[ruby-core:122832] [Ruby Bug#21504] [Ractor] Process.waitpid blocks ractor, new NT doesn't pick up other ractors

21 Jul 2025

      Issue #21504 has been updated by luke-gru (Luke Gruber).

This is actually a more pervasive problem than I first realized, because only sometimes does `IO#read` register with the timer thread. With a plain `IO#read` without arguments, it does not wake the timer thread and can block the nt forever as well.

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Bug #21504: [Ractor] Process.waitpid blocks ractor, new NT doesn't pick up other ractors
https://bugs.ruby-lang.org/issues/21504#change-114121

* Author: luke-gru (Luke Gruber)
* Status: Open
* Assignee: ractor
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
The following code hangs when run with `RUBY_MAX_CPU=2 make run`:

Note: `RUBY_MAX_CPU` is set to 2 so that only 1 non-main ractor can run at once.

test.rb:
```ruby
rs = []
2.times do |i|
  rs << Ractor.new(i) do |i|
    if i == 0
      io = IO.popen("ruby -e 'sleep'")
      Process.wait(io.pid) # block forever
    else
      sleep 1 # make sure first ractor blocks forever first
      $stderr.puts "Running r #{i}"
      100_000.times do
        [nil] * 1_000
      end
      $stderr.puts "done r #{i}"
    end
  end
end

while rs.size == 2
  r, obj = Ractor.select(*rs)
  rs.delete(r)
end
```

The timer thread should create a new NT to compensate for the dedicated task, and the new NT should be able to pick up the other runnable ractor.

In contrast, the following works fine:

```ruby
rs = []
2.times do |i|
  rs << Ractor.new(i) do |i|
    if i == 0
      r, w = IO.pipe
      r.read(1) # block forever
    else
      sleep 1 # make sure first ractor blocks forever first
      $stderr.puts "Running r #{i}"
      100_000.times do
        [nil] * 1_000
      end
      $stderr.puts "done r #{i}"
    end
  end
end

while rs.size == 2
  r, obj = Ractor.select(*rs)
  rs.delete(r)
end
```

-- 
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