[ruby-core:123990] [Ruby Bug#21759] `test_fork.rb:78` is failing with `RUBY_MN_THREADS=1`

3 Dec 2025

      Issue #21759 has been reported by hsbt (Hiroshi SHIBATA).

----------------------------------------
Bug #21759: `test_fork.rb:78` is failing with `RUBY_MN_THREADS=1`
https://bugs.ruby-lang.org/issues/21759

* Author: hsbt (Hiroshi SHIBATA)
* Status: Open
* ruby -v: ruby 4.0.0dev (2025-12-01T18:02:35Z master f92001344d) +YJIT +MN +PRISM [arm64-darwin25]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
The following failure of `make btest` is happend with `RUBY_MN_THREADS=1`.

```
#289 test_fork.rb:78:
     def now = Process.clock_gettime(Process::CLOCK_MONOTONIC)

     Thread.new do
       loop { sleep 0.0001 }
     end

     10.times do
       pid = fork{ exit!(0) }
       deadline = now + 10
       while true
         _, status = Process.waitpid2(pid, Process::WNOHANG)
         break if status
         if now > deadline
           Process.kill(:KILL, pid)
           raise "failed"
         end
         sleep 0.001
       end
       unless status.success?
         raise "child exited with status #{status}"
       end
   (...snip)
  #=> #<Assertion::Timeout: timed out after 180 seconds> (expected "ok")  [Bug #20670]
```

This failure occurred 4 times out of 5 times. But I couldn't reproduce this when I removed `RUBY_MN_THREADS` at 5 times.

-- 
https://bugs.ruby-lang.org/