Lazy import traceback points to another module's declaration

[pablogsal]

With #158521 applied:

A.py:

lazy import xml.missing
import B

xml.missing

B.py:

lazy import xml.missing

Run python A.py. The chained ImportError points to B.py:1, but it should point to the declaration in A.py:1.

Reproduced on 3.16.0a0, Linux. Follow-up to the discussion on #158521.

[utam-1]

Hello @pablogsal and @StanFromIreland, this would be my first interaction with the CPython community, I have recently cloned the repo and built it locally.
I have been following this issue along with #158521 , and I think this is caused by register_lazy_on_parent which is present in import.c, coz PyDict_SetItem is being called unconditionally, which is resulting in the overwrite. Maybe it is plausible to have a safeguard as follows? ( Hopefully, my understanding is correct on this )

PyObject *is_existing = NULL; // We use this to track the changes.
if (source == Py_None) {
    err = PyDict_SetDefaultRef(submodules, child, value, NULL);
} else {
    if (PyDict_GetItemRef(submodules, child, &existing) < 0) {
        err = -1; // Error handling
} 
else if (existing == NULL || is_existing == Py_None) {
        err = PyDict_SetItem(submodules, child, value); // Only then enter new data.
} 
    // If we have a lazy import object already
else {
        err = 0; // Do nothing and send to back to original
    }
    Py_XDECREF(is_existing);
}

I have reproduced the bug locally and applied the above guard, and it seems to be working:
Windows 11, Version: 23H2

.\PCbuild\amd64\python.exe A.py
Traceback (most recent call last):
  File "C:\Users\uttam\Desktop\cpython\A.py", line 1, in <module>
    lazy import xml.missing
ImportError: lazy import of 'xml.missing' raised an exception during resolution

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Users\uttam\Desktop\cpython\A.py", line 3, in <module>
    xml.missing
ModuleNotFoundError: No module named 'xml.missing'

I think the maintainers have already started working on this, so it wouldn't be appropriate for me to open a PR. My intention with this comment is to learn about the internals of CPython and just say Hi in general : ).