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Avoid slowing down AR object initialization
2d73f5a forces AR to enter the `define_attribute_methods` method whenever it instantiate a record from the `init_with` entry point. This is a potential performance hotspot, because `init_with` is called from all `find*` family methods, and `define_attribute_methods` is slow because it tries to acquire a lock on the mutex everytime it is entered. By using [DCL](http://en.wikipedia.org/wiki/Double-checked_locking), we can avoid grabbing the lock most of the time when the attribute methods are already defined (the common case). This is made possible by the fact that reading an instance variable is an atomic operation in Ruby. Credit goes to Aaron Patterson for pointing me to DCL and filling me in on the atomicity guarantees in Ruby. [*Godfrey Chan*, *Aaron Patterson*]
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