在 Ruby 中为互斥锁使用类实例变量 [英] Using class instance variable for mutex in Ruby

问题描述

注意:下面显示的代码摘要不是我遇到问题的代码的提炼.因为其他人已经回答了，所以我把这个原始摘要留在这里，但是实际代码显示在我在下面提供的答案中.

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 someone else already answered, but the actual code is shown in the answer I've provided below.

我无法将其隔离为一个小的失败测试用例，但我遇到了以下一般构造的失败:

I haven't been able to isolate this to a small failing test case, but I'm getting a failure with the following general construct:

class Foo

  @mutex = Mutex.new

  ....

  def self.bar
    @mutex.synchronize { ... }
  end

end

如果我创建多个调用 Foo.bar 的线程，有时 @mutex 将在 bar 中评估为 nil.如果我使用常量(例如 MUTEX)而不是实例变量，我就没有这个问题.

If I create multiple threads invoking Foo.bar, sometimes @mutex will evaluate to nil in bar. If I use a constant (e.g. MUTEX) instead of an instance variable, I don't have this problem.

我不知道这是否重要，但我在多核机器上运行 JRuby.

I don't know if it's significant, but I'm running on JRuby on a multi-core machine.

对于如何隔离问题的任何解释或帮助，我将不胜感激.

I'd appreciate any explanation or help in how to isolate the problem.

更新:我相信这与自动加载有关.使用 Rails，我能够在 Rails 自动加载的目录之一中使用 foo.rb 的以下内容重现类似的问题:

Update: I believe this is related to autoloading. With Rails, I was able to reproduce a similar problem with the following contents of foo.rb in one of the directories Rails autoloads from:

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

当我在 Rails 控制台中执行以下操作时:

When I then execute the following in the Rails console:

1.upto(4).map { Thread.new { Foo.bar }}.map(&:join)

我收到以下错误:

RuntimeError: Circular dependency detected while autoloading constant Foo
    from /Users/palfvin/.rvm/gems/jruby-1.7.10@javlats/gems/activesupport-4.0.1/lib/active_support/dependencies.rb:461:in `load_missing_constant'
    from /Users/palfvin/.rvm/gems/jruby-1.7.10@javlats/gems/activesupport-4.0.1/lib/active_support/dependencies.rb:184:in `const_missing'
    from (irb):1:in `evaluate'

这种行为在 CRuby (MRI Ruby) 中是相同的.

and this behavior is the same in CRuby (MRI Ruby).

推荐答案

类变量会发生这种情况吗?@@mutex.在线程之间创建新的类实例可能存在竞争条件，并且 @mutex 的新副本尚未准备好.然而，常量和类变量在类和子类的副本之间共享.另外，如果你把 @mutex 初始化代码放在一个 memoized 方法中，例如:

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

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