gistfile1.rb

require 'tempfile'
require 'thread'

$stdout_mutex = Mutex.new

def thread_log(msg)
  $stdout_mutex.synchronize do
    puts "#{Time.now} #{Thread.current} -- #{msg}"
    STDOUT.flush
  end
end

def do_flock(lockfile)
  File.open(lockfile, 'w+') do |f|
    thread_log("Acquiring lock")

    f.flock(File::LOCK_EX)

    thread_log("Acquired lock")
    thread_log("Sleeping 10")
    sleep(10)

    # Not strictly necessary as exiting the block closes the fd
    # which releases the lock
    f.flock(File::LOCK_UN)
  end
end

lockfile = Tempfile.new("test_flock")
begin

  t1 = Thread.new { do_flock(lockfile.path) }
  t2 = Thread.new { do_flock(lockfile.path) }
  t3 = Thread.new do
    loop do
      thread_log("Still active")
      sleep(1)
    end
  end
  t1.join
  t2.join

ensure
  lockfile.close
  lockfile.unlink
end