[MacRuby-devel] Concurrency bug in ControlTower

[Laurent Sansonetti]

On Jan 26, 2011, at 12:49 AM, Charles Oliver Nutter wrote:
> On Mon, Jan 24, 2011 at 11:13 AM, Joshua Ballanco <jballanc at gmail.com> wrote:
>> Regarding the potential bug...well...where to begin? So, yes, MacRuby blocks
>> have the same semantics as regular Ruby blocks, *except* when they are
>> dispatched through GCD. In that case, ivars are shared, but local variable
>> get copied. I'm sure Laurent can explain why better than I, but it has to do
>> with the semantics of libdispatch and the uncertainty inherent in when a
>> dispatched block or function will execute (i.e. if local variables were not
>> copied during dispatch, they might go out of scope and be collected before
>> GCD ever gets around to running the code).
> 
> Wow, that's very surprising. I'm not sure I agree with bending Ruby
> semantics so drastically, even to help concurrency. Or at least, I'd
> expect other threaded scenarios to be consistent:
> 
> ~ ➔ ruby -ve "a = 0; Thread.new { a += 1 }.join; p a"
> MacRuby 0.8 (ruby 1.9.2) [universal-darwin10.0, x86_64]
> 1
> 
> ~ ➔ ruby -ve "a = 0; q = Dispatch::Queue.concurrent; q.sync {a += 1}; p a"
> MacRuby 0.8 (ruby 1.9.2) [universal-darwin10.0, x86_64]
> 0
> 
> The implicitness in being able to mutate the surrounding scope is
> certainly problematic. This is one reason Java's anonymous inner
> classes require that referenced variables be declared final...to
> indicate they can't be mutated by the body of the anonymous inner
> class's methods.
> 
> The result is that people end up using one-element arrays and the
> like, but people find ways around anything.
> 
> So I suppose this applies to anything in the surrounding scope,
> including visibility, $~, and so on?

No, only locals and dynamic (block) variables.

To be honest I always disliked this semantic change too. I think it was a mistake to add it. It will probably be reverted for 1.0.

Laurent
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.macosforge.org/pipermail/macruby-devel/attachments/20110126/e58ae6c1/attachment.html>