gh-126105: Fix crash in ast module, when ._fields is deleted

[sobolevn]

This PR changes the logic a bit, but it looks like a correct thing to me.
Previously we were handling NULL of fields case for remaining_fields

cpython/Python/Python-ast.c

Lines 5089 to 5098 in 9b14083

	if (fields) {
	numfields = PySequence_Size(fields);
	if (numfields == -1) {
	goto cleanup;
	}
	remaining_fields = PySet_New(fields);
	}
	else {
	remaining_fields = PySet_New(NULL);
	}

But, we didn't really handle the fields itself.
Further calls assume that fields cannot be NULL.

The other way of fixing this is to add a default for fields like:

    if (fields == NULL) {
        fields = PyList_New(0);
        if (fields == NULL) {
            goto cleanup;
        }
    }

In this case ast.AST(arg=1) with no _fields will produce:

DeprecationWarning: ast.AST.__init__ got an unexpected keyword argument 'arg'. Support for arbitrary keyword arguments is deprecated and will be removed in Python 3.15.

It does not seem correct to me, because _fields is a part of our API. Since AST objects are mostly internal, there are no real cases of missing _fields.

• Issue: Segmentation fault after deleting __fields__ attribute from ast.AST #126105

[picnixz]

I'm wondering why we have:

if (PyObject_GetOptionalAttr((PyObject*)Py_TYPE(self), state->_fields, &fields) < 0) {

This could be because some AST nodes do not have a _fields attribute but then, why don't we simply change it to an eager PyObject_GetAttr?

[sobolevn]

Docs give me more confidence: https://docs.python.org/3/library/ast.html#ast.AST._fields

Each concrete class has an attribute _fields which gives the names of all child nodes.

[sobolevn]

I don't think that idle_tests are related to this change. But, I will take a look.

[Eclips4]

LGTM. Thanks!

[JelleZijlstra]

A simpler fix would have been this, right?

    if (PyObject_GetOptionalAttr((PyObject*)Py_TYPE(self), state->_fields, &fields) <= 0) {

However, your fix is better.

[miss-islington-app]

Thanks @sobolevn for the PR, and @Eclips4 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12, 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @sobolevn and @Eclips4, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b2eaa75b176e07730215d76d8dce4d63fb493391 3.12

[bedevere-app]

GH-126130 is a backport of this pull request to the 3.13 branch.

[Eclips4]

I'll take care of backport to 3.12.

[bedevere-app]

GH-126132 is a backport of this pull request to the 3.12 branch.

[sobolevn]

@Eclips4 thank you for your help!
@JelleZijlstra @picnixz thanks for the quick reviews!