Bug #21812: Kernel#sleep without arguments returns immediately when subprocess exits in another thread (regression in Ruby 4.0)

• Status: Assigned
• Assignee: luke-gru (Luke Gruber)
• Target version: -
• ruby -v: ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[jaimevelaz (Jaime Velaz)]

Description

In Ruby 4.0.0, Kernel#sleep without arguments returns 0 immediately when a subprocess (spawned via backticks in another thread) exits. This is a regression from Ruby 3.4.8 where sleep blocks indefinitely as expected.

Note: sleep(N) with an argument is not affected.

---

Steps to reproduce

# test_sleep.rb
Thread.new do
  sleep 0.3
  `echo hello`  # Spawns subprocess
  puts "Subprocess exited"
end

puts "Main thread sleeping..."
result = sleep  # Should block forever
puts "sleep returned: #{result.inspect}"

Run with:
$ ruby test_sleep.rb

---

Actual result (Ruby 4.0.0):

Main thread sleeping...
Subprocess exited
sleep returned: 0

The program exits immediately after the subprocess completes.

---

Expected result (Ruby 3.4.8 behavior):

Main thread sleeping...
Subprocess exited
(process blocks indefinitely until interrupted by signal)

The program should block forever until explicitly interrupted by SIGINT/SIGTERM.

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned
• Assignee set to luke-gru (Luke Gruber)

Thanks for the nice catch.

git bisect points to 8d8159e7d87e4fd1594ce2fad3d2653e47fb1026. The changes in that commit seem relevant to this issue.

@luke-gru (Luke Gruber) Could you take a look?