From: Brian Candler <b.candler@...>
Date: 2010-04-28T23:06:41+09:00
Subject: Re: Mutex

Oliver Cat wrote:
> What interests me is the 'matches?' method, specifically the
> mutex.synchronize block. I'm wondering what is the benefit of this code,
> and how it works?
> 
> Is it so other attempts to call Blacklist.retrieve_ips are blocked, and
> if so, should the mutex be implemented in the Blacklist class?

Only one instance of the code inside @mutex.synchronize { ... } can be 
executing at any one time ("mutex" = "mutual exclusion")

Without this mutex, it would be possible that two threads which called 
BlacklistConstraint#matches? concurrently, when the cache is more than 1 
hour old, could end up calling Blacklist.retrieve_ips concurrently.

This might not be catastrophic if Blacklist.retrieve_ips were 
thread-safe, but it would be wasteful. (Presumably 
Blacklist.retrieve_ips is an expensive operation, given that it is being 
cached)
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