Re: Need help merging unrelated histories

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Fri, May 24 2019, Robert Dailey wrote:

> Everything I'm going to describe is related to this repository:
>
> https://github.com/powervr-graphics/Native_SDK
>
> This repo has several distinct branches. None of them seem to be tied
> to each other. Instead of having a `master` where they branched off
> each of their releases (e.g. 3.1, 3.2, 4.0), it looks like they made a
> copy of previous branches with no ancestry and then continued coding
> on that.
>
> What I'm trying to do is go back and rebase all of the X.X branches
> onto a new `master` branch. I started off with this:
>
> $ git checkout --orphan master
>
> Then I committed a `.gitattributes`. Next, I rebased the first
> (oldest) release branch:
>
> $ git rebase -i --onto master origin/3.0 --root
>
> Then I merged:
>
> $ git checkout master && git merge --no-ff -
>
> Next, I did 3.1:
>
> $ git rebase -i --onto master origin/3.0 origin/3.1 -X theirs
>
> Using interactive mode, Git is smart enough to detect duplicate
> commits and eliminates those, even though the 2 branches do not share
> a merge base. I continued doing this all the way up to rebasing 4.3,
> but that's when things got tough. I see a lot of 'UD', 'UA', 'AU',
> 'AA' merge conflicts. These are obviously due to the fact that the
> branches aren't connected. But I expected was that `-X theirs` would
> always favor what's on the branch being rebased. However, it seems
> this only affects modified conflicts, not adds or deletes.
>
> I was trying to find a way to bulk-resolve these. I mean, if git sees
> a file added on the left AND the right, I want the right one (theirs;
> the one coming from the 4.3 branch). Even though the branches are
> unrelated in terms of their history, I want the net effect of the
> rebase to essentially reflect the files on 4.3 itself. If a file isn't
> present on HEAD, delete it. If a file exists on REBASE_HEAD but not on
> HEAD, then add it. If the same file exists on both, favor the one on
> REBASE_HEAD.
>
> But I don't see a way of doing that. I tried `git checkout --theirs .`
> and `git checkout REBASE_HEAD -- .` but this doesn't work with all
> conflict types.
>
> Can anyone provide some advice on how to properly restructure this
> repository to create some ancestry, as if all along a `master` existed
> and all release branches were based on this in a linear fashion?

We don't have a merge strategy to do this, but should. I had a WIP patch
for this here that I haven't picked up:
https://public-inbox.org/git/87sh7sdtc1.fsf@evledraar.gmail.com/

But you can emulate the same thing with "read-tree" and
"commit". I.e. manually craft a commit with plumbing that diverges from
the "master" branch, then instead of stock "rebase" write some loop
where you keep using "read-tree" to read what you want to stage into the
index from the existing branched commits, then the equivalent of "git
commit -c" etc. to commit it.