[ruby-core:119001] [Ruby master Bug#20670] fork deadlocks in child process due to timer thread

Issue #20670 has been updated by k0kubun (Takashi Kokubun).

Backport changed from 3.1: DONTNEED, 3.2: DONTNEED, 3.3: REQUIRED to 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONE

ruby_3_3 commit:66312ad913d67bfd3c2c83b174eabf537f5def84.

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Bug #20670: fork deadlocks in child process due to timer thread
https://bugs.ruby-lang.org/issues/20670#change-109576

* Author: jhawthorn (John Hawthorn)
* Status: Closed
* ruby -v: ruby 3.3.4 (2024-07-09 revision be1089c8ec) [x86_64-linux]
* Backport: 3.1: DONTNEED, 3.2: DONTNEED, 3.3: DONE
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We've been seeing an occasional failure in the Rails CI related to a test which forks and I managed to reduce it to the following reproduction.

``` ruby
Thread.new do
  loop { sleep 0.0001 }
end

1000.times do
  pid = fork{}
  Process.waitpid(pid)
rescue Exception
  Process.kill(:KILL, pid)
  raise
end
```

This hangs on Ruby 3.3 and HEAD (_fairly_ reliably), but completes always on Ruby 3.2

In a debugger it seems like the timer thread acquires `vm->ractor.sched.lock` in the parent process just as the process is forking. The child process then ends up stuck inside of `thread_sched_atfork` when trying to acquire the same lock.

I've opened https://github.com/ruby/ruby/pull/11356 with a fix



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