Re: Newbie question regarding 3way merge order.

From: Raimund Berger <raimund.berger@gmail.com>
Date: Feb 1, 2009, 14:13 UTC
Message-ID: <87eiyib5y8.fsf@gigli.quasi.internal>
In-Reply-To: <20090201064543.6117@nanako3.lavabit.com>

Nanako Shiraishi <nanako3@lavabit.com> writes:

Show 45 quoted lines

> Quoting "Raimund Berger" <raimund.berger@gmail.com> writes:
>
>> I'm myself, especially since a conflicting rebase leaves the index in an
>> "unmerged" state. Much like a regular merge does. It's still all
>> assumptions though, or maybe I'm missing documentation .... (?)
>
> The rebase command you run with neither -m nor -i option replays
> your work on top of the upstream by running git-am with the --3way
> option.
>
> This commit introduced the feature.
>
> commit 7f59dbbb8f8d479c1d31453eac06ec765436a780
> Author: Junio C Hamano <junkio@cox.net>
> Date:   Mon Nov 14 00:41:53 2005 -0800
>
>     Rewrite rebase to use git-format-patch piped to git-am.
>     
>     The current rebase implementation finds commits in our tree but
>     not in the upstream tree using git-cherry, and tries to apply
>     them using git-cherry-pick (i.e. always use 3-way) one by one.
>     
>     Which is fine, but when some of the changes do not apply
>     cleanly, it punts, and punts badly.
>
>     [omitting the rest]
>
> The message talks about what was wrong with the original, what
> benefit it gives the users, and how to use it, but it doesn't
> discuss how the magic works in detail.
>
> Junio much later describes how it works, taking a real-world example
> in this message:
>
> http://thread.gmane.org/gmane.comp.version-control.git/46569/focus=46609
>
> In short, it works by applying your changes as patches but when a
> patch doesn't apply it falls back to a simplified three-way merge.
>
> http://thread.gmane.org/gmane.comp.version-control.git/100579/focus=100602
>
> I think Junio misremembered the history in his last message in the thread.
> He says that rebase was originally a format-patch piped to am, but
> before the commit 7f59dbbb8f8d479c1d31453eac06ec765436a780 it was
> done by a series of cherry-pick.

Ugh. OK, that really explains a lot alright. Especially that fundamental change in rebase behavior makes it quite clear that the various techniques for "merging" content might not be as consistent as I had hoped. In my ideal world they'd only differ in how they map out commit history, but obviously that's not the case.

To me, this really leaves questions open regarding a hassle free workflow. Since one apparently can't be confident that conflict resolutions obtained in some testing or early integration branch will easily carry over to stable it looks like special measures need to be taken, like strict observation of merge order/technique and employment of replay tools.

So serialization apparently can't entirely be lost, and where e.g. CVS kind of enforces it at an early stage through expensive branching it seems git still requires some of the same coordination just at a different level resp. point in time.

Hey, thanks a bunch for taking the time carrying those links together. They've been very informative.