[ruby-core:105392] [Ruby master Bug#18189] `rb_cString` can be NULL during `Init_Object`

Issue #18189 has been updated by ioquatix (Samuel Williams).


The order of operations between here, for anything involving strings, could be a problem:

```
    CALL(Object);
    CALL(top_self);
    CALL(Encoding);
    CALL(Comparable);
    CALL(Enumerable);
    CALL(String);
```

----------------------------------------
Bug #18189: `rb_cString` can be NULL during `Init_Object`
https://bugs.ruby-lang.org/issues/18189#change-93805

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
It's possible for `rb_cString` to be NULL during `Init_Object` and thus `Init_class_hierarchy` which means that `rb_fstring_lit`, which invokes `setup_fake_str`, invokes `RBASIC_SET_CLASS_RAW(..., NULL)` (or possibly just something totally random if it's not zero initialized!).

Later on in `register_fstring` we have an assertion which also fails to detect the abnormality:

```
assert(RBASIC_CLASS(args.fstr) == rb_cString);
```

Because both are NULL. Oops.

It seems that later on, `rb_cString` is set on that specific fstring. But in my own usage of `rb_define_module_under` during `InitVM_Object`, this creates invalid class names which fail when passed into Ruby land.



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