gh-156909: Fix AST.__repr__ call getting the _fields attribute

[johnslavik]

Proof this works:

❯ cat t.py
import ast

class FieldsMissingMeta(type):
    def __getattribute__(self, name):
        if armed and name == '_fields':
            # PyObject_GetOptionalAttr() returns 0 now, *fields is NULL.
            # The returned sentinel 0 is not handled.
            raise AttributeError
        return type.__getattribute__(self, name)

class FieldsMissing(ast.Del, metaclass=FieldsMissingMeta):
    pass

armed = False  # don't raise during construction
f = FieldsMissing()
armed = True  # raise in repr()
repr(f)  # problem is in ast_repr_max_depth()

❯ ./python t.py
Traceback (most recent call last):
  File "/Volumes/projects/python/cpython.gh-156021/t.py", line 17, in <module>
    repr(f)  # problem is in ast_repr_max_depth()
    ~~~~^^^
  File "/Volumes/projects/python/cpython.gh-156021/t.py", line 8, in __getattribute__
    raise AttributeError
AttributeError: type object 'FieldsMissing' has no attribute '_fields'

❯ every-python run main -- python t.py
Segmentation fault: 11     every-python run main -- python t.py

(every-python = https://github.com/savannahostrowski/every-python)

I didn't include a regression test as I find it quite unrealistic. LMK if you'd like the regrtest added anyway.

I also think we can skip news as this isn't a realistic crash.

• Issue: ast.AST.__repr__ can crash on missing _fields #156909

[johnslavik]

I'll take care of the backports if there are any conflicts 👍🏽

[miss-islington-app]

Thanks @johnslavik for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @johnslavik and @encukou, I could not cleanly backport this to 3.15 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 1bfcc3cc871d3fab21b77cdc47cdbfcf89fd8b5b 3.15

[miss-islington-app]

Sorry, @johnslavik and @encukou, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 1bfcc3cc871d3fab21b77cdc47cdbfcf89fd8b5b 3.14

[miss-islington-app]

Sorry, @johnslavik and @encukou, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 1bfcc3cc871d3fab21b77cdc47cdbfcf89fd8b5b 3.13

[encukou]

Thank you!

I'll take care of the backports if there are any conflicts 👍🏽

There'll be conflicts for 3.15, but once that gets merged you can tell Miss Islington to backport from there to 3.13 & 3.14.