D2 weak references

[Jason House]

Jason House Wrote:

> 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?

Here's the implementation I'm currently thinking of:

weak ref construction:
Copy pointer into struct without casting
Call rt_attach_dispose_event

weak ref dereference:
copy value into temp var
** query gc to see if garbage **
If not garbage, Recopy value and return
If garbage set value = null

dispose of weakly referenced obj:
set value = null

The emphasized line is impossible according to the publicized gc interface. Given the delayed finalization, I don't see any way around it.