[ruby-core:105393] [Ruby master Bug#18189] `rb_cString` can be NULL during `Init_Object`

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


I see, there is some kind of patch up:

```
    rb_cString  = rb_define_class("String", rb_cObject);
    assert(rb_vm_fstring_table());
    st_foreach(rb_vm_fstring_table(), fstring_set_class_i, rb_cString);
```

So for some things it's working but for `rb_define_module_under` it seems not working.

----------------------------------------
Bug #18189: `rb_cString` can be NULL during `Init_Object`
https://bugs.ruby-lang.org/issues/18189#change-93806

* 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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