[ruby-core:101797] [Ruby master Bug#17489] Ractor segfaults

[shatrov@me.com]

Issue #17489 has been reported by kirs (Kir Shatrov).

----------------------------------------
Bug #17489: Ractor segfaults
https://bugs.ruby-lang.org/issues/17489

* Author: kirs (Kir Shatrov)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
I was able to boil it down to this script:

```ruby
require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem "sinatra", "~> 2.1"
end

require 'sinatra'

class App < Sinatra::Base
  get '/' do
    200
  end
end

module Sinatra
  class Base
    class << self
      # patch it to avoid hitting other unsafe things
      def setup_default_middleware(builder)
        builder.use Rack::NullLogger
      end
    end
  end
end

workers = 2.times.map do
  Ractor.new { App.new }
end

Ractor.select(*workers)
```

Even though the code doesn't make a lot of sense and Sinatra has other stuff that's not compatible with Ractor, I would not expect user-level Ruby code to segfault.


I have attach the crash report and stderr output.

---Files--------------------------------
segfault.dump (44.8 KB)
ruby_2020-12-29-002108_Kirs-MacBook-Pro-2.crash (27.9 KB)


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