PidLock

gistfile1.rb

class PidLock
  def initialize(name, &block)
    @name = name
    if can_execute?
      set_running
      block.call
    else
      puts "the pidfile #{pidfile} is locked by process #{current_pid}"
    end
  end

  def can_execute?
    return true if current_pid == 0
    Process.kill(0, current_pid)
    false
  rescue Errno::ESRCH, RangeError => e
    true
  end

  def current_pid
    timeout ||= 2
    IO.read(pidfile).to_i
  rescue Errno::ENOENT => e
    File.open(pidfile, 'w')
    retry if (timeout -= 1) > 0
  end

  def set_running
    File.open(pidfile, 'w') {|f| f.write("#{Process.pid}") }
    nil
  end

  def pidfile
    "#{@name}.pid"
  end
end

namespace "workers" do
  task "prova" do |t|
     PidLock.new(t.name) do
       sleep(10)
     end
  end
end

# pigoz@NAVI ~/dev » rake workers:prova &
# [1] 1083
# pigoz@NAVI ~/dev » rake workers:prova
# the pidfile workers:prova.pid is locked by process 1083
# pigoz@NAVI ~/dev »
# [1]  + 1083 done       rake workers:prova                    

@francescoagati: possiamo usare questo per non eseguire + rake task dello stesso tipo contemporaneamente.