Subject: Re: Sharing merge conflict resolution between multiple developers

From: Junio C Hamano <[email-masked]>
Date: Mon, 11 Aug 2014 11:44:11 -0700

Chris Packham <[email-masked]> writes:

> Is there any way where we could share the conflict resolution around
> but still end up with a single merge commit.

One idea that immediately comes to me is to use something like
"rerere" (not its implementation and storage, but the underlying
idea) enhanced with the trick I use to fix-up merges in my daily
integration cycle (look for "merge-fix" in howto/maintain-git.txt
in Documentation/).

> developer A:
>   git merge $upstream
>   <conflicts>

And then commit this immediately, together with conflict markers
(i.e. "commit -a"), and discard it with "reset --hard HEAD^" *after*
storing it somewhere safe.  And then redo the same merge, resolve
the conflicts and commit the usual way.

The difference between the final conflict resolution and the
original conflicted state can be used as a reference for others to
redo the same conflict resolution later elsewhere.  That can most
easily be done by creating a commit that records the final state
whose parent is the one you recorded the initial conflicted state.

So, the "recording" phase may go something like this:

    git checkout $this
    git merge $that
    git commit -a -m 'merge-fix/$this-$that preimage'
    git branch merge-fix/$this-$that
    git reset --hard HEAD^
    git merge $that
    edit
    git commit -a -m 'merge $that to $this'
    git checkout merge-fix/$this-$that
    git read-tree -m -u HEAD $this
    git commit -a -m 'merge-fix/$this-$that postimage'

The rough idea is "git show merge-fix/$this-$that" will show the
"patch" you can apply on top of the conflicted state other people
would get by running "git merge $that" while on "$this" branch.

"rerere" essentially does the above recording (and replaying)
per-path and it comes with a clever indexing scheme to identify
which previous conflict resolution would apply to the conflicts you
see in your working tree.