Check if TCPServer's port-finding ability is safe with parallelism via fork

port-finder.rb

# ruby port-finder.rb | cut -f1 -d: | sort -n | uniq -c
#
# Verdict: it seems to be safe on Mac OS X, but it's *not safe* on Linux,
# where causal testing confirms that two subprocesses could pick the same port.

require 'socket'

module PortFinder
  def self.call
    server = TCPServer.new('127.0.0.1', 0)
    server.addr[1]
  ensure
    server.close if server
  end
end

pids = 7.times.map do |num|
  fork do
    srand
    20.times do
      sleep((rand/3) + 0.01)
      puts "%d: found by process %d" % [
        PortFinder.call,
        num
      ]
    end
  end
end

pids.each do |pid|
  Process.wait pid
end