From: Robert Klemme <shortcutter@...>
Date: 2009-05-14T20:37:49+09:00
Subject: Re: Monitor Error when Joining Threads

2009/5/14 Jeff Shantz <rubyforum@jeffshantz.com>:
> I realize this is an old question, but I'm replying in case anyone else
> encounters this error, since I just found the fix to it.
>
> You need to make a call to super() in your constructor.
>
> So, the following:
>
>> class StatsTracker < Monitor
>>   include Singleton
>>   ...
>>   def foo
>>     synchronize do
>>       # do something
>>     end
>>   end
>>   ...
>> end
>
> should be changed to:
>
>  class StatsTracker < Monitor
>  include Singleton
>
>   def initialize
>     super()
>   end
>
>   def foo
>     synchronize do
>       # do something
>     end
>   end
>   ...
>  end
>
> The same is true if you're using MonitorMixin.  For instance,
>
> class StatsTracker
>  include Singleton
>  include MonitorMixin
>
>   def initialize
>     super()
>   end
>
>   def foo
>     synchronize do
>       # do something
>     end
>   end
>   ...
>  end
>
> Hope that helps someone.

Just an additional note: the reason why the exception appears to come
out of Thread.join is this:

irb(main):004:0> t = Thread.new { raise ArgumentError, "test" }
=> #<Thread:0x100ea960 run>
irb(main):005:0> t.join
ArgumentError: test
        from (irb):4:in `block in irb_binding'
irb(main):006:0>

In other words: since the exception is not caught in the thread,
thread exits but the exception survives.  When you do the join, it
will come to haunt you.  This does make sense when you consider
Thread.value:

irb(main):006:0> t = Thread.new { "result" }
=> #<Thread:0x100e0b54 run>
irb(main):007:0> t.value
=> "result"
irb(main):008:0> t = Thread.new { raise ArgumentError, "test" }
=> #<Thread:0x10016aac run>
irb(main):009:0> t.value
ArgumentError: test
        from (irb):8:in `block in irb_binding'
irb(main):010:0>

Kind regards

robert


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