Errors during `array` module import result in double-DECREF

[stestagg]

Crash report

What happened?

In DEBUG build:

import collections.abc

del collections.abc.MutableSequence
import array

or:

sys.setrecursionlimit(25)
import array

generates a double DECREF on the array ArrayType object due to poisoned module state:

Python/gc.c:96: gc_decref: Assertion "gc_get_refs(g) > 0" failed: refcount is too small
object type name: type
object repr     : <class 'array.array'>
Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed

Analysis

So, array_modexec creates the ArrayType and assigns it to array state:

cpython/Modules/arraymodule.c

Line 3400 in 836b206

CREATE_TYPE(m, state->ArrayType, &array_spec);

It then goes on to do some other setup tasks, and if any of these fail, it DECREF's state->ArrayType but leaves the type object on the state struct:

cpython/Modules/arraymodule.c

Lines 3409 to 3421 in 836b206

	PyObject *mutablesequence = PyImport_ImportModuleAttrString(
	"collections.abc", "MutableSequence");
	if (!mutablesequence) {
	Py_DECREF((PyObject *)state->ArrayType);
	return -1;
	}
	PyObject *res = PyObject_CallMethod(mutablesequence, "register", "O",
	(PyObject *)state->ArrayType);
	Py_DECREF(mutablesequence);
	if (!res) {
	Py_DECREF((PyObject *)state->ArrayType);
	return -1;
	}

Returning -1 error code.

PyModule_ExecDef passes the -1 up the chain:

cpython/Objects/moduleobject.c

Line 793 in 0514489

return -1;

which ends up with an exception in _bootstrap.py:

cpython/Lib/importlib/_bootstrap.py

Lines 910 to 915 in 0514489

	except:
	try:
	del sys.modules[spec.name]
	except KeyError:
	pass
	raise

At this point, the array module refcount falls and it's cleaned up, and array_clear calls:

cpython/Modules/arraymodule.c

Line 3346 in 2d7a74e

Py_CLEAR(state->ArrayType);

because the ArrayType pointer is still in the module state, this generates the double DECREF

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/main-dirty:1034e73, Jul 6 2026, 11:11:46) [Clang 22.1.6 ]

Linked PRs

• gh-153210: Fix array module import crash under a memory pressure #153238
• gh-153210: Do not DECREF ArrayType in array_modexc when errors occur #153239

[sobolevn]

Indeed, I also found several other problems with the setup. I will send a PR, thanks for the report! 👍

[stestagg]

@sobolevn Haha, we got there at the same time :), I'll withdraw mine