Re: RefTree: Alternate ref backend

On Thu, Dec 17, 2015 at 02:28:01PM -0800, Shawn Pearce wrote:

On Thu, Dec 17, 2015 at 2:10 PM, Jeff King [off-list ref] wrote:

quoted

On Thu, Dec 17, 2015 at 01:02:50PM -0800, Shawn Pearce wrote:

quoted

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.

quoted

 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. :)

quoted

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.

quoted

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