Re: Regarding "git log" on "git series" metadata

On Fri, Nov 04, 2016 at 03:49:07PM -0400, Jeff King wrote:

On Fri, Nov 04, 2016 at 12:19:55PM -0700, Jacob Keller wrote:

quoted

I agree with your assessment here. The main difficulty in implementing
gitrefs is to ensure that they actually do get picked up by
reachability checks to prevent dropping commits. I'm not sure how easy
this is, but I would much rather we go this route rather than
continuing along with the hack. This seems like the ideal solution,
since it solves the entire problem and doesn't need more hacks bolted
on.

I think the main complication is that the reachability rules are used
during object transfer. So you'd probably want to introduce some
protocol extension to say "I understand gitrefs", so that when one side
says "I have sha1 X and its reachable objects", we know whether they are
including gitrefs there. And likewise receivers with
transfer.fsckObjects may complain about the new gitref tree mode
(fortunately a new object type shouldn't be needed).

You might also want fallback rules for storing gitrefs on "old" servers
(e.g., backfilling gitrefs you need if the server didn't them in the
initial fetch). But I guess storing any gitrefs on such a server is
inherently dangerous, because the server might prune them at any time.

So perhaps a related question is: how can gitrefs be designed such that
existing servers reject them (rather than accepting the push and then
later throwing away half the data). It would be easy to notice in the
client during a push that we are sending gitrefs to a server which does
not claim that capability. But it seems more robust if it is the server
who decides "I will not accept these bogus objects".

This seems like the critical problem, here.  The parent hack I used in
git-series might be a hack, but it transparently works with old servers
and clients.  So, for instance, I can push a git-series ref to github,
with no changes required on github's part.  If git added gitrefs, and I
started using them in git-series, then that'd eliminate parent hack and
allow many standard git tools to work naturally on git-series commits
and history, but it'd also mean that people couldn't push git-series
commits to any server until that server updates git.

That said, I'd *love* to have gitrefs available, for a wide variety of
applications, and I can see an argument for introducing them and waiting
a few years for them to become universally available, similar to the
process gitlinks went through.

But I'd also love to have a backward-compatible solution.

- Josh Triplett