Re: [PATCH] technical doc: add a design doc for the evolve command

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 19, 2018, 04:15 UTC
Message-ID: <xmqqwop9d7oq.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <CAPL8ZisfhNqN3gh0E_=mwXuBPGRZ9fKgyQ=HWNFx_9m2job-=g@mail.gmail.com>

Stefan Xenos <sxenos@google.com> writes:

Show 5 quoted lines

> The scenario you describe would not produce an origin edge in the
> metacommit graph. If the user amended X, there would be no origin
> edges - just a replacement. If you cherry-picked Z you'd get no
> replacements and just an origin. In neither case would you get both
> types of parent.

OK, that makes things a lot simpler.

I can see why we want to record "commit X obsoletes commit Y" to help the "evolve" feature, which was the original motivation this started the whole discussion. But it is not immediately obvious to me how it would help to have "Z was cherry-picked from W" in "evolve".

The whole point of cherry-picking an old commit W to produce a new commit Z is because the developer wanted to use the change between W^ and W in a context that is quite different from W^, so it would make no sense to "evolve" anything that was built on top of W on top of Z.

It is of course OK to build a different feature that can take advantage of the cherry-pick information on top of the same meta commit concept in later steps, and to ensure that is doable, the initial meta commit design must be done in a way that is flexible enough to be extended, but it is not clear to me if this "origin" thing is "while this does not have much to do with 'evolve', let's throw in fields that would help another feature while we are at it" or "in addition to 'X obsoletes Y', we need the cherry-pick information for 'evolve' feature because..." (and because it is not clear, I am assuming that it is the former). If we can design the "evolve" thing with only the "contents" and "obsoletes", that would allow us to limit the scope of discussion we need to have around meta commit and have something that works earlier, wouldn't it?

Thanks.