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The following script can sometimes trigger a crash:
```ruby
GC.stress = true
class Array
def foo(bool)
if bool
@a = 1
@b = 2
@c = 1
else
@c = 1
end
end
end
obj = []
obj.foo(true)
obj2 = []
obj2.foo(false)
obj3 = []
obj3.foo(true)
```
This is because vm_setivar_default calls rb_ensure_generic_iv_list_size
to resize the iv list. However, the call to gen_ivtbl_resize reallocs
the iv list, and then inserts into the generic iv table. If the
st_insert triggers a GC then the old iv list will be read during
marking, causing a use-after-free bug.
Co-Authored-By: Jemma Issroff <jemmaissroff@gmail.com>