From: Jeff King <peff@peff.net>
Date: Tue, 12 Jun 2012 19:41:26 GMT
Subject: Re: Keeping unreachable objects in a separate pack instead of loose?
Message-ID: <20120612194126.GA17519@sigill.intra.peff.net>
In-Reply-To: <alpine.LFD.2.02.1206121536570.23555@xanadu.home>

On Tue, Jun 12, 2012 at 03:39:05PM -0400, Nicolas Pitre wrote:

> > So the race window depends on the time it takes "git prune" to run.
> > 
> > I wonder if git-prune could do a double-check of the refs. Something
> > like:
> > 
> >   1. calculate reachability on all refs
> > 
> >   2. read list of objects to prune, and make a list of unreachable ones
> > 
> >   3. calculate reachability again (which should be very cheap, because
> >      you can stop when you get to an object you have already seen)
> > 
> >   4. Drop any objects found in (3) from the list in (2), and delete
> >      items from your list
> > 
> > But I think that still has a race where objects are created before
> > step 2, but are not actually referenced until after step 3. I think
> > doing it safely may actually require a repo-wide prune lock.
> 
> Yeah... that's what I was thinking too.  Maybe we're making our life 
> overly miserable by trying to avoid any locking here.

I think I would be OK with "prune" locking, as long as everything else
was able to happen simultaneously. Especially if we can keep prune's
lock as short as possible through double-reads or similar tricks (like
we do for ref updates).

-Peff