[ruby-core:124147] [Ruby Bug#21759] `test_fork.rb:78` is failing with `RUBY_MN_THREADS=1`

11 Dec 2025

      Issue #21759 has been updated by luke-gru (Luke Gruber).

Status changed from Open to Closed
Assignee changed from ractor to luke-gru (Luke Gruber)

This has most likely been fixed by https://github.com/ruby/ruby/pull/15392. I can't reproduce the failure on master and the fix was targeting a race condition in the thread scheduler in MN thread mode.

----------------------------------------
Bug #21759: `test_fork.rb:78` is failing with `RUBY_MN_THREADS=1`
https://bugs.ruby-lang.org/issues/21759#change-115602

* Author: hsbt (Hiroshi SHIBATA)
* Status: Closed
* Assignee: luke-gru (Luke Gruber)
* 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.

-- 
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