Set socket timeout in Ruby via SO_RCVTIMEO socket option

Question

I\'m trying to make sockets timeout in Ruby via the SO_RCVTIMEO socket option however it seems to have no effect on any recent *nix operating system.

Using Ruby\'s T

Answer 1

I think you're basically out of luck. When I run your example with strace (only using an external server to keep the output clean), it's easy to check that setsockopt is indeed getting called:

$ strace -f ruby foo.rb 2>&1 | grep setsockopt
[pid  5833] setsockopt(5, SOL_SOCKET, SO_RCVTIMEO, "\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 16) = 0

strace also shows what's blocking the program. This is the line I see on the screen before the server times out:

[pid  5958] ppoll([{fd=5, events=POLLIN}], 1, NULL, NULL, 8

That means that the program is blocking on this call to ppoll, not on a call to recvfrom. The man page that lists socket options (socket(7)) states that:

Timeouts have no effect for select(2), poll(2), epoll_wait(2), etc.

So the timeout is being set but has no effect. I hope I'm wrong here, but it seems there's no way to change this behavior in Ruby. I took a quick look at the implementation and didn't find an obvious way out. Again, I hope I'm wrong -- this seems to be something basic, how come it's not there?

One (very ugly) workaround is by using dl to call read or recvfrom directly. Those calls are affected by the timeout you set. For example:

require 'socket'
require 'dl'
require 'dl/import'

module LibC
  extend DL::Importer
  dlload 'libc.so.6'
  extern 'long read(int, void *, long)'
end

sock = Socket.new(Socket::AF_INET, Socket::SOCK_STREAM, 0)
timeval = [3, 0].pack("l_l_")
sock.setsockopt Socket::SOL_SOCKET, Socket::SO_RCVTIMEO, timeval
sock.connect( Socket.pack_sockaddr_in(1234, '127.0.0.1'))

buf = "\0" * 1024
count = LibC.read(sock.fileno, buf, 1024)
if count == -1
  puts 'Timeout'
end

This code works here. Of course: it's an ugly solution, which won't work on many platforms, etc. It may be a way out though.

Also please notice that this is the first time I do something similar in Ruby, so I'm not aware of all the pitfalls I may be overlooking -- in particular, I'm suspect of the types I specified in 'long read(int, void *, long)' and of the way I'm passing a buffer to read.