Crash in genericaliasobject.c when tuple_extend() fails under allocation failure

Crash report

What happened?

While investigating allocation-failure paths in Objects/genericaliasobject.c, I found a NULL dereference in subs_tvars().

When tuple_extend() fails, it returns -1 after _PyTuple_Resize() fails. _PyTuple_Resize() clears its out-parameter (*pv = NULL) on failure, so subargs is guaranteed to be NULL.

However, subs_tvars() unconditionally calls:

if (j < 0) {
    Py_DECREF(subparams);
    Py_DECREF(subargs);
    return NULL;
}

As a result, Py_DECREF(subargs) dereferences a NULL pointer and crashes the interpreter instead of propagating the MemoryError.

Reproducer

A debug build with _testcapi is required.

import _testcapi
from typing import TypeVarTuple

Ts = TypeVarTuple("Ts")
alias = dict[str, tuple[*Ts]]
key = (int, str)

_testcapi.set_nomemory(24, 25)
try:
    alias[key]
finally:
    _testcapi.remove_mem_hooks()

The exact allocation index may vary between builds, so sweeping a range of indices is recommended.

Observed result

ASan reports:

AddressSanitizer: SEGV on unknown address 0x000000000000

#0 _Py_IsImmortal
#1 Py_DECREF
#2 subs_tvars (Objects/genericaliasobject.c)
#3 _Py_subs_parameters
#4 ga_getitem

Root cause

tuple_extend() immediately returns -1 when _PyTuple_Resize() fails. _PyTuple_Resize() sets its out-parameter to NULL on failure, so subargs is guaranteed to be NULL when control reaches the error path. Calling Py_DECREF(subargs) therefore dereferences a NULL pointer.

The issue appears related to gh-148222, which removed the same Py_DECREF() pattern from _Py_make_parameters() after _PyTuple_Resize() failure, but this analogous path in subs_tvars() remained unchanged.

Removing Py_DECREF(subargs); causes the reproducer to raise MemoryError instead of crashing.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

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

Python 3.16.0a0 (heads/investigate-genericalias-oom-null-decref-dirty:7ce7f0bd851, Aug 1 2026) [GCC 13.3.0]

Linked PRs

• gh-155055

Inizia in Objects/genericaliasobject.c, in subs_tvars() e tuple_extend(), quindi esegui il riproduttore fornito del fallimento di allocazione di _testcapi su una build di debug. Verifica che il percorso di errore non vada più in crash e propaghi MemoryError; confronta la gestione analoga in _Py_make_parameters() e controlla il lavoro collegato gh-155055.