[ruby-core:116183] [Ruby master Bug#20181] Process.wait(-1) doesn't report exited child processes if WAITPID_USE_SIGCHLD is enabled

Issue #20181 has been reported by stanhu (Stan Hu).

----------------------------------------
Bug #20181: Process.wait(-1) doesn't report exited child processes if WAITPID_USE_SIGCHLD is enabled
https://bugs.ruby-lang.org/issues/20181

* Author: stanhu (Stan Hu)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [aarch64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
>From Ruby 2.6 to 3.2, `Process.wait(-1)` doesn't return in a timely manner if a spawned, detached process is still running. The following script exits immediately with Ruby 3.3, but hangs for 10 minutes (the length of the `sleep`) in Ruby 2.6 to 3.2:

```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..."

  # This works
  # puts Process.wait2(forked_pid)

  # The spawned process has to exit before this returns in Ruby 3.1 and 3.2
  pid, status = Process.wait2(-1)
  puts "Exited PID: #{pid}, status: #{status}"
end

process_killer = Thread.new do
  puts "Killing #{forked_pid}"
  system("kill #{forked_pid}")
end

child_waiter.join
process_killer.join
```

In Ruby 3.2, we see:

```
detaching PID 8
Waiting for child process to die...
Killing 11
<process hangs here>
```

In Ruby 3.3, this exits immediately:

```
detaching PID 9
Waiting for child process to die...
Killing 11
Exited PID: 11, status: pid 11 SIGTERM (signal 15)
```

However, if I switch the `Process.wait(-1)` to `Process.wait(forked_pid)`, Ruby 3.2 works fine.

I've validated that this problem goes away if I disable `WAITPID_USE_SIGCHLD`:

```diff
diff --git a/vm_core.h b/vm_core.h
index 1cc0659700..0e7d1643fe 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -126,7 +126,7 @@
 #endif
 
 /* define to 0 to test old code path */
-#define WAITPID_USE_SIGCHLD (RUBY_SIGCHLD || SIGCHLD_LOSSY)
+#define WAITPID_USE_SIGCHLD 0
 
 #if defined(SIGSEGV) && defined(HAVE_SIGALTSTACK) && defined(SA_SIGINFO) && !defined(__NetBSD__)
 #  define USE_SIGALTSTACK
```

This was first reported in the Puma issue tracker (https://github.com/puma/puma/issues/3313), and another contributor documented long-standing issues with `Process.wait` in the past: https://github.com/dentarg/gists/tree/master/gists/ruby-bug-15499#ruby--puma-bug

In Ruby 2.6, https://github.com/ruby/ruby/commit/054a412d540e7ed2de63d68da753f585ea6616c3 introduced a mechanism for `rb_waitpid` that uses `SIGCHLD` for blocking `wait` calls, and this might have introduced this bug. Ruby 2.5 doesn't appear to have this problem.

In Ruby 3.3, this `SIGCHLD` implementation was dropped in https://github.com/ruby/ruby/pull/7476 and https://github.com/ruby/ruby/pull/7527, so Ruby 3.3 no longer appears affected.





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