[ruby-core:109162] [Ruby master Bug#18902] Thread#value crash if the thread was killed because of a fork

Issue #18902 has been reported by byroot (Jean Boussier).

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Bug #18902: Thread#value crash if the thread was killed because of a fork
https://bugs.ruby-lang.org/issues/18902

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.0dev, ruby 3.1.2p20, ruby 3.0.3p157, ruby 2.7.5p203 
* Backport: 2.7: REQUIRED, 3.0: REQUIRED, 3.1: REQUIRED
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Reproduction script 

```ruby
thr = Thread.new { sleep 10} 
fork do
  p thr.status # =>false
  p thr.join # => #<Thread:0x00000001008ae480 /tmp/thread-join.rb:1 dead>
  p thr.value # Assertion Failed: ./vm_core.h:1303:VM_ENV_FLAGS:FIXNUM_P(flags) (or crash if RUBY_DEBUG isn't set)
end
```

What happens is that `rb_thread_t->value` is initialized to `Qundef`, so when the thread is killed by a fork, `Thread#value` tries to return `Qundef` which laters crash the VM.

I have a patch incomming.




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