[ruby-core:101940] [Ruby master Bug#17489] Ractor segfaults

Issue #17489 has been updated by inversion (Yura Babak).


During my Ractors tests I had segfaults many times and easy to reproduce (in my environment). 
The same code crashes on Linux and Windows10 (sometimes with other numbers of iterations or workers).
More Ractors seems to increase the segfault rate.
And if I add `GC.disable` — no crash.

So I was looking for a place where to add a report with my code samples.
This issue looks like a good place for this.

**Test cases:**
crash_reused_sha2.rb
https://gist.github.com/Inversion-des/fcc7dbb2d9a77a7dba540c29077e5ebf 
(90% crash, if I uncomment GC.disable — no crash)

crash_encrypt_big_data_often.rb
https://gist.github.com/Inversion-des/a9dbde4eacfa859e5bd7cc8fd8a8049f
(50% crash, again uncomment GC.disable — no crash)

Win10_crash_report.txt
https://gist.github.com/Inversion-des/c79ea00407c579b83ebbe00541e20c45

If it doesn't crash for you — try to increase number of iterations and data size.

My environment:
**Linux CentOS 7.9 (VPS)**
4-core CPU 2.6GHz
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]
**Windows 10**
2-core CPU 3.6GHz
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x64-mingw32]



----------------------------------------
Bug #17489: Ractor segfaults
https://bugs.ruby-lang.org/issues/17489#change-89787

* Author: kirs (Kir Shatrov)
* Status: Assigned
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
* 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 attached 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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