[ruby-core:101920] [Ruby master Bug#17420] Unsafe mutation of $" when doing non-RubyGems require in Ractor

Issue #17420 has been updated by ko1 (Koichi Sasada).


Current `require` behavior with ractors are not well considered and we need to consider it.

> Maybe a solution would be to do all require in the main Ractor?

Yes, This is one good option. But not sure it is only one solution...

If we allow to require from non-main ractors, the only problem is `$LOADED_FEATURES`?

----------------------------------------
Bug #17420: Unsafe mutation of $" when doing non-RubyGems require in Ractor
https://bugs.ruby-lang.org/issues/17420#change-89766

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: ruby 3.0.0dev (2020-12-16T10:12:48Z master a9a7f4d8b8) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
With an empty file `a.rb`:
```
$ ruby --disable-gems -e 'Ractor.new { puts $" }.take'
-e:1:in `block in <main>': can not access global variables $" from non-main Ractors (RuntimeError)
```
That is expected, given the rules for global variables.

```
ruby --disable-gems -e 'Ractor.new { require "./a.rb"; }.take; p $"'
[... , "/home/eregon/a.rb"]
```
Is it OK that the Ractor can do `require`, which does modify `$"`?

I think it's not, and it might lead to segfaults if e.g. the main Ractor mutates `$"` in parallel to some other Ractor doing `require`.

Probably `require` needs to be forbidden in non-main Ractors (it does mutate `$"`, so it's logical), or there needs to be always VM-global synchronization on any access to `$"` (otherwise, segfaults are possible).
The latter doesn't seem reasonable, especially when considering the user might do `$".each { ... }`.

---

Note that RubyGems' `require` does not work on non-main Ractors (pretty much expected given it depends on a lot of global state):
```
$ ruby -e 'Ractor.new { require "./a.rb"; }.take'
<internal:/home/eregon/prefix/ruby-master/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:37:in `require': can not access non-shareable objects in constant Kernel::RUBYGEMS_ACTIVATION_MONITOR by non-main ractor. (NameError)
```

This probably also has consequences for `autoload`.
Maybe the `zeitwerk` gem can help with the mode to resolve all autoload at once.



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