Re: Leveraging --rebase-merges --update-refs mechanism to rebase several branches in one run

[Yann Dirson <ydirson@free.fr>]

Johannes Sixt wrote:
> Am 06.01.24 um 20:05 schrieb Yann Dirson:
> > The "core + 1 variant" case pretty much works out of the box, with
> > --rebase-merges
> > and --update-refs generating a perfect instructions sheet.
> > 
> > But if I was to rebase just one variant while rewriting the core
> > branch, obviously
> > all other variants would still fork off the pre-rewrite core
> > branch, and we'd loose
> > all chances of automating the same work on the other variants.
> > 
> > OTOH, if I get `git-rebase` to generate the instruction sheets for
> > those other
> > variants first, strip them (manually) from the common part, and
> > insert them in the
> > instruction sheet of my "core + 1 variant" case ... I do get the
> > whole of my branches
> > rebased together, and sharing the updated core.
> 
> Not a complete automation, but... You can merge all variant branches
> into a temporary branch (or detached HEAD), even if that are merely
> -s
> ours merges, and then rebase the temporary branch with
> --rebase-merges
> --update-refs. This will generate the instruction sheet that you
> want.
> You can remove the final merge instructions (the temporary ones) from
> the instruction sheet if you do not want them to be executed.

Nice idea, and this is indeed automatable for the most part, Q&D PoC below.

There are a few things I can see missing in this PoC:

- removal of the final merge from instruction sheet

  Could be done by wrapping $EDITOR - I'm not particularly fond doing things
  behind the user's back, but I lack better ideas.

- restoration of HEAD

  In the general case it cannot be done from the script, so we would naturally
  want to do that from the instruction sheet?

  While I was at manually removing the final merge, I experimented with changing
  the "reset onto" to "reset <a branch name>", but that resulted in moving HEAD
  to the pre-rebase version of the requested branch.

- When aborting the rebase HEAD still points to the extra merge

  This is indeed a special case of the above, where instruction sheet cannot
  be used, and where the script could help since we won't be in the middle of
  a rebase when git-rebase stops.

  There does not seem to be any documented exit-code protocol to tell the
  git-rebase caller the user aborted.  I guess "HEAD pointing to this commit"
  could be used to identify the abort.



---- 8< ---- git-rebase-batch
#!/bin/bash
set -e

die() {
    echo >&2 "ERROR: $0: $*"
    exit 1
}

REBASE_OPTS=(--interactive --rebase-merges --update-refs)
# all args before "--" are passed to git-rebase
while [ $# -ge 1 ]; do
    case "$1" in
        --) shift; break;;
        *) REBASE_OPTS+=("$1"); shift;;
    esac
done

[ $# -ge 3 ] || die "need cutting-point and at least 2 refs to rebase"
CUT="$1"
shift

git checkout --detach "$CUT"
git merge -s ours "$@" -m "temporary handle for all rebased branches"
git rebase "${REBASE_OPTS[@]}" "$CUT" HEAD

# here we can be in the middle of interactive rebase, cannot perform
# any kind of cleanup (which would include restoring HEAD ref to its
# original destination)
---- 8< ----