Issue #17477 has been reported by kirs (Kir Shatrov).

----------------------------------------
Bug #17477: Ractor and pp incompatibility
https://bugs.ruby-lang.org/issues/17477

* Author: kirs (Kir Shatrov)
* Status: Open
* Priority: Normal
* ruby -v: 3.0.0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
```ruby
r = Ractor.new do
  pp("foobar")
end
Ractor.select(r)
```

fails with:

```
#<Thread:0x00007f973414a050 run> terminated with exception (report_on_exception is true):
<internal:/opt/rubies/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:164:in `ensure in require': can not access non-shareable objects in constant Kernel::RUBYGEMS_ACTIVATION_MONITOR by non-main ractor. (Ractor::IsolationError)
	from <internal:/opt/rubies/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:167:in `require'
	from notractor.rb:8:in `block in <main>'
<internal:/opt/rubies/3.0.0/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. (Ractor::IsolationError)
	from notractor.rb:8:in `block in <main>'
<internal:ractor>:345:in `select': thrown by remote Ractor. (Ractor::RemoteError)
	from notractor.rb:11:in `<main>'
<internal:/opt/rubies/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:164:in `ensure in require': can not access non-shareable objects in constant Kernel::RUBYGEMS_ACTIVATION_MONITOR by non-main ractor. (Ractor::IsolationError)
	from <internal:/opt/rubies/3.0.0/lib/ruby/3.0.0/rubygems/core_ext/kernel_require.rb>:167:in `require'
	from notractor.rb:8:in `block in <main>'
<internal:/opt/rubies/3.0.0/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. (Ractor::IsolationError)
	from notractor.rb:8:in `block in <main>'
```

This is easy to fix by calling `require 'pp'` in the top from the main Ractor - but I don't think it is expected for developers. We should optimize for developer's happiness and either make it just work or make it clear that everything should be required beforehand.



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