From: Brian Candler <b.candler@...>
Date: 2010-04-07T22:25:15+09:00
Subject: Re: modifying a Hash in one process when .each is running in another

Nathan Macinnes wrote:
> I want one process to continually loop through a list of objects (in
> the form of a hash), while another process continually refreshes that
> list. I've done it the most obvious way below, but are there any
> pitfalls here? I don't really know how the each method works, so will
> it be looking for keys that may not be there anymore? Would I be
> better off doing some kind of merge of the old hash with the new,
> rather than replacing the old hash entirely?

Fortunately you are not replacing or modifying the hash at all (and 
modifying a hash while iterating through it is a really bad idea)

Rather, you are updating the local variable 'myHash' to point to a new 
hash. The tester thread, once it has started iterating through the old 
hash, will continue to do so until it gets to the end.

You could make this even more explicit by eliminating the 'terminate' 
variable:

tester = Thread.new {
  while myHash
    myHash.each ...
  end
}

...
myHash = nil
tester.join

But is there a particular reason to do this using threads? It would be 
simpler like this:

while Time.now < time_to_finish
  myHash = MyClass.generate_list_of_objects
  myHash.each do |key,value|
    value.test
  end
end

Note that MRI won't give you thread concurrency across multiple cores, 
although JRuby does.
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