsyncIteration` is raised by a synchronous `anext` #131670

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CPython versions tested on:

Operating systems tested on:

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

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

anext(A(), default) crashes when StopAsyncIteration is raised by a synchronous __anext__ #131670

Description

Crash report

What happened?

CPython versions tested on:

Operating systems tested on:

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

Linked PRs

Activity

picnixz commented on Mar 24, 2025

sobolevn commented on Mar 24, 2025

sobolevn commented on Mar 24, 2025

picnixz commented on Mar 24, 2025

picnixz commented on Mar 24, 2025

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`anext(A(), default)` crashes when `StopAsyncIteration` is raised by a synchronous `anext` #131670