[ruby-talk:15567] detecting dead sockets?

Joel and Mathieu, thanks much for the help yesterday.  

Still working on the hello world chatserver - what I'm having a problem
with is figuring out when a user drops off, how to close their socket
gracefully.  When the last user disconnects, the loop that reads input
from them starts spewing "nil".  I've tried everything I can think of to
test for this(without doing something icky like if input == "nil"), but
I haven't found anything that will actually determine whether the user
is still there.  Code below:

require 'socket'
require 'thread'
require 'user.rb'

users = []
port = 4242
server = TCPServer.new('localhost', port)


while (session = server.accept)
    users << User.new(session)
    users.last.thread = Thread.new(users.last) { |user|
      user.linenum = users.length
      user.address = user.socket.addr
      puts "connected: #{user.to_s}"
        while user.socket # <------  this is what i need help with...
          input = user.socket.gets
          puts input
          users.each {|user| user.write(input) }
        end
      user.socket.close
      users.delete_at(linenum - 1)
    }
end


Thanks,
David