Re: Git Evolve

From: Stefan Xenos <sxenos@google.com>
Date: Sep 30, 2018, 20:17 UTC
Message-ID: <CAPL8ZiuzxmgNRE5mVy64xpm_bwbEdkV_X_cAW9B=6LU1T_YF9w@mail.gmail.com>
In-Reply-To: <xmqqbm8f952b.fsf@gitster-ct.c.googlers.com>

> If we lose the Change-Id: footer without adding any new cruft in the
> commit object header, that would be a great success.  It would
> be a failure if we end up touching the object header.

Yes, I was thinking along the same lines.

If obsolescence graph is stored as part of the commit header, it would get unconditionally shared whenever that commit is pushed, and I would consider it a desirable property if users could choose whether to push just a commit or a the obsolescence graph that contains the commit. Sometimes the reason you're amending a commit may be because the original contained content that shouldn't be pushed.

Putting it in the header would also either cause it to get retained forever by git gc or create a situation where we permit dangling references - and neither seem desirable to me. It would be nice if users could retain the obsolescence graph for stuff a user is actively working on, but we could discard it (with 0 cost) for commits they're done with or were never interested in editing.

On Sat, Sep 29, 2018 at 5:55 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 43 quoted lines

> Stefan Xenos <sxenos@google.com> writes:
>
>> What is the evolve command?
>> ...
>> - Systems like gerrit would no longer need to rely on "change-id" tags
>> in commit comments to associate commits with the change that they
>> edit, since git itself would have that information.
>> ...
>> Is anyone else interested in this? Please email me directly or on this
>> list. Let's chat: I want to make sure that whatever we come up with is
>> at least as good as any similar technology that has come before.
>
> As you listed in the related technologies section, I think the
> underlying machinery that supports "rebase -i", especially with the
> recent addition of redoing the existing merges (i.e. "rebase -i
> -r"), may be enough to rewrite the histories that were built on top
> of a commit that has been obsoleted by amending.
>
> I would imagine that the main design effort you would need to make
> is to figure out a good way to
>
>  (1) keep track of which commits are obsoleted by which other ones
>      [*1*], and
>
>  (2) to figure out what histories are still to be rebuilt in what
>      order on top of what commit efficiently.
>
> Once these are done, you should be able to write out the sequence of
> instructions to feed the same sequencer machinery used by the
> "rebase -i" command.
>
> [Side note]
>
> *1* It is very desirable to keep track of the evolution of a change
>     without polluting the commit object with things like Change-Id:
>     and other cruft, either in the body or in the header.  If we
>     lose the Change-Id: footer without adding any new cruft in the
>     commit object header, that would be a great success.  It would
>     be a failure if we end up touching the object header.
>
>
>
>