class RFlow
  class PIDFile
    attr_reader :pid_file_path

    def initialize(pid_file_path)
      @pid_file_path = pid_file_path
    end

    def validate?
      if current_process?
        RFlow.logger.warn "Already running #{read.to_s}, not writing PID to file '#{to_s}'"
        return nil
      elsif running?
        error_message = "Already running #{read.to_s}, possibly stale PID file '#{to_s}'"
        RFlow.logger.error error_message
        raise ArgumentError, error_message
      elsif exist?
        RFlow.logger.warn "Found stale PID #{read.to_s} in PID file '#{to_s}', removing"
        unlink
      end
      true
    end

    def read
      return nil unless File.exist? pid_file_path
      File.read(pid_file_path).to_i
    end

    def write(pid=$$)
      return unless validate?

      RFlow.logger.debug "Writing PID #{pid} file '#{to_s}'"
      pid_fp = begin
                 tmp_pid_file_path = File.join(File.dirname(pid_file_path), ".#{File.basename(pid_file_path)}")
                 File.open(tmp_pid_file_path, File::RDWR|File::CREAT|File::EXCL, 0644)
               rescue Errno::EEXIST
                 retry
               end
      pid_fp.syswrite("#{pid}\n")
      File.rename(pid_fp.path, pid_file_path)
      pid_fp.close

      pid
    end

    def exist?
      File.exist? pid_file_path
    end

    def running?
      return false unless exist?
      pid = read
      return false unless pid
      Process.kill(0, pid)
      pid
    rescue Errno::ESRCH, Errno::ENOENT
      nil
    end

    def current_process?
      read == $$
    end

    def unlink
      File.unlink(pid_file_path)
    end

    # unlinks a PID file at given if it contains the current PID still
    # potentially racy without locking the directory (which is
    # non-portable and may interact badly with other programs), but the
    # window for hitting the race condition is small
    def safe_unlink
      (current_process? and unlink) rescue nil
    end

    def signal(sig)
      Process.kill(sig, read)
    end

    def to_s
      File.expand_path(pid_file_path)
    end
  end
end