gh-86298: Ensure that __loader__ and __spec__.loader agree in warnings.warn_explicit() (GH-97803)

In `_warnings.c`, in the C equivalent of `warnings.warn_explicit()`, if the module globals are given (and not None), the warning will attempt to get the source line for the issued warning.  To do this, it needs the module's loader.

Previously, it would only look up `__loader__` in the module globals.  In https://github.com/python/cpython/issues/86298 we want to defer to the `__spec__.loader` if available.

The first step on this journey is to check that `loader == __spec__.loader` and issue another warning if it is not.  This commit does that.

Since this is a PoC, only manual testing for now.

```python
# /tmp/foo.py
import warnings

import bar

warnings.warn_explicit(
    'warning!',
    RuntimeWarning,
    'bar.py', 2,
    module='bar knee',
    module_globals=bar.__dict__,
    )
```

```python
# /tmp/bar.py
import sys
import os
import pathlib

# __loader__ = pathlib.Path()
```

Then running this: `./python.exe -Wdefault /tmp/foo.py`

Produces:

```
bar.py:2: RuntimeWarning: warning!
  import os
```

Uncomment the `__loader__ = ` line in `bar.py` and try it again:

```
sys:1: ImportWarning: Module bar; __loader__ != __spec__.loader (<_frozen_importlib_external.SourceFileLoader object at 0x109f7dfa0> != PosixPath('.'))
bar.py:2: RuntimeWarning: warning!
  import os
```

Automerge-Triggered-By: GH:warsaw

1 files changed, 10 insertions, 3 deletions

diff --git a/Python/_warnings.c b/Python/_warnings.c
index 1b9e107..0d4c50f 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -977,6 +977,7 @@ warnings_warn_impl(PyObject *module, PyObject *message, PyObject *category,
 static PyObject *
 get_source_line(PyInterpreterState *interp, PyObject *module_globals, int lineno)
 {
+    PyObject *external;
     PyObject *loader;
     PyObject *module_name;
     PyObject *get_source;
@@ -984,12 +985,18 @@ get_source_line(PyInterpreterState *interp, PyObject *module_globals, int lineno
     PyObject *source_list;
     PyObject *source_line;
 
-    /* Check/get the requisite pieces needed for the loader. */
-    loader = _PyDict_GetItemWithError(module_globals, &_Py_ID(__loader__));
+    /* stolen from import.c */
+    external = PyObject_GetAttrString(interp->importlib, "_bootstrap_external");
+    if (external == NULL) {
+        return NULL;
+    }
+
+    loader = PyObject_CallMethod(external, "_bless_my_loader", "O", module_globals, NULL);
+    Py_DECREF(external);
     if (loader == NULL) {
         return NULL;
     }
-    Py_INCREF(loader);
+
     module_name = _PyDict_GetItemWithError(module_globals, &_Py_ID(__name__));
     if (!module_name) {
         Py_DECREF(loader);