D2 weak references

[Jason House]

Sean Kelly Wrote:

> Daniel Keep wrote:
> > 
> > Sean Kelly wrote:
> >> Jason House wrote:
> >>> Jason House wrote:
> >>>
> >>>> Tango's GC (and therefore
> >>>> druntime's GC) has an explicit notification mechanism.
> >>> I guess I shouldn't have assumed that the features of tango became
> >>> part of druntime.  I don't see any notification mechanism :(
> >> Same as Tango:
> >>
> >> alias void delegate(Object) DEvent;
> >> extern (C) void rt_attachDisposeEvent(Object h, DEvent e);
> >> extern (C) void rt_detachDisposeEvent(Object h, DEvent e);
> >>
> >> I should probably expose these in core.runtime.
> > 
> > How are these events dispatched?  I remember a discussion about race
> > conditions if these events were fired off after all threads had been
> > resumed; you could have the event zero out the weak ref AFTER something
> > else had dereferenced it.
> 
> It's the responsibility of the person making the weak ref to make it 
> thread-safe if that's a design consideration.  If the cleanup is 
> performed before the threads are restarted and the weak ref (or 
> signal/slot mechanism) uses mutexes then a deadlock is possible.  At 
> least this approach provides the option for correct code to actually work.
> 
> > If that's still a possibility, maybe you could add a case where if the
> > delegate's funcptr is null, it just assumes the context pointer points
> > to a void*.sizeof word and zeroes it out.  You could do that without
> > resuming threads.
> 
> This is all tied into the finalizer.  To do what you suggest the GC 
> would have to perform two sweeps--one before resuming threads and one after.

If I understand you correctly, a weak reference library needs to query the GC to see if an object is marked for collection. How do I do this?