Re: RefTree: Alternate ref backend

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".  But you could not push the whole thing up to
another server and expect it to hold the whole graph.

Which is not strictly necessary, but to me seems like the real advantage
of using git objects versus some other system.

Of course, the lack of reachability has advantages, too. You can
drop commits pointed to by old reflogs without rewriting the ref
history. Unfortunately you cannot expunge the reflogs at all. That's
good if you like audit trails. Bad if you are worried that your reflogs
will grow large. :)

By storing all references in a single tree, atomic transactions are
possible. Its a simple compare-and-swap of a single 40 byte SHA-1.
This of course leads to a bootstrapping problem, where do we store the
40 byte SHA-1? For this example its just $GIT_DIR/refs/txn/committed
as a classical loose reference.

Somehow putting it inside `refs/` seems weird to me, in an infinite
recursion kind of way.  I would have picked $GIT_DIR/REFSTREE or
something. But that is a minor point.

Configuration:

  [core]
    repositoryformatversion = 1
  [extensions]
    refsBackendType = RefTree

The semantics of extensions config keys are open-ended. The
formatVersion=1 spec only says "if there is a key you don't know about,
then you may not proceed". Now we're defining a refsBackendType
extension. It probably makes sense to write up a few rules (e.g., is
RefTree case-sensitive?).

-Peff