From: mental@...
Date: 2006-01-26T01:35:44+09:00
Subject: Re: LazyLoad

Quoting Erik Veenstra <google@erikveen.dds.nl>:

> > On the other end of things -- one issue that I've still got
> > to address for myself in lazy.rb is threadsafety. So you've
> > improved on mine in that respect.
>
> Well, there is a problem with my locking. Do you remember that
> a set both Branch@items and Branch@snapshots in Branch#load,
> like this?:
>
>  @snapshots  = lazy {load; @snapshots}
>  @items      = lazy {load; @items}
>
> Both Lazy objects don't share the same Mutex object, so,
> effectively, under these circumstances, there's no locking.

Well, the way I would do it is to treat load as a single computation
(which I'd be inclined to do for conceptual reasons, thread safety
aside).

That means something like:

 blah       = lazy {load; [ @snapshots, @items ]}
 @snapshots = lazy { blah[0] }
 @items     = lazy { blah[1] }

(note that Array#[] forces the promise, so we add extra laziness)

One computation, one promise, one mutex.

-mental