PyObject_CallOneArg() usage triggers assert

[ariebovenberg]

Bug Description

I'm getting an error using PyObject_CallOneArg() and I think this may be a bug. Feel free to close this issue if it isn't 👀

Steps to Reproduce

a minimal-ish example:

let zoneinfo_module = PyImport_ImportModule("zoneinfo\0".as_ptr().cast::<c_char>());
let zoneinfo_class =
    PyObject_GetAttrString(zoneinfo_module, "ZoneInfo\0".as_ptr().cast::<c_char>());
let zoneinfo_instance = PyObject_CallOneArg(
    zoneinfo_class,
    PyUnicode_FromString("Europe/Amsterdam\0".as_ptr().cast::<c_char>()),
);

this triggers the assert below. Indeed the n is set to 9223372036854775809 (max i64 + 1). The function seems to usually get smaller values like 0, or 3 (corresponding to the number of arguments).

// abstract_.rs
pub unsafe fn PyVectorcall_NARGS(n: size_t) -> Py_ssize_t {
    assert!(n <= (PY_SSIZE_T_MAX as size_t));
    (n as Py_ssize_t) & !PY_VECTORCALL_ARGUMENTS_OFFSET
}

The large value can be traced back directly to PyObject_CallOneArg

// in abstract_.rs
pub unsafe fn PyObject_CallOneArg(func: *mut PyObject, arg: *mut PyObject) -> *mut PyObject {
    assert!(!arg.is_null());
    let args_array = [std::ptr::null_mut(), arg];
    let args = args_array.as_ptr().offset(1); // For PY_VECTORCALL_ARGUMENTS_OFFSET
    let tstate = PyThreadState_GET();
    let nargsf = 1 | PY_VECTORCALL_ARGUMENTS_OFFSET;
    _PyObject_VectorcallTstate(tstate, func, args, nargsf as size_t, std::ptr::null_mut())
}

where 1 | PY_VECTORCALL_ARGUMENTS_OFFSET results in this massive number. Patching let nargsf = 1; gets things working—but it should obviously be derived from PY_VECTORCALL_ARGUMENTS_OFFSET in some way.

I'm still pretty new to the C API, so it isn't obvious to me what the answer is 😬. Of course I'd be happy to submit a PR if that'd help 🙂

Backtrace

No response

Your operating system and version

MacOS 14.4.1

Your Python version (python --version)

Python 3.12.0

Your Rust version (rustc --version)

1.77.2

Your PyO3 version

master branch

How did you install python? Did you use a virtualenv?

pyenv virtualenv 3.12.0 foo
pyenv local foo

I then added my code to the string_example example in the pyo3 repo

Additional Info

No response

[birkenfeld]

Thanks for the report! #4104 should fix this.