[ruby-core:117883] [Ruby master Bug#20490] Process.waitpid2(-1, Process::WNOHANG) misbehaves on Ruby 3.1 & 3.2 with detached process

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


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

```diff
diff --git a/process.c b/process.c
index 354e16fd26..bcf7f5aaf4 100644
--- a/process.c
+++ b/process.c
@@ -1291,6 +1299,9 @@ waitpid_wait(struct waitpid_state *w)
         if (w->ret == -1) w->errnum = errno;
     }
     else if (w->options & WNOHANG) {
+        if (w->pid < 0) {
+            need_sleep = TRUE;
+        }
     }
     else {
         need_sleep = TRUE;
```


----------------------------------------
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-108298

* 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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