Single instance check with ruby and windows

single_instance_windows.rb

# context: I need to be reasonably sure that not two instances of a ruby script
# run at the same time, on Windows. What follows seems to work.

# tested on ruby 1.9.3p125 (2012-02-16) [i386-mingw32]

def log(msg)
  puts "#{$$} #{msg}"
end

def do_the_work_with_lock
  File.open("test.lock", File::RDWR|File::CREAT, 0644) {|f|
    log "Attempting to lock..."
    if (f.flock(File::LOCK_EX | File::LOCK_NB) == false)
      log "Another instance is already running! Exiting"
      abort
    end
    log "Lock acquired"
    f.rewind
    f.write($$)
    f.flush
    10.times do |i|
      log "Doing work... #{i}"
      sleep(1)
    end
    log "Exiting"
    sleep(1)
  }
end

do_the_work_with_lock

Years ago (there still was Ruby 1.8.6 with the OneClick Installer) I tried to achieve the same thing but by using the win32-mutex gem and a global mutex, see this gist. I'm not sure if win32-mutex has been updated for 1.9 and can be used with the Ruby Installer though. Mutexes can also be subject to some restrictions due to access rights, but anyway it worked for me.

@nrk thanks for sharing this! Didn't think about using win32-mutex. Looks like a release occurred recently (0.3.2) so my guess is that this is still likely to work on Ruby 1.9.

Good to know :)

@thbar to be honest I've never tried that script to check the state of processes running under different users sessions since I just needed to ensure that only one instance was running inside the same user session, so you probably might want to test this condition just to make sure that it meets your requirements:-)

@nrk sure :)