[ruby-core:117884] [Ruby master Bug#20490] Process.waitpid2(-1, Process::WNOHANG) misbehaves on Ruby 3.1 & 3.2 with detached process

15 May 2024

      Issue #20490 has been updated by stanhu (Stan Hu).

I've opened fixes here:

* 3.2: https://github.com/ruby/ruby/pull/10771
* 3.1: https://github.com/ruby/ruby/pull/10772

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Bug #20490: Process.waitpid2(-1, Process::WNOHANG) misbehaves on Ruby 3.1 & 3.2 with detached process
https://bugs.ruby-lang.org/issues/20490#change-108299

* Author: stanhu (Stan Hu)
* Status: Open
* ruby -v: ruby 3.2.4 (2024-04-23 revision af471c0e01) [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
This is a follow-up issue for a bug that I thought was fixed in https://bugs.ruby-lang.org/issues/19837 and duplicated in https://bugs.ruby-lang.org/issues/20181.

The following script doesn't terminate quickly in Ruby 3.1.5 and 3.2.4, even with the patches to address https://bugs.ruby-lang.org/issues/19837. It works fine in Ruby 3.3. It appears that the `Process::WNOHANG` argument passed to `Process.wait2` causes this script to spin until the child process stops:

```ruby
#!/bin/env ruby

Process.spawn({}, "sh -c 'sleep 600'").tap do |pid|
  puts "detaching PID #{pid}"
  Process.detach(pid)
end

forked_pid = fork do
  loop { sleep 1 }
end

child_waiter = Thread.new do
  puts "Waiting for child process to die..."

  # The spawned process has to exit before this returns in Ruby 3.1 and 3.2
  loop do
    pid, status = Process.wait2(-1, Process::WNOHANG)

    puts "Exited PID: #{pid}, status: #{status}"

    break if pid
    sleep 1
  end
end

process_killer = Thread.new do
  puts "Killing #{forked_pid}"
  system("kill #{forked_pid}")
end

child_waiter.join
process_killer.join
```

If I drop the `Process::WNOHANG` argument, it works fine.

-- 
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