threadsafe! in ActiveSupport::Cache::Store extends all instances of the class, and breaking all but the one it was called on

When you call threadsafe! on an instance of ActiveSupport::Cache::Store it extends the class with the ThreadSafe module, thus effecting all future instances, not just the current instance.

I don't think this is the desired behavior, but either way it ends up breaking all future instances because the @mutex instance variable will be null for them.

Instead, I think the author meant to "extend" the instance of the class with the ThreadSafe module.

Here is a monkey patch that does this:

module ActiveSupport module Cache

class Store
  def threadsafe!
    @mutex = Mutex.new
    self.extend ThreadSafety
    self
  end
end

end end

It appears that edge rails assumes all caches (except memory cache) are thread safe by default and so this is no longer an issue.