From: "Iñaki Baz Castillo" <ibc@...>
Date: 2011-01-13T03:39:09+09:00
Subject: Re: Exception Handling in Blocks

2011/1/12 Bryan Richardson <btrichardson@gmail.com>:
> I'm using the Memcache protocol class in EventMachine to make calls into
> a memcache server. The way the Memcache protocol class works is the user
> calls the 'get' function with the key they want a value for and provide
> a block to which the value for that key is passed. The Memcache protocol
> object then registers a callback with EventMachine, in which the value
> returned from memcache is then yielded to the user's callback.
>
> So here's my question... if in the block I pass to the Memcache protocol
> object I raise an error, how does the error get handled? In the code
> example at https://gist.github.com/776547 the error raised does NOT get
> rescued.

Note that, even if you see the code as "sequential" it is not as
EventMachine works on iterations. So when you call memcache.get(:foo)
most probably a deferable object is created so the callback function
is called in a *later* iteration of EventMachine, so the begin-rescue
code has already been bypassed.

Test the following:

----------------------------------------------
require 'rubygems'
require 'eventmachine'

EM.run do
  memcache = EM::Protocols::Memcache.connect('127.0.0.1', 11211)

  begin
    memcache.get(:foo) do |value|
      if value.nil?
        raise Exception, "Tag :foo not found"
      end

      puts "Value Returned: #{value}"

      EM.stop_event_loop
    end
  rescue Exception => ex
    puts 'Failed to get tag...'
    exit
  end

  puts "This shouldn't be printed as the above code will raise, but
this does appear because  memcache.get(:foo) returns inmediately and
its result is processed in a later iteration of the EM reactor"

end
----------------------------------------------


-- 
Iñaki Baz Castillo
<ibc@aliax.net>