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Rebasing a feature branch

A feature branch was pushed, so upstream has it as a PR. Then upstream's main moved on, and the monorepo's main pulled that in. The PR is now behind main: GitHub says "This branch is out-of-date with the base branch". To bring it up to date, feature is rebased onto main. m adds lock.txt:

The monorepo, a with two children, p on main and b on feature, which is H; upstream, k, which is U(H), with two children, m on main and b prime, which is T, R and feature

The rebase strategies​

Four branches take part, two in each repository, and git-splice syncs the monorepo's folder with the upstream branch of the same name. Rebasing feature onto main is plain Git, and it's one operation that can be done in either repository. Do it in one of them only, then carry the result across with git-splice, in this order:

The two rebase strategies, each with the monorepo main and feature on the left and the upstream main and feature on the right. Rebase via upstream: 1, rebase or merge the upstream feature onto main, for example with Update branch; 2, on feature, git splice pull into the monorepo feature. Rebase via monorepo: 1, on main, git splice pull from the upstream main; 2, on feature, git rebase main; 3, git splice push --force to the upstream feature

  1. Rebase via upstream: click "Update branch" on GitHub, or rebase feature onto main in a clone of the upstream, then git splice pull on feature. The simple default.
  2. Rebase via monorepo: git rebase main on feature, then git splice push --force. For when GitHub can't rebase, e.g. on a conflict, which you then resolve with the whole monorepo to build and test against. It rewrites the monorepo's feature and overwrites the PR, so it's for branches nobody else pushes to.

Don't mix the strategies: a rebase in the monorepo, then a pull of the PR that was never rebased, sends the old commits back to the PR.

Output​

rebase-setup.bash builds this state:

$ build_scenario scenario_feature_branch_behind_default_branch
$ git -C "$UPSTREAM" log --graph --format=%s main feature
* m: bump dep
| * b: feature change
|/  
* k: seed

git-splice compares feature with upstream's feature only, and those match. That the PR is behind main is a matter between the two branches of each repository:

$ git splice status
On branch feature

  up to date  vendor/a