Bug #20490: Process.waitpid2(-1, Process::WNOHANG) misbehaves on Ruby 3.1 & 3.2 with detached process

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.2.4 (2024-04-23 revision af471c0e01) [x86_64-linux]
• Backport: 3.1: WONTFIX, 3.2: DONE, 3.3: DONTNEED

[stanhu (Stan Hu)]

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:

#!/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.

[stanhu (Stan Hu)]

I think this patch in the ruby_3_2 branch fixes the problem:

diff --git a/process.c b/process.c
index 354e16fd26..52d49948a5 100644
--- a/process.c
+++ b/process.c
@@ -1290,7 +1290,7 @@ waitpid_wait(struct waitpid_state *w)
     if (w->ret) {
         if (w->ret == -1) w->errnum = errno;
     }
-    else if (w->options & WNOHANG) {
+    else if (w->options & WNOHANG && w->pid > 0) {
     }
     else {
         need_sleep = TRUE;

[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

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN to 3.1: WONTFIX, 3.2: REQUIRED, 3.3: DONTNEED

Thank you for your report. I will look the PRs.
I will set the status of this ticket because of this is a backport ticket.
And currently 3.1 branch is under the security maintenance phase, so I fill the Backport field for 3.1 with "WONTFIX". cc: @hsbt (Hiroshi SHIBATA) (as the new 3.1 branch maintainer).

[nagachika (Tomoyuki Chikanaga)]

• Status changed from Open to Closed

[nagachika (Tomoyuki Chikanaga)]

Currently the backport PR for ruby_3_2 was made by @kjtsanaktsidis (KJ Tsanaktsidis).
https://github.com/ruby/ruby/pull/10787
But in my understanding, it was still in the works.

[kjtsanaktsidis (KJ Tsanaktsidis)]

Apologies I missed this - I just rebased the branch, I think it should be OK for a stable backport to 3.2.

[nagachika (Tomoyuki Chikanaga)]

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

Merged at https://github.com/ruby/ruby/commit/65fed1c3e439bc47bcf6ec884431a86cb9ebd1dc. Thank you!