D2 weak references

[Sean Kelly]

== Quote from Jason House (jason.james.house at gmail.com)'s article
> Jarrett Billingsley Wrote:
> > On Mon, Apr 20, 2009 at 5:03 PM, Sean Kelly <sean at invisibleduck.org> wrote:
> >
> > > The horrible thing about all this is it makes using a multithreaded
> > > WeakRef horribly slow.  Even if the GC facility were provided for
> > > what you want to do you're talking about acquiring a mutex in
> > > the GC for every deref operation.  What a mess.
> >
> > Then take a hint from D2 and make them thread-local by default ;)  But
> > provide a SharedWeakRef for the cases where you need it.
> I'm pretty sure the GC can bypass thread locality for mark and sweep, so thread local won't help.
Anything that stops the sweep from occurring in parallel with a thread using the data bypasses this
issue.

Yeah.  The unfortunate tradeoff is that finalizers are run while all these
app threads are suspended, and if these finalizers enter a synchronized
block they're likely to deadlock.  This happened enough with the Phobos
GC that I was motivated to make the change in the first place.  It's
unfortunate, but at least WeakRef is the only victim of the current
approach that I'm aware of.