[ruby-core:122132] [Ruby Bug#21343] Namespace: singleton classes of root namespace leak between namespaces

Issue #21343 has been reported by jhawthorn (John Hawthorn).

----------------------------------------
Bug #21343: Namespace: singleton classes of root namespace leak between namespaces
https://bugs.ruby-lang.org/issues/21343

* Author: jhawthorn (John Hawthorn)
* Status: Open
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
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Ruby classes all have singleton classes, those singleton classes have singleton classes, and so on with infinite nesting. Since we don't have infinite resources, we create these lazily.

Because of this, only a certain number of singleton classes exist in the root namespace. At a certain depth, mutable singleton classes end up shared between namespaces.

``` ruby
File.write("/tmp/ntest.rb", <<~'RUBY')
  p String.singleton_class.singleton_class::Foo
  String.singleton_class.singleton_class::Bar = 456
RUBY

String.singleton_class.singleton_class::Foo = 123

ns = Namespace.new
ns.require("/tmp/ntest.rb")

p String.singleton_class.singleton_class::Bar
```


```
RUBY_NAMESPACE=1 ruby nstest2.rb
ruby: warning: Namespace is experimental, and the behavior may change in the future!
See doc/namespace.md for known issues, etc.
123
456
```



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