From: Massimiliano Mirra <list@...>
Date: 2001-12-04T23:51:47+09:00
Subject: [ruby-talk:27450] Checking objects in a drb net

Yes, yes, DRb again, don't beat me, folks.

I'm trying to make my app (an ICQ LAN-only clone) a bit real-world
compliant, and must handle the case where a client is shutted down
without first calling its logout method first (because of a kill
signal, of a crash, or whatever).  In other words the server has to
handle refs to remote objects that stopped existing without telling
it.

Here is a snippet that reproduces the error.

Server part:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
require 'drb/drb'
require 'drb/eq'

class Server
  def initialize
    @users = []
  end

  def register(attempting)
    @users.push(attempting)
  end

  def unregister(user)
    @users.delete(user) if @users.include?(user)
  end

  def user_list
    @users.collect{|user| user.name}.sort
  end
end

DRb.start_service('druby://localhost:9000', Server.new)
gets
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Client part:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
require 'drb'

class Client
  include DRbUndumped

  attr_reader :name
  def initialize(name)
    @name = name
    DRb.start_service
    @server = DRbObject.new(nil, 'druby://localhost:9000')
  end
  
  def login
    @server.register(self)
  end

  def logout
    @server.unregister(self)
  end

  def get_user_list
    puts @server.user_list
  end
end

name = ARGV.shift || 'joe'
client = Client.new(name)
client.login
puts client.get_user_list
# client.logout
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1) I start the server on a console.

2) I start a client on another console.  It logs in by asking the
server to register itself in @users.  Then it queries the server for a
list of logged users.  It exits without having logged out.
  
3) I start another client.  When it queries for logged users, the
server cannot access to the former client referenced in @users and
this is the result:

$ ./client.rb foo
(druby://localhost:9000) /usr/local/lib/site_ruby/1.6/drb/drb.rb:275:in `open': Connection refused - "connect(2)" (Errno::ECONNREFUSED)
        from (druby://localhost:9000) /usr/local/lib/site_ruby/1.6/drb/drb.rb:275:in `initialize'
        from (druby://localhost:9000) /usr/local/lib/site_ruby/1.6/drb/drb.rb:256:in `new'
        from (druby://localhost:9000) /usr/local/lib/site_ruby/1.6/drb/drb.rb:256:in `open'
        from (druby://localhost:9000) /usr/local/lib/site_ruby/1.6/drb/drb.rb:210:in `method_missing'
        from (druby://localhost:9000) ./server.rb:20:in `user_list'
        from (druby://localhost:9000) ./server.rb:20:in `collect'
        from (druby://localhost:9000) ./server.rb:20:in `user_list'
        from ./client.rb:24:in `get_user_list'
        from ./client.rb:32

Just checking for remote_object.nil? in Server does not do the trick,
presumably because the reference is still there even if the object is
not.  Is there a way out?

Massimiliano