Question

I’m trying to write a simple script which can execute mongodb server in the background. Currently I use Process.spawn method. It works but I have to wait some time for mongod to be process fully operational (boot process is completed and the database is waiting for new connections).

  def run!
    return if running?

    FileUtils.mkdir_p(MONGODB_DBPATH)

    command = "mongod --port #{port} --dbpath #{MONGODB_DBPATH} --nojournal"
    log_file = File.open(File.expand_path("log/test_mongod.log"), "w+")
    @pid = Process.spawn(command, out: log_file)

    # TODO wait for the connection (waiting for connections on port xxxx)
    sleep 2

    yield port if block_given?
  end

Here is the full this script: https://github.com/lucassus/mongo_browser/blob/master/spec/support/mongod.rb#L22

Is it somehow possible to remove this ugly arbitrary sleep 2 from this code?

My first guess is to connect a pipe to the spawned process and wait until “waiting for connections on port xxxx” message is written to the pipe. But I don’t know how to implement it.

Answer 1

Here is a pattern for waiting on some output from a child process:

def run_and_wait_for_this regexp_to_wait_for, *cmd
  rd, wr = IO.pipe
  pid = Process.spawn(*cmd, out: wr)
  pid_waiter = Thread.new { Process.wait(pid); wr.close }
  thr = Thread.new do
    buffer = ''
    until buffer =~ regexp_to_wait_for
      buffer << rd.readpartial(100)
    end
  end
  thr.join
rescue EOFError
ensure
  rd.close
end

run_and_wait_for_this( /waiting for connections on port xxxx/, 'mongo', '--port', port, '--dbpath', MONGODB_PATH, '--nojournal' )

It blocks until the process flushes the expected output into the pipe.