Description

maurycy

opened on Apr 26, 2026

Issue body actions

Bug description:

As pointed out by @htjworld in #148088 (comment):

I was wondering about the lazy init pattern in a free-threaded build. If two threads enter build_struct_time concurrently with st->time_struct_time == NULL, both would call PyImport_ImportModuleAttrString and one overwrite would leave a reference with an incremented refcount that never gets decref'd.

I noticed arraymodule.c and _decimal.c have the same pattern, so maybe this is just accepted. But _asynciomodule.c initializes eagerly in module_init() which avoids this structurally. Was lazy chosen deliberately here over eager init in init_state()?

While I've addressed this in #148088, I think that this is a valid concern, and import caches should use CAS, just like:

cpython/Modules/_sre/sre.c

Lines 1173 to 1188 in 7e0a0be

	PyObject *func = FT_ATOMIC_LOAD_PTR(module_state->compile_template);
	if (func == NULL) {
	func = PyImport_ImportModuleAttrString("re", "_compile_template");
	if (func == NULL) {
	return NULL;
	}
	#ifdef Py_GIL_DISABLED
	PyObject *other_func = NULL;
	if (!_Py_atomic_compare_exchange_ptr(&module_state->compile_template, &other_func, func)) {
	Py_DECREF(func);
	func = other_func;
	}
	#else
	Py_XSETREF(module_state->compile_template, func);
	#endif
	}

If there's interest, I can do it.

(It seems similar to #149000, so perhaps a more thorough check warranted.)

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Reacted by htjworld

Activity

1. maurycy

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 26, 2026
2. maurycy

   changed the title [-]Possible lazy import caches reference leaks in free-threaded builds[/-] [+]`arraymodule`, `_decimal`: possible lazy import caches reference leaks in free-threaded builds[/+] on Apr 26, 2026
3. htjworld commented on Apr 26, 2026

   

   htjworld

   on Apr 26, 2026

   Contributor

   More actions

   Hi @maurycy — interested in helping with this one. Happy to follow your lead since you opened it, but a couple of things I could pick up:

   • Take a first pass at the CAS fix for arraymodule.c or _decimal.c, whichever you're not already on
   • Grep around for other sites with the same pattern and post a checklist here

   Or if you'd rather drive both, happy to just review. Let me know what works.
4. maurycy commented on Apr 26, 2026

   

   maurycy

   on Apr 26, 2026

   ContributorAuthor

   More actions

   Hi @maurycy — interested in helping with this one. Happy to follow your lead since you opened it, but a couple of things I could pick up:

   • Take a first pass at the CAS fix for arraymodule.c or _decimal.c, whichever you're not already on
   • Grep around for other sites with the same pattern and post a checklist here

   Or if you'd rather drive both, happy to just review. Let me know what works.

   I'm not a core dev! Let's wait for a green light from the higher ups :-)

   Reacted by htjworld
5. serhiy-storchaka

   added

   topic-free-threading

   on Jul 3, 2026

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