gistfile1.txt

# begin
#   if lock = Lock.acquire('indexing')
#     ...
#   end
# ensure
#   lock.release if lock
# end

# Host: MacBook 2.4GHz Core 2 Duo
#   Sqlite -> 120 acquires+unlocks per second
#   MySQL  -> ???

class Lock < ActiveRecord::Base

  # Find the lock with the given name and aquire it. If successful, return
  # the lock object (so we can later unlock it), otherwise nil
  def self.acquire(name)
    lock = self.find_or_create_by_name(name)
    return lock.acquire() && lock || nil
  end

  # Ackquire the lock. Return a boolean indicating whether we own the lock
  # or not.
  def acquire
    return self.class.update_all([
      "time = ?, owner = ?", Time.now, self.class.owner
    ], [
      "name = ? AND owner IS NULL", name]
    ) == 1
  end

  # Release the lock.
  def release
    return self.class.update_all([
      "time = ?, owner = NULL", Time.now
    ], [
      "name = ? AND owner = ?", name, self.class.owner]
    ) == 1
  end

private

  # We use a combination of hostname and PID to identify ourselves. This
  # makes it easy to identify the exact process who holds the lock in case
  # something goes wrong (the process crashes).
  def self.owner
    return "#{Socket.gethostname}:#{Process.pid}"
  end

end