[dmd-concurrency] Smoke test

[Sean Kelly]

On Jan 7, 2010, at 5:28 PM, Walter Bright wrote:
> 
> Having a per-thread gc is an optimization, not a fundamental feature of the concurrency model. For one thing, it precludes casting data to immutable. For another, it may result in excessive memory consumption as one thread may have a lot of unused data in its pool that is not available for allocation by another thread.

I agree completely that having a per-thread GC is simply an optimization.  I just brought it up because it's the simplest way I've come up with to think about what "shared" means.  Put another way, I think of shared as controlling the access control pool the data lives in, and the lifetime of that data.  If I see "T v1" then I should be able to infer that v1 is only visible to the current thread and will go away when the thread terminates.  Similarly, if I see "shared T v2" I should be able to infer that v2 is globally visible and may remain until process termination.

I feel like I'm not explaining myself very well, but that's the best I can do at the moment.  As a related issue, I have a feeling that the following is a bad idea, but I haven't come up with a good explanation for why yet, maybe simply the principle of least surprise?:

class C
{
    shared int x;
}

auto c = new C;
sendRefToAnotherThread( c ); // fails, c is local
sendToAnotherThread( &c.x ); // succeeds, c.x is shared