Race condition in Search.ozf

[Harmon Nine]

Actually, we tried an approach like this as well.  Unfortunately, there
is still a race, as there is a chance, however small, that the stop
method could be called in between the checking of the 'stopped' flag and
the call to the "next" (or other) method in the Search.object subclass.
This approach decreases the probability that the "stop" will succeed,
but it's still not 100%, and thus still probabilistic.

Of course, the mods to make "stop" deterministic might break some
existing code.  So it might be better to put it into a new class like
"SearchDet.oz". This is what we did for our project.

Regards,
-- Harmon

-----Original Message-----
From: hackers-bounces at mozart-oz.org
[mailto:hackers-bounces at mozart-oz.org] On Behalf Of Raphael Collet
Sent: Wednesday, February 01, 2006 9:36 AM
To: hackers at mozart-oz.org
Subject: Re: Race condition in Search.ozf

Yves Jaradin wrote:
> an alternate solution would be to first stop the loop in thread1 and 
> then to stop the (possibly running) search. (See code below)
> To avoid a potential race-condition (thread1 at position (2) when 
> StopCalc is called), the stopping is retried until confirmed.
> Cells could be used if you want to resume the computation later.

There is a much clearer solution.  Just extend Search.object, redefine 
stop(), and provides an explicit reset() method:

local
    SearchObject = Search.object
in
    class BetterSearchObject from SearchObject
       attr stopped: false
       meth next($)
	 if @stopped then stopped else SearchObject,next($) end
       end
       meth nextS($)
	 if @stopped then stopped else SearchObject,nextS($) end
       end
       meth nextP($)
	 if @stopped then stopped else SearchObject,nextP($) end
       end
       meth last($)
	 if @stopped then stopped else SearchObject,last($) end
       end
       meth lastS($)
	 if @stopped then stopped else SearchObject,lastS($) end
       end
       meth lastP($)
	 if @stopped then stopped else SearchObject,lastP($) end
       end
       meth stop
	 stopped := true   SearchObject,stop
       end
       meth reset
	 stopped := false
       end
    end
end


Cheers,
raph

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