From: "luke-gru (Luke Gruber) via ruby-core" <ruby-core@...>
Date: 2025-07-18T21:17:52+00:00
Subject: [ruby-core:122821] [Ruby Bug#21504] [Ractor] Process.waitpid blocks ractor, new NT doesn't pick up other ractors

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


This is due to IO (ex: IO#read) registering wait events with the timer thread. When it does this, it wakes the timer thread up. This makes the timer thread check if any new NT needs to be created, and creates one if necessary. A quick fix would be to wake the timer thread when calling Process#waitpid, but there could be other methods affected as well.

----------------------------------------
Bug #21504: [Ractor] Process.waitpid blocks ractor, new NT doesn't pick up other ractors
https://bugs.ruby-lang.org/issues/21504#change-114108

* 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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