Re: RefTree: Alternate ref backend

From: Mike Hommey <mh@glandium.org>
To: Shawn Pearce <spearce@spearce.org>
Cc: Jeff King <peff@peff.net>, git <git@vger.kernel.org>,
	David Turner <dturner@twopensource.com>,
	Michael Haggerty <mhagger@alum.mit.edu>
Subject: Re: RefTree: Alternate ref backend
Date: Fri, 18 Dec 2015 10:36:03 +0900	[thread overview]
Message-ID: <20151218013603.GA13717@glandium.org> (raw)
In-Reply-To: <CAJo=hJsSgU6yOFZMac85jkOtw9TXWXh0Ext4-Gb1TsSXqROn4g@mail.gmail.com>

On Thu, Dec 17, 2015 at 02:28:01PM -0800, Shawn Pearce wrote:
> On Thu, Dec 17, 2015 at 2:10 PM, Jeff King <peff@peff.net> wrote:
> > On Thu, Dec 17, 2015 at 01:02:50PM -0800, Shawn Pearce wrote:
> >
> >> I started playing around with the idea of storing references directly
> >> in Git. Exploiting the GITLINK tree entry, we can associate a name to
> >> any SHA-1.
> >
> > Gitlink entries don't imply reachability, though. I guess that doesn't
> > matter if your ref backend says "no, really, these are the ref tips, and
> > they are reachable".
> 
> Exactly. This works with existing JGit because it swaps out the ref
> backend. When GC tries to enumerate the roots (current refs), it gets
> these through the ref backend by scanning the tree recursively. The
> packer itself doesn't care where those roots came from.
> 
> Same would be true for any other pluggable ref backend in git-core. GC
> has to ask the ref backend, and then trust its reply. How/where that
> ref backend tracks that is an implementation detail.
> 
> >  But you could not push the whole thing up to
> > another server and expect it to hold the whole graph.
> 
> Correct, pushing this to another repository doesn't transmit the
> graph. If the other repository also used this for its refs backend,
> its now corrupt and confused out of its mind. Just like copying the
> packed-refs file with scp. Don't do that. :)
> 
> > Which is not strictly necessary, but to me seems like the real advantage
> > of using git objects versus some other system.
> 
> One advantage is you can edit HEAD symref remotely. Commit a different
> symlink value and push. :)
> 
> I want to say more, but I'm going to hold back right now. There's more
> going on in my head than just this.
> 
> > Of course, the lack of reachability has advantages, too. You can
> > drop commits pointed to by old reflogs without rewriting the ref
> > history.
> 
> Yes.

Related thread: "Allowing weak references to blobs and strong references
to commits" http://marc.info/?l=git&m=142779648816577&w=2

Mike