From: Guillaume Marcais <guslist@...>
Date: 2005-04-04T23:28:06+09:00
Subject: Re: Handling Timeout::Error from TCPSocket

On Sat, 2005-04-02 at 20:05 +0900, Pat Maddox wrote:

> 	def execute
> 		status = timeout(@timeoutval) {
> 			socket = TCPSocket.new(@host, @port) rescue false
> 		
> 			socket.close if socket
> 			return socket != false
> 		} rescue Timeout::Error
                    ^^^^^^^^^^^^^^^^^^^^^

This probably doesn't do what you expect. The argument to rescue for the
inline version is the value to return in case an exception is raised and
caught. Which is different from the block version where it is an
exception class.
The block version with no argument or the inline version filters all
exception that inherit from StandardError. But Timeout::Error does not
inherit from StandardError. So it punches right through.
The code you wrote returns the class Timeout::Error if a StandardError
is raised.

Example:
$ irb
irb(main):001:0> require 'timeout'
=> true
irb(main):002:0> 1 rescue 0
=> 1
irb(main):003:0> raise "bad" rescue 0
=> 0
irb(main):004:0> raise "Bad" rescue Timeout::Error
=> Timeout::Error
irb(main):005:0> raise Timeout::Error, "Catch me if you can" rescue 0
(irb):5:in `irb_binding': Catch me if you can (Timeout::Error)
        from /usr/lib/ruby/1.8/irb/workspace.rb:52:in `irb_binding'
        from /usr/lib/ruby/1.8/irb/workspace.rb:52

If you look at Sam's code snippet, he correctly uses begin/rescue/end to
catch the Timeout::Error exception. 

A final note on the inline rescue: it has a higher precedence than =, so
you would need parenthesis to assign the rescue value:

$ irb
irb(main):001:0> a = 0
=> 0
irb(main):002:0> a = raise "bad" rescue 1
=> 1
irb(main):003:0> a
=> 0
irb(main):004:0> a = (raise "bad" rescue 1)
=> 1
irb(main):005:0> a
=> 1

Hope it helps,
Guillaume.