[ruby-core:105058] [Ruby master Bug#18129] Ractor-incompatible global variables can be accessed through alias

Issue #18129 has been reported by rm155 (Rohit Menon).

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Bug #18129: Ractor-incompatible global variables can be accessed through alias
https://bugs.ruby-lang.org/issues/18129

* Author: rm155 (Rohit Menon)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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Some global variables are not allowed in non-main Ractors. For example, the following code does not run:
``` ruby
Ractor.new do
  $/
end.take
#=> can not access global variables $/ from non-main Ractors (Ractor::IsolationError)
```


This error message is expected because $/ is not allowed outside the main Ractor.

However, if the variable is aliased, it sometimes bypasses this. For example, the following code sometimes runs unexpectedly without error:
``` ruby
alias $alias_global_variable $/
Ractor.new do
  $alias_global_variable
end.take
```


Sometimes, it correctly fails with the message “can not access global variables $alias_global_variable from non-main Ractors (Ractor::IsolationError)”. However, because the global variable is sometimes accessed without error, this could lead to data races.




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