From: Dominik Werder <13thmonkey@...>
Date: 2006-03-31T20:38:43+09:00
Subject: sockets and threads

Hello,

I observed that even if every thread in a program creates it's own
Socket, but all to the same Server, then this is not thread-safe.

It is the same situation as if every thread shared the same Socket
without any mutexes. I observed this with TCPSockets, UNIXSockets ans
UDPSockets.

I run Kernel 2.6.14.2, ruby 1.8.4 (2005-12-24) [i686-linux]


The short source code below produces the following output on the server.
If I enabled the mutex (the 3 lines commented out) it would work better.

Got: thread[16] says 65
Got: thread[19] says 68thread[17] says 20
Got:
Got: thread[24] says 97
Got: thread[8] says 64thread[26] says 17thread[13] says 43
Got:
Got:
Got: thread[7] says 84
Got: thread[0] says 78thread[18] says 37
Got: thread[27] says 98
Got:
Got: thread[18] says 13
Got: thread[11] says 19
Got: thread[1] says 91thread[26] says 18
Got:
Got: thread[1] says 53
Got: thread[19] says 42


# First start the server with:
# ruby tcpsockets.rb server
# Then start the client which spawns several threads:
# ruby tcpsockets.rb


require 'socket'
require 'monitor'

if ARGV[0] == 'server'
  threads = []
  server = TCPServer.new 'localhost', 8889
  putslock = Monitor.new
  while socket = server.accept
    t = Thread.new(socket) do |sock|
      while str = sock.gets
        putslock.synchronize do
          puts "Got: #{str}"
        end
      end
    end
    threads << t
  end
  threads.each do |t| t.join end
else
  Thread.abort_on_exception = true
  #socketlock = Monitor.new
  threads = []
  30.times do |i|
    t = Thread.new(i) do |ii|
      count = 0
      socket = TCPSocket.new 'localhost', 8889
      puts "Using socket #{socket.object_id}"
      while count < 10
        #socketlock.synchronize do
          socket.puts "thread[#{ii}] says #{Kernel.rand 100}"
          socket.flush
        #end
        sleep Kernel.rand * 0.3
        count += 1
      end
    end
    threads << t
  end
  threads.each do |t| t.join end
end




Is this known/intended behavior?

thanks and a good day!
Dominik