threader.rb

#!/usr/bin/env ruby
#
# Thread, fork, and try to trigger deadlocks
#

def make_noise
  $stdout.write("\000")
end

[1,2].each do
  Thread.new do
    while true
      make_noise
    end
  end
end

while true
  pid = fork { make_noise }
  Process.waitpid(pid)
end

This will cause deadlocks in the forked processes in MRI 1.9.3, but not in 1.8.7 or 2.0+.

It surfaces with any output file, with or without flushing, and starting from 2 extra threads in the main process (1 extra thread only does not trigger the issue).

1.9.3-p545 fixes the issue.