Using class instance variable for mutex in Ruby

Question

Note: The code summary shown below is not a distillation of the code that I had the problem with. I\'ve left this original summary here since someo

Answer 1

Does it happen with a class variable? @@mutex. There might be a race condition with making new class-instances between threads and the new copy of @mutex isn't ready yet. Constants and class variables however, are shared between copies of the class and subclasses. Also, what if you put the @mutex init code in a memoized method such as:

def self.mutex
  @mutex ||= Mutex.new
end

Answer 2

While autoloading is indeed not thread safe in Rails like it is in Ruby 1.9 (per Is autoload thread-safe in Ruby 1.9?), the problem I encountered was not due to that problem and the code I had was not an instance of the code I showed above, but rather an instance of the following:

class Foo
  @mutex = Mutex.new
  def self.bar
     @mutex.synchronize { }
  end
end

class Foobar < Foo ; end

Foobar.bar

The problem is that when executing method from a superclass, the value of self remains unchanged, so the value of @mutex within Foo.bar is interpreted in the context of the Foobar object, not the value of the Foo object.

This problem can be avoided by using a class variable (e.g. @@mutex) for the mutex.