From: Leslie Viljoen <leslieviljoen@...>
Date: 2008-06-11T07:43:44+09:00
Subject: Re: Non blocking UDP

On Wed, Jun 11, 2008 at 12:37 AM, Bill Kelly <billk@cts.com> wrote:
>
> From: "Leslie Viljoen" <leslieviljoen@gmail.com>
>>
>> I need to make a UDP server that waits for clients to log on and then
>> drives
>> communication from the server side. This means that if a client doesn't
>> ack
>> subsequent comms from the server, the server needs to resend those
>> comms a few times before giving up.
>>
>> I have gotten as far as a successful UDP server, but it blocks the thread
>> forever, and having a server constantly breaking out of
>> socket.recvfrom using Timeout
>> does not seem to work at all (port never seems open). I need a UDP wait
>> with a timeout.
>>
>> I have looked at several UDP examples, and tried the sparse docs here:
>> http://ruby-doc.org/stdlib/libdoc/socket/rdoc/classes/Socket.html#M004528
>>
>> - but no luck yet. I tried recvfrom_nonblock as a guess but it seems to
>> block anyway.
>
> Over the years, I've never been 100% successful getting nonblock
> semantics to work with UDP sockets on ruby.  I end up with code like:
>
>
> require 'timeout'
> require 'fcntl'
>
> UDP_RECV_TIMEOUT = 0.5  # seconds
>
> # ...
>
>    begin
>    timeout(0.17) {
>     sock = UDPSocket.open
>     if defined? Fcntl::O_NONBLOCK
>       if defined? Fcntl::F_GETFL
>         sock.fcntl(Fcntl::F_SETFL, sock.fcntl(Fcntl::F_GETFL) |
> Fcntl::O_NONBLOCK)
>       else
>         sock.fcntl(Fcntl::F_SETFL, Fcntl::O_NONBLOCK)
>       end
>     end
>     n = sock.send(cmd, 0, @server_addr, @server_port)
>    }
>    rescue Timeout::Error
>      File.open("error.log", "a") {|f| f.puts "[#{Time.now}] q2cmd: timeout
> in UDP open/send"}
>    end
>
>     if select([sock], nil, nil, UDP_RECV_TIMEOUT)
>       begin
>         # note, some linux kernel versions will select() positive for a UDP
>         # packet, but the packet has a bad checksum, and when we do
> recvfrom()
>         # the packet is thrown out, and we are blocked.  (I think, due to
> ruby's
>         # internals, even though we're setting NONBLOCK here, doesn't help,
>         # for some reason... i think this was explained on ruby-talk.)
>         # Thus the 'timeout'.
>         timeout(0.17) {
>           resp = sock.recvfrom(65536)
>         }
>       rescue Timeout::Error
>         $stdout.puts "q2cmd: Timeout::Error in sock.recvfrom !"
>       end
>     end
>
>
> . . . and it still blocks occasionally, even though the socket is
> in nonblocking mode.
>
>
> I'm currently rewriting one of my UDP apps using eventmachine
> http://rubyeventmachine.com/  ... which presumably will solve
> the blocking issue.

Wow. I looked briefly at Eventmachine but got the idea it was for TCP only.
If it can do UDP I'll definitely use that. Thanks a lot!

Les