I seem to be having a bad day with the built in Timeout class in Ruby. There are two problems; one is pretty innocuous, the other … not so much.

When you’re using Timeout, you’ll typically wrap the block of code you’re wanting to guard like this:

require 'timeout'

begin
  Timeout.timeout(10) do
    # Block of code
  end
rescue Timeout::Error => e
  puts "Execution expired"
end

Your block of code will run for up to (approximately) 10 seconds and, if it hasn’t completed in that time, will raise the Timeout::Error exception. Pretty straightforward.

The innocuous issue is just one trying to make me mistrust my memory. In Ruby 1.8.x, Timeout::Error inherits from Interrupt, so it’s inheritance from Exception goes along the lines of:

Timeout::Error < Interrupt < SignalException < Exception

The key thing to note here is that it doesn’t inherit directly from StandardError and so a blank rescue block won’t catch it:

begin
  Timeout.timeout(10) { sleep 20 }
rescue
  puts "On Ruby 1.8.x I won't catch the timeout exception."
end

However, on Ruby 1.9.2, Timeout::Error inherits from RuntimeError, so in the above code example, the rescue block will get called. That’s annoying, but it’s not like it’s the only incompatible change between Ruby 1.8.x and Ruby 1.9, so I’m OK with that. Plus, non-specific rescue blocks like that are a bad smell anyway.

The slightly more insidious problem needs further explanation. Come back again later on and I’ll tell you all about it. In fact, it’s already here, at Ruby Timeout Woes, Part 2.