From: Robert Klemme <shortcutter@...>
Date: 2009-10-08T05:25:05+09:00
Subject: Re: errors with monitor

On 10/07/2009 05:28 AM, Russell Fulton wrote:
> getting this error in a synchronize call:
> 
> /usr/lib/ruby/1.8/monitor.rb:212:in `mon_enter': undefined method `+'
> for nil:NilClass (NoMethodError)
>   from /usr/lib/ruby/1.8/monitor.rb:236:in `synchronize'
> 
> 
> class definition is here:
> 
> class SyncPipe < IO
>   include MonitorMixin
> 
>   def initialize( p )
>     @file = IO.popen( p )
>   end
> 
>   def puts( output)
>     synchronize do
>       @file.puts(output)
>     end
>   end
> 
>   def close
>     @file.close
>   end
> end
> 
> The idea is to have a pipe that threads can write their output too
> without interfering with each other....
> 
> Any idea what is going wrong. I had this working at one stage, sigh...

I believe there are some issues with your code.  The one which stroke me 
first is that you do not guard #close which can likely result in issues.

Then you inherit IO and leave most methods untouched which also means 
they are not guarded.  Btw, I don't believe that you actually need to 
inherit class IO.

IIRC IO.popen with just a command name will open only one pipe for 
reading stdout so your writing might not succeed anyway.

That's all not necessarily related to your issue.  The show stopper is 
that you do not invoke super() in #initialize and thus prevent 
initialization of the mixin:


irb(main):021:0> robert@fussel:~$ irb1.8 -r monitor
irb(main):001:0> class X
irb(main):002:1> include MonitorMixin
irb(main):003:1> def foo
irb(main):004:2> synchronize { puts "yy" }
irb(main):005:2> end
irb(main):006:1> end
=> nil
irb(main):007:0> X.new.foo
yy
=> nil
irb(main):008:0> class X
irb(main):009:1> def initialize; end # no call to super
irb(main):010:1> end
=> nil
irb(main):011:0> X.new.foo
NoMethodError: undefined method `+' for nil:NilClass
	from /usr/lib/ruby/1.8/monitor.rb:215:in `mon_enter'
	from /usr/lib/ruby/1.8/monitor.rb:240:in `synchronize'
	from (irb):4:in `foo'
	from (irb):11
	from :0
irb(main):012:0>


Here's a slightly different approach that takes all these points into 
account:

class SyncPipe
   include MonitorMixin

   def initialize(*a)
     super()
     @io = IO.popen(*a)
   end

   def method_missing(*a, &b)
     synchronize do
       @io.send(*a, &b)
     end
   end
end

or even more general

class SyncIO
   include MonitorMixin

   def initialize(io)
     super()
     @io = io
   end

   def method_missing(*a, &b)
     synchronize do
       @io.send(*a, &b)
     end
   end
end

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/