Import `_datetime` in sub-interpreters in the same time may crash the process

[aisk]

Crash report

What happened?

from concurrent.futures import InterpreterPoolExecutor


executor = InterpreterPoolExecutor()


def f():
    import _datetime
    print(_datetime)


executor.submit(f)
executor.submit(f)

executor.shutdown()

This code may lead to a interpreter crash:

PS C:\Users\xxx\Source\cpython> .\python.bat .\a.py
Running Debug|x64 interpreter...
<module '_datetime' (built-in)>  # The first sub-interpreter's output
Assertion failed: !managed_static_type_index_is_set(self), file C:\Users\xxxxx\Source\cpython\Objects\typeobject.c, line 317

Please note that there is a similar issue may crash the interpreter but in the sestructing phase: #136423

My analyze

The _datetime module will call _PyStaticType_InitForExtension to initialize some static types. Look at it's implementation:

cpython/Objects/typeobject.c

Lines 9269 to 9273 in 490eea0

	int
	_PyStaticType_InitForExtension(PyInterpreterState *interp, PyTypeObject *self)
	{
	return init_static_type(interp, self, 0, ((self->tp_flags & Py_TPFLAGS_READY) == 0));
	}

It's using tp_flags Py_TPFLAGS_READY to determine if this type is already initialized. And at last it will call managed_static_type_state_init with the initialized flag.

If the type flag Py_TPFLAGS_READY was not set before, and another sub-interpreter has been initialized, finished, and has set the Py_TPFLAGS_READY flag, this assertion will be triggered:

cpython/Objects/typeobject.c

Lines 316 to 317 in 490eea0

	if (initial) {
	assert(!managed_static_type_index_is_set(self));

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS, Windows

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

Python 3.15.0a0 (heads/main:ba9c1986305, Jul 8 2025, 22:13:18) [MSC v.1943 64 bit (AMD64)]

Linked PRs

• gh-136421: Fix crash when _datetime is been initialized in multiple sub-interpreters #136422
• gh-136421: Load _datetime static types during interpreter initialization #136583
• gh-136421: Fix crash in _datetime when initialized/finalized concurrently #136620
• [3.14] gh-136421: Load _datetime static types during interpreter initialization (GH-136583) #136943

[aisk]

I have an idea to address this. In the _datetime module's exec phase, we can add a lock to protect these static type initialize process been called in the same time.

I don't know if there is a better way to do this, but I think operating on the same (static) object in parallel with multiple reads and writes should be protected by a lock, and it works well on my machine, and I think the static type's initialize process is very fast and will only execute once (per sub-interpreter), so there will be no performance concerns.

The PR is on the way.

[aisk]

cc @ericsnowcurrently

[ZeroIntensity]

As I said in the PR, I think we need a more general fix that prevents static types from being concurrently initialized. There was a prior attempt at #129829.

[neonene]

The immediate workaround in the given script is to import _datetime module first.