Re: [PATCH 2/5] add object-cache infrastructure

From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, "Jakub Narebski" <jnareb@gmail.com>,
	"Ted Ts'o" <tytso@mit.edu>,
	"Jonathan Nieder" <jrnieder@gmail.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Clemens Buchacher" <drizzd@aon.at>
Subject: Re: [PATCH 2/5] add object-cache infrastructure
Date: Mon, 11 Jul 2011 20:03:04 -0400	[thread overview]
Message-ID: <20110712000304.GA32276@sigill.intra.peff.net> (raw)
In-Reply-To: <7vk4bo9ze5.fsf@alter.siamese.dyndns.org>

On Mon, Jul 11, 2011 at 04:21:54PM -0700, Junio C Hamano wrote:

> Actually I do not think identifying the ones that can safely skipped is
> such a big issue. The case I am most concerned about is when you see that
> "two reverted back to one" (which you obviously want to avoid, to keep the
> effect of the commit the upstream has to have "two" on that line), but at
> the same time when you do not agree with the change that the upstream took
> for the _current commit_ you are replaying (i.e. you want the final result
> to have "one", not "modified one" which the upstream has applied).

I'm not sure there's a general solution to that. You can't keep the
commit you want intact, because you are rebasing and therefore building
on top of the other broken commit. So in a history like:

    B'--C'
   /
  A--B--C

You really want to perform the transformation of B to B', but on top of
C (i.e., "git checkout C; git diff B' B | git apply"). But if B and C
are textually related, it's going to conflict horribly. And I don't
think there is a general solution short of a darcs-style patch algebra.

-Peff