Provides a system wide file lock to ensure no more than X process is running at the same time.

process_semaphore.rb

#!/usr/bin/env ruby

# Provides a system wide file lock to ensure no more
# than X process is running at the same time.

# Example:
#
# process_semaphore!({
#   prefix: "/tmp/my_process",
#   limit: 2
# })

# Calling this at the beginning of your script will ensure
# no more than 2 process with the same prefix are currently
# running. Any exceding process will simply exit(0).

require 'pathname'

def process_semaphore! options
  prefix = options.fetch(:prefix)
  limit = options.fetch(:limit, 1)

  # Lock file
  lock_file = Pathname.new("#{prefix}.#{$$}")
  at_exit { lock_file.unlink if lock_file.exist? }

  # Check running processes, remove stale pid files
  running = Dir.glob("#{prefix}.*").sort.select do |path|
    begin
      # Try to lock the file to see if the process is still running
      if File.open(path).flock(File::LOCK_EX | File::LOCK_NB)
        puts "#{path} present but not locked, deleting"
        File::unlink(path)
        false
      else
        true
      end
    rescue Errno::ENOENT # The file is deleted
      false
    end
  end.size

  # Quit if above limit
  exit if running >= limit

  # Apply lock
  file = lock_file.open(File::RDWR | File::CREAT)
  file.flock(File::LOCK_EX | File::LOCK_NB)
end