`_interpchannels`: missing `PyErr_NoMemory()` causes `SystemErroron` OOM in `{create,close}()`

[lpyu001]

Bug report

Bug description:

Bug description

parent issue:#146102
refs:https://gist.github.com/devdanzin/d9975354c27e57f2589acfd0a777a2f1

Several allocation failure paths in Modules/_interpchannelsmodule.c
return NULL or -1 without setting an exception.

This violates CPython's C API convention that an error return should have
an exception set. In release builds, this can surface as:

SystemError: <built-in function ...> returned NULL without setting an exception

instead of the expected MemoryError. In debug builds, the same paths may
trip assert(PyErr_Occurred()) in handle_channel_error().

Affected functions

The affected allocation sites are:

Function	Triggered via
_channelends_new()	_channels.create()
_channel_new()	_channels.create()
_channelref_new()	_channels.create()
_channel_set_closing()	_channels.close(send=True) on a non-empty channel

Nearby helper functions in the same file already handle this correctly by
calling PyErr_NoMemory() on allocation failure, for example:

Function
_channelitem_new()
_channelqueue_new()
_channelend_new()

Reproducer

import _testcapi
import _interpchannels as _channels
from concurrent.interpreters import _crossinterp

REPLACE = _crossinterp._UNBOUND_CONSTANT_TO_FLAG[_crossinterp.UNBOUND]

# Case 1: _channels.create()
for n in range(1, 20):
 _testcapi.set_nomemory(n, n + 1)
 try:
 cid = _channels.create(REPLACE)
 except MemoryError:
 pass
 except SystemError as exc:
 print(f"[create] n={n}: {type(exc).__name__}: {exc}")
 else:
 _channels.destroy(cid)
 finally:
 _testcapi.remove_mem_hooks()

# Case 2: _channels.close(send=True) on a non-empty channel
for n in range(1, 30):
 cid = _channels.create(REPLACE)
 _channels.send(cid, b"spam", blocking=False)
 _testcapi.set_nomemory(n, n + 1)
 try:
 _channels.close(cid, send=True)
 except MemoryError:
 pass
 except SystemError as exc:
 print(f"[close] n={n}: {type(exc).__name__}: {exc}")
 finally:
 _testcapi.remove_mem_hooks()
 try:
 _channels.close(cid, force=True)
 except Exception:
 pass
 try:
 _channels.destroy(cid)
 except Exception:
 pass

Sample output on main, CPython 3.16.0a0, macOS, release build:

[create] n=1: SystemError: <built-in function create> returned NULL without setting an exception
[create] n=3: SystemError: <built-in function create> returned NULL without setting an exception
[create] n=4: SystemError: <built-in function create> returned NULL without setting an exception
[close] n=1: SystemError: <built-in function close> returned NULL without setting an exception

Expected behavior

All affected allocation failure paths should raise MemoryError, consistent
with nearby helper functions and CPython's general C API error handling
contract.

Fix

Call PyErr_NoMemory() before returning an error from the affected
allocation failure paths.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-150213: Fix OOM error handling in _interpchannels #150214

[ZeroIntensity]

@ericsnowcurrently I'm wondering whether it's worth keeping _interpchannels around at all. It seems to be a maintenance burden, quite out of date, and we don't use it anywhere right now.

I think interpreter queues and the cross-interpreter protocol are sufficient for sharing things long-term. I'd be in favor of removing the module altogether and just sticking with those.

[lpyu001]

fix in #151239