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 Timeout module is not an option as it requires spawning and joining threads for each timeout which can become expensive. In applications that require low socket timeouts and which have a high number of threads it essentially kills performance. This has been noted in many places including Stack Overflow.

I've read Mike Perham's excellent post on the subject here and in an effort to reduce the problem to one file of runnable code created a simple example of a TCP server that will receive a request, wait the amount of time sent in the request and then close the connection.

The client creates a socket, sets the receive timeout to be 1 second, and then connects to the server. The client tells the server to close the session after 5 seconds then waits for data.

The client should timeout after one second but instead successfully closes the connection after 5.

#!/usr/bin/env ruby
require 'socket'

def timeout
  sock = Socket.new(Socket::AF_INET, Socket::SOCK_STREAM, 0)

  # Timeout set to 1 second
  timeval = [1, 0].pack("l_2")
  sock.setsockopt Socket::SOL_SOCKET, Socket::SO_RCVTIMEO, timeval

  # Connect and tell the server to wait 5 seconds
  sock.connect(Socket.pack_sockaddr_in(1234, '127.0.0.1'))
  sock.write("5\n")

  # Wait for data to be sent back
  begin
    result = sock.recvfrom(1024)
    puts "session closed"
  rescue Errno::EAGAIN
    puts "timed out!"
  end
end

Thread.new do
  server = TCPServer.new(nil, 1234)
  while (session = server.accept)
    request = session.gets
    sleep request.to_i
    session.close
  end
end

timeout

I've tried doing the same thing with a TCPSocket as well (which connects automatically) and have seen similar code in redis and other projects.

Additionally, I can verify that the option has been set by calling getsockopt like this:

sock.getsockopt(Socket::SOL_SOCKET, Socket::SO_RCVTIMEO).inspect

Does setting this socket option actually work for anyone?

Answer 1

You can do this efficiently using select from Ruby's IO class.

IO::select takes 4 parameters. The first three are arrays of sockets to monitor and the last one is a timeout (specified in seconds).

The way select works is that it makes lists of IO objects ready for a given operation by blocking until at least one of them is ready to either be read from, written to, or wants to raise an error.

The first three arguments therefore, correspond to the different types of states to monitor.

• Ready for reading
• Ready for writing
• Has pending exception

The fourth is the timeout you want to set (if any). We are going to take advantage of this parameter.

Select returns an array that contains arrays of IO objects (sockets in this case) which are deemed ready by the operating system for the particular action being monitored.

So the return value of select will look like this:

[
  [sockets ready for reading],
  [sockets ready for writing],
  [sockets raising errors]
]

However, select returns nil if the optional timeout value is given and no IO object is ready within timeout seconds.

Therefore, if you want to do performant IO timeouts in Ruby and avoid having to use the Timeout module, you can do the following:

Let's build an example where we wait timeout seconds for a read on socket:

ready = IO.select([socket], nil, nil, timeout)

if ready
  # do the read
else
  # raise something that indicates a timeout
end

This has the benefit of not spinning up a new thread for each timeout (as in the Timeout module) and will make multi-threaded applications with many timeouts much faster in Ruby.